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CORNELL 

UNIVERSITY 

LIBRARY 




Cornell University Library 
QH 471.G29 1890 



Evolution of sex. 




3 1924 024 756 193 






THE 

EVOLUTION OF SEX. 



Professor PATRICK GEDDES 



J. ARTHUR THOMSON. 



ILLUSTRATED. 




NEW YORK : 

THE HUMBOLDT PUBLISHING CO., 

28 LAFAYETTE PLACE. 



/\.q^g<^n^ 



PREFACE. 



JN course of the preparation of critical summaries, such as the 
articles "Reproduction" or "Sex," contributed by one of us to 
the " Encyclopedia Britannica," or the account of recent progress 
annually prepared for the Zoological Record by the other, we have not 
only naturally accumulated considerable material toward a general 
theory of the subject, but have come to take up an altered and 
unconventional view upon the general questions of biology, particularly 
upon that of the factors of organic evolution. Hence this little book 
has the difficult task of inviting the criticism of the biological student, 
although primarily addressing itself to the general reader or beginner. 
The specialist, therefore, must not expect exhaustiveness, despite a 
good deal of small type and bibliography, over which other readers 
(for whose sakes technicalities have also been kept down as much as 
possible) may lightly skim. 

Our central thesis has been, in the first place, to present an outline 
of the main processes for the continuance of organic life with such 
unity as our present knowledge renders possible; and in the second, 
to point the way toward the interpretation of these processes in those 
ultimate biological terms which physiologists are already reaching as 
regards the functions of individual life, — those of the constructive and 
destructive changes (anabolism and katabolismj of living matter or 
protoplasm. 

But while Books I. and II. are thus the more important, and such 
chapters as "Hermaphroditism," "Parthenogenesis," "Alternation 
of Generations," have only a subordinate and comparatively technical 
interest, it will be seen that our theme raises nearly all the burning 
questions of biology. Hence, for instance, a running discussion and 
criticism of the speculative views of Professor Weismann, to which 
their very recent introduction to English readers has awakened so wide 
an interest. At once of less technical difficulty, and in some respects 
even wider issues, is the discussion of Mr. Darwin's theory of sexual 
selection, reopened by the other leading contribution to the year's 
biological literature which we owe to Mr. Alfred Russel Wallace.* 
Besides entering this controversy at the outset of the volume, we have 
in the sequel attempted to show that the view taken of the processes 

* "Darwinism." No. 115 and No. 116 of the Humboldt Library. 



IV PREFACE. 

concerned with the maintenance of the species leads necessarily to a 
profound alteration of our views regarding its origin, although the vast 
problems thus raised necessarily remain open for fuller separate treat- 
ment. It is right, however, to say that the restatement of the theory 
of organic evolution, for which we here seek to prepare (that not of 
indefinite but definite variation, with progress and survival essentially 
through the subordination of individual struggle and development to 
species-maintaining ends), leads us frankly to face the responsibility of 
thus popularizing a field of natural knowledge from which there are so 
many superficial reasons to shrink, and which knowledge and ignor- 
ance so commonly conspire to veil. For if not only the utmost 
degeneracy be manifestly connected with the continuance of organic 
species, but also the highest progress and blossoming of life in all its 
forms, of man or beast or flower, it becomes the first practical applica- 
tion of biological science not only to investigate and map out these 
two paths of organic progress, but to illuminate them. Hence we 
have attempted to indicate the application of the general organic 
survey, which has been our main theme, to such questions as those of 
human population and progress, although here, more even than else- 
where, our treatment can be at best only suggestive, not exhaustive. 
While limits of space have made it impossible to give the botanical 
side of our subject its proportionate share of attention, our illustrations 
of the essential facts are sufficient so show the parallelism of the repro- 
ductive processes throughout Nature. 

It remains to express our thanks to Professor F. Jeffrey Bell for 
some valuable suggestions while the work was passing through the 
press; to Mr. G. F. Scott-Elliot for assistance in summarizing certain 
portions of the literature; and to our engravers, Messrs. Harry S. 
Percy, F. V. M'Combie, and G. A. Morison, especially to the first- 
named, who has executed the great majority of our illustrations with 
much care and skill. 

PATRICK GEDDES. 

J. ARTHUR THOMSON. 



CONTENTS. 



BOOK I.— MALE AND FEMALE. 

CHAPTER I. 

The Sexes and Sexual Selection - - . _ ^-i, 

I. Primary and secondary sexual characters. 
II. Illustrations from Darwin. 

III. Darwin's explanation by sexual selection. 

IV. Criticisms of sexual selection — 

(a) Wallace. 

(5) Brooks. 

(c) St George Mivart. 

(rf) Others. 



CHAPTER II. 

The Sexes, and Criticism of Sexual Selection - - 14-26 

I. Search for a broader basis. 

II. Differences in general habit, &c. Males active, females 
passive. 

III. Differences in size. Males smaller, females larger. 
Pigmies and exceptions. 

IV. Secondary differences in color, skin, &c. Males 
katabolic, females anabolic. 

V. Sexual selection : Its limits as an explanation. 

Postulate of extreme aesthetic sensi- 
tiveness. 

Darwin and Wallace combined and 
supplemented. 

Sexual selection a minor accelerant, 
natural selection a retarding action, 
on constitutional differentiation. 



CHAPTER III. 

The Determination, qf Sex (Hypotheses and Obser- 
vations) --------- 27-35 

I. The period at which sex is settled. Ploss, Sutton, 
Laulani^, &c. 



VI CONTENTS. 

II. Over five hundred theories suggested — 

Theological. 

Metaphysical. 

Statistical and hypothetical. 

Experimental. (Chap. IV.) 

III. Theory of male and female ova requires analysis. 

IV. Theory of "polyspermy," or multiple fertilization, 

dismissed. 
V. The theory of age of elements allowed. Thury, 
I Hensen, &c. 

i VI. Theory of parental age of secondary moment. Hofacker 

and Sadler. 

VII. Theories of ' ' comparative vigor, ' ' &c. , require analysis. 

VIII. Theory of Starkweather, — many factors combined under 

"superiority." 

IX. Darwin's position. 

X. Diising's synthetic treatment, and theory of self-regu- 
lation of numbers. 
XI. The sexes of twins. 



CHAPTER IV. 

The Determination of Sex (Constructive Treatment) - - 36-48 

I. Nutrition as a factor determining sex. Favorable 

nutrition tends to females. 

(a) Yung's tadpoles. 

(b) Cases of bees. 

(c) Von Siebold's observations. 

(d) Case of aphides. 

(e) Caterpillars. 
(jD Crustaceans. 
{£■) Mammals. 

(A) Human species. 
(i) Plants. 

II. Temperature as a factor. Favorable conditions tend to 

females. 

III. Summary of factors : — 

(a) Nutrition, age, &c., of parents affecting — 
(4) Condition of sex-cells, followed by — 
(c) Environment of embryo. 
rV. General conclusion : — Anabolic conditions favor pro- 
duction of females, katabolic conditions males. 
V. Hence corroboration of conclusion of Chap. II., that 
females were preponderatingly anabolic, males 
katabolic. 
VI. Note on Weismann's theory of heredity. 



CONTENTS. vii 

BOOK II.— ANALYSIS OF SEX— ORGANS, TISSUES, 

CELLS. 

CHAPTER V. 

_, PAGE 

Sexual Organs and Tissues ---... 51-59 

I. Essential sexual organs of animals. 
II. Associated ducts. 

III. Origin of yelk-glands, &c. 

IV. Organs auxiliary to impregnation. 
V. Egg-laying organs. • 

VI. Brood-pouches. 

CHAPTER VI. 
Hermaphroditism ---.... 60-76 

I. Definition of hermaphroditism ; its varied forms. 
II. Embryonic hermaphroditism. Ploss, Laulani6, Sutton. 

III. Casual or abnormal hermaphroditism, from jelly-fish to 

mammal. 

IV. Partial hermaphroditism, from butterflies to birds. 

V. Normal adult hermaphroditism, from sponges to toads. 
VI. Degrees of normal hermaphroditism. 
VII. Self-fertilization and its preventives. 
VIII. Complemental males — cirripedes and Myzostomata. 
IX. Conditions of hermaphroditism ; its association with, 
passivity and parasitism. 
X. Origin of hermaphroditism ; the primitive condition ; 
persistence and reversion. 

CHAPTER VII. 

The Sex-Elements (General and Historical) - - - 77^1 

I. The ovum-theory. 

II. The history of embryology, " evolution " and " epigenesis." 
Harvey's epigenesis and prevision of ovum- 
theory. 
Malpighi and early observers. 
Preformation school ; " evolution " according to 
Haller, Bonnet, and Buffon ; ovists and ani- 
malculists. 
Wolff's demonstration of epigenesis. 
Wolff's successors. 

III. The cell-theory. 

IV. The protoplasmic movement. 

V. Protozoa contrasted with Metazoa ; the making of the 

"body." 
VI. General origin of the sex-cells in sponges. 

" " " " ccelenterates. 

" " " " other Metazoa. 

VII. Early separation of the sex-cells in a minority of cases. 



Vlll CONTENTS. 

PAGB: 

VIII. " Body" versus reproductive cells, and the continuity 
of the latter. 
Owen. 
Haeckel. 
Rauber. 
Brooks. 
Jager. 
Galton. 
Nussbaum. 
IX. Weismann's theory of the continuity of the germ-plasma. 



CHAPTER VIII. 

The Egg-Cell or Ovum ------ 92-102 

I. Structure of ovum — 

Cell substance and protoplasm. 

Nucleus and chromatin. 
II. Growth of ovum — 

Transition from amoeboid to encysted phase. 

III. The yelk — 

Its threefold mode of origin. 

Its diffuse, polar, or central disposition., 

Resulting influence on segmentation. 

IV. Composite ova. 
V. Egg-envelopes — 

(a) From ovum itself 
(d) From surrounding cells. 
(c) From special glands. 
VI. Birds' eggs — 

Concrete illustration of facts and problems. 
VII. Chemistry of the ovum — 

Its capital of anastates. 
VIII. Maturation of ovum — 

Occurrence, formation, history of polar globules ; 
parthenogenetic ova. 
IX. Theories of polar globules — 

1. Minot, Balfour, Van Beneden, &c. 

2. Biitschli, Hertwig, Boveri, &c. 

3. Weismann. 

CHAPTER IX. 

The Male-Cell OR Sperm ------ 103-1 ro 

I. General contrast between sperm and ovum — 

An index to contrast between male and female. 
II. History of discovery — 

(a) Hamm and Leeuwenhoek. 

(6) Animalculists. 

(c) Classed as Entozoa or parasites. 

{d) Kolliker's demonstration of cellular origin. 



CONTENTS. ix 



III. Structure of sperm — 

"Head," "tail," "middle portion," &c. 

IV. Physiology of sperm — 

Locomotor energy and persistent vitality. 
V. Origin of sperm — 

Theory of spermatogenesis. 
VI. Further comparison of sperm and ovum — 

Processes comparable with formation of polar 
globules. 
VII. Chemistry of the sperm. 



CHAPTER X. 

Theory of Sex : Its Nature and Origin - - . . . 111-126- 
I. Suggested theories of male and female — 
Rolph. 
Minot. 
Brooks. 
II. Nature of sex — seen in Sex-cells. 

The cell-cycle. 
Protoplasmic interpretation. 

III. Problem of origin of sex. 

IV. Incipient sex among plants. 
V. Incipient sex among animals. 

VI. Corroborative illustrations 
VII. General conclusions from foregoing chapters. 



BOOK III.— PROCESSES OE REPRODUCTION. 
CHAPTER XI. 

Sexual Reproduction -- 129-14& 

I. Different modes of reproduction. 
II. Facts involved in sexual reproduction. 

III. Fertilization in plants — 

From Sprengel to Strasburger. 

IV. Fertilization in higher animals — 

From Martin Barry and Biitschli, to Van Bene- 
den and Boveri. 
V. Fertilization in Protozoa. 
VI. Origin of fertilization — 
(a) Plasmodium. 
(6) Multiple conjugation. 
(c) Ordinary conjugation. 
{d) Union of incipiently dimorphic cells. 
(e) Fertilization by differentiated sex-cells. 
VII. Hybridization in animals and plants. 



X CONTENTS. 

CHAPTER XII. 

rAGE 

Theory op Fertiljzation -------- 147-157 

I. Old theorie<; — 

{a) Ovist, (b) animalculists, (c) the "aura 
seniinalis. ' ' 

II. Modem morphological theories — 

{a) Nuclei all-important. Hertwig, Stras- 

burger, &c. 
(6) Cell-substance also important. Nussbaum, 
Boveri, &c. 
(II. Modem physiological theories — 

Sachs, De Bary, Marshall Ward, &c. 
Cienkowski and Rolph. 
Weismann's view. 

Critique and statement of present theory. 
fV. Use of fertilization to the species — 
(a) Rejuvenescence — 

Van Beneden and Biitschli. 
Galton and Hensen. 
Weismann's critique. 
(6) The observations of Maupas. 
(c) A source of variation. Brooks and Weismann. 

CHAPTER XIII. 
■Degenerate Sexual Reproduction or Parthenogenesis 158-174 

I. History of discovery 
II. Degrees of parthenogenesis — 

Artificial, pathological, occasional, partial, sea- 
sonal, total. 

III. Occurrence in animals — 

Rotifers, crustaceans, insects. 

IV. Occurrence in plants — 

Phanerogams and fungi. 
V. The ofTspring of parthenogenesis. 
VI. Effects on the species. 
VII. Peculiarities of parthenogenetic ova — 

Weismann's discovery. 
VIII. Theory of parthenogenesis — 
Minot and Balfour. 
Rolph and Strasburger. 
Weismann. 
The present. 
IX. Origin of parthenogenesis. 
X. Case of bees. 

CHAPTER XIV. 
Asexual Reproduction ----... 



175-18* 

I. Artificial division. 

II. Regeneration. 

III. Degrees of asexual reproduction. 

IV. Asexual reproduction in plants and animals. 



CONTENTS. XI 

CHAPTER XV. 

PAGE 

Alternation of Generations - - 185-199 

I. History of discovery. 
II. Rhythm between sexual and asexual reproduction. 

III. Alternation between sexual and degenerate sexual repro- 

duction. 

IV. Combination of both these alternations. 

V. Alternation of juvenile parthenogenetic reproduction, 
with the adult sexual process. 

VI. Alternation of parthenogenesis and ordinary sexual 
reproduction. 

VII. Alternation of different sexual generations. 

VIII. Occurrence of these alternations in animals. 

IX. Occurrence of alternations in plants. 

X. The problem of heredity in alternating generations. 

XI. Hints as to the rationale of alternation. 

XII. Origin of alternation of generations. 

BOOK IV.— THEORY OF REPRODUCTION. 

CHAPTER XVI. 

Growth and Reproduction -------- 203-214 

I. Facts of growth. 
II. Spencer's analysis. 

III. Cell-division. 

IV. Protoplasmic restatement. 

V. Antithesis between growth and reproduction. 
VI. The contrast in the individual — 

(a) In distribution of organs. 
{b) In the periods of life. 
VII. The contrast between asexual and sexual reproduction. 

CHAPTER XVII. 
Theory of Reproduction — co«/2«K^(/ ... - 2i5-22» 

I. The essential facts in reproduction. 
II. The beginning of reproduction. 

III. Cell-division. 

IV. Gradations from asexual severance to liberation of sex. 

cells. 
V. The close connection between reproduction and death. 
VI. Reproduction as influenced by the environment. 
VII. General conclusion. 



xii CONTENTS. 

CHAPTER XVIII. 

PACE 

Special Physiology of Sex and Reproduction • - 221-243 

I. The continuity of germ-plasma. 

II. Sexual maturity. 

III. Menstruation. 

IV. Sexual Union. ' 

V. Parturition. 
VI. Early nutrition. 

VII. Lactation. 

VIII. Other secretions. 

IX. Incubation. 

X. Nemesis of reproduction. 

XI. Love and death, or organic immortality. 

CHAPTER XIX. 

Psychological and Ethical Aspects - - - . 244-260 

I. Common ground between animals and men. 

II. The love of mates. 

III. Sexual attraction. 

IV. Intellectual and emotional diflferences between the 

sexes. 
V. Love for offspring. 

VI. Criminal habit of the cuckoo. 
VII. Egoism and altruism. 

CHAPTER XX. 

Laws of Multiplication ---... 261-275 

I. Rate of reproduction and rate of increase. 

II. History of discussion. 

III. Spencer's analysis ; individuation and genesis. 

IV. Spencer's application to man. 

V. General statement of the population question, 

VI. Sterility. 

CHAPTER XXI. 

The Reproductive Factor in Evolution - - . 276-289 

I. General history of evolution. 

II. The reproductive factor so far as hitherto recognized. 
III. Suggested lines of further construction. 



BOO K I 



THE SEXES AND SEXUAL SELECTION. 



THE EVOLUTION OF SEX. 



CHAPTER I. 

THE SEXES AND SEXUAL SELECTION. 

'pHAT all higher animals are represented by distinct male and 
female forms is one of the most patent facts of observation, 
striking enough in many a beast and bird to catch any eye, and 
familiarly expressed in not a few popular names which contrast the two 
sexes. In lower animals, the contrast, and indeed the separateness, of 
the sexes often disappears; yet even naturalists have sometimes 
mistaken for different species what were afterwards recognized to be 
but the male and female of a single form. 

L Primary and Secondary Characters. — When we pass 
from this commonplace of observation and experience to inquire 
more precisely into the differences between the sexes, we speedily 
recognize that these are of very different degrees. In some cases no 
marked differences whatever are, recognizable; thus a male starfish 
or sea-urchin looks exactly like the female, and a carefiil examination 
of the essential reproductive organs is requisite to determine whether 
these respectively produce male elements or eggs. In other cases — for 
instance, in most reptiles — no external differences are at all striking; 
but the aspect of the internal organs, both essential and auxiliary to 
reproduction, at once settles the question. In a great number of 
cases, again, the sexes resemble one another closely, but each has 
certain minor structural features at once decisive as to its respective 
maleness or femaleness. Thus in the males there are frequendy 
prominent organs used in sexual union, while the peculiar functions 
of the females are indicated in the special egg-laying or young-feeding 
organs. All such characters, directly associated with the essential 
functions of the sexes, are included under the title oi primary sexual 
characters. 

Of less real importance, though often much more striking, are the 
numerous distinctions in size, color, skin, skeleton, and the like, which 
often signalize either sex. These are termed secondary sexual 
characters; for though they will be shown in some cases at least to be 



THE EVOLUTION OF SEX. 



truly part and parcel with the male or female constitution, they are 
only of secondary importance in the reproductive process. The beard 
of man and the mane of the lion, the antlers of stags and the tusks of 
elephants, the gorgeous plumage of the peacock or of the bird-of- 
paradise, are familiar examples of secondary sexual characters in males. 
Nor are the females lacking in special characteristics, which serve as 



Fig. I. — Male and Female Bird-of-Paradise {Paradisea minor). 
From Catalogue of Zoological Museum, Dresden. 

indices of their true nature. , Large size is one of the commonest of 
these; while in some few cases the excellencies of color and other 
adornments are possessed by the females rather than by their mates. 

The whole subject of secondary sexual characters has found its most 
extensive treatment in Darwin's " Descent of Man" ; and to that work, 
therefore, the more so as its limits exceed those of the present volume, 
the reader must be assumed to make reference. All that can be here 



THE SEXES AND SEXUAL SELECTION. 




attempted is an illustration, by representative cases, of the main differ- 
ences between the sexes ; from which we shall pass to Darwin' s inter- 
pretation, and, after a fresh survey, to the explanation by which we 
propose to supplement his theory. 

II. Illustrations from Darwin. — -Among invertebrates, prom- 
inent secondary sexual characters are rarely exhibited outside the 
great division of jointed-footed animals or arthropods. There, how- 
ever, among crustaceans and spiders, but especially among insects, 
beautiful illustrations abound. Thus the great claws of crabs are 
frequently much larger in the males; and male spiders often differ 
from their fiercely coy mates, in smaller size, darker colors, and some- 
times in the power of producing rasping sounds. Among insects, the 
males are frequently distinguished by brighter colors attractively dis- 
played, by weapons utilized in disposing of their rivals, and by the 
exclusive possession of the power of noisy love- calling. Thus, as the 
Greek observed, the cicadas "live 
happy, having voiceless wives." 
Not a few male butterflies are 
preeminently more brilliant than 
the females; and many male 

beetles fight savagely for the Fig. ..-Wingrf Male and wingless Female of a 
^ o ^ certain iiioVn^Urgyia antigua). — trom Leunis. 

possession of their mates. 

Passing to backboned animals, we find that among fishes the males 
are frequently distinguished by bright colors and ornamental append- 
ages, as well as by structural adaptations for combat. Thus the 
' ' gemmeous dragonet ' ' ( Callionymus lyrd) is flushed with gorgeous 
color, in great contrast to the ' ' sordid ' ' female, and is further adorned 
by a graceful elongation of the dorsal fin. In many cases, as in the 
sea-scorpion {Cottiis scorpius), or in the stickleback {Gasterosteus) , it 
is only at the reproductive period that the males are thus transformed, 
literally putting on a wedding-garment. Every one knows, on the 
other hand, the hooked lower jaw of the male salmon, which comes to 
be of use in the furious charges between rivals ; and this is but one 
illustration of many stn^ictures utilized in the batde for mates. In 
regard to amphibians, it is enough to recall the notched crests and 
lurid coloring of our male newts, and the indefatigable serenading 
powers of male frogs and toads, to which the females are but weakly 
responsive. Among reptiles, differences of this sort are comparatively 
rare, but male snakes have often more strongly-pronounced tints, and 
the scent-glands become more active during the breeding season. In 
this, as in many other cases, love has its noisy prayer replaced by the 
silent appeal of fragrant incense. Among lizards, the males are often 
more brightly decorated, the splendor of their colors being frequently 



THE EVOLUTION OF SEX. 



exaggerated at pairing-time. They may be further distinguished by 
crests and wattlelike pouches; while horns, probably used in fighting, 
are borne by some male chameleons. 

It is among birds, however, that the organic apparatus of courtship 
is most elaborate. The males very generally excel in brighter colors 
and ornaments. Beautiful plumes, elongated feathery tresses, brightly- 
colored combs and wattles, topknots, and curious markings, occur 
with marvelous richness of variety. These are frequently displayed by 
their proud possessers before the eyes of their desired mates, with 
mingled emotions of eager love and pompous vanity ; or it may be to 
the subtier charms of music that the wooers mainly trust. During the 



St 












r t 



=&' 



» 1 



Fig. 3. — Male and Female Blackcocks. 

breeding season, the males are jealously excited and pugnacious, while 
some have special weapons for dealing directly with their rivals. The 
differences between the magnificent male birds-of-paradise and their 
sober mates, between the peacock with his hundred eyes and the plain 
peahen, between the musical powers of male and female songsters, are 
very familiar facts. Or again, the combs and "gills" of cocks, the 
"wattles" of turkey-cocks, the immense topknot of the male 
umbrella-bird {Cephalopterus orttaius), the throat-pouch of the 
bustard, — -illustrate another series of secondary sexual characters. 
The spurs of cocks and allied birds are the most familiar illustrations 
of weapons used by the males in fighting with rivals. As in other 
animals, it is important to notice that male birds often acquire their 
special secondary characters — such as color, markings, and special 
forms of feathers — only as they approach sexual maturity, and some- 
times retain them in all their glory only during the breeding season. 



THE SEXES AND SEXUAL SELECTION. 




Among mammals, which stand in so many ways in marked 
contrast to birds, the law of batde much more than the power of 
charming decides the problem of courtship. Thus most of the strik- 
mg secondary characters of male mammals are weapons. Yet there 
are crests and tufts of hair, and other acknowledgments of the beauty- 
test, while the incense of odoriferous glands is a frequent means of 
sexual attraction. The colors, too, of die males are often more 
sharply contrasted, and there are minor differences, in voice and the 
like, which can not be ignored. Of weapons, the larger canine teeth 
of many male animals, such as boars; the special tusks of, for instance, 
the elephant and narwhal; 

the antlers of stags, all but ^ ^ \ ')^!^\ 

exclusively restricted to the 
combative sex ; the horns of 
antelopes, goats and sheep, 
oxen and the like, — which 
at least predominate in the 
males, — are well known 
illustrations. The manes 

of male lions, bisons, and f,g. 4- The development of antlers i„ the successive years 
baboons, the beards of Cer- °f a stag's hfc, or in the general history of stags. 

, From Carus Sterne. 

tain goats, the crests along 

the backs of some antelopes, the dewlaps of bulls, — illustrate another 
set of secondary characters. The odoriferous glands of many mammals 
are more developed in the males, and become specially functional 
during the breeding season. This is well illustrated in the case of 
goats, deer, shrew-mice, elephants. The differences in color are 
slight compared with those seen between the sexes in birds, but in 
not a few orders the distinction is marked enough, — males being, in 
the great majority of cases, the more strongly and brilliantly colored. 
Among monkeys the difference in color in the bare regions, and the 
subtler decorations in the arrangement of the hair on the face, are 
often very conspicuous. 

III. Darwin's Explanation. — Sexual Selection. — Darwin 
started from the occurrence of such variations, in structure and habit, 
as might be useful either for attraction between the sexes or in the 
direct contests of rival males. The possessers of these variations 
succeeded better than their neighbors in the art of courtship; the 
factors which constituted success were transmitted to the offspring; 
and, gradually, the variations were established and enhanced as second- 
ary sexual characters of the species. The process by which the 
possessers of the fortunate excellencies of beauty and strength outbid 



8 THE EVOLUTION OF SEX. 

or overcome their less endowed competitors, he termed "sexual 
selection." It is only fair, however, to state Mr. Darwin's case by- 
direct quotation. 

Sexual selection "depends on the advantage which certain indi- 
viduals have over others of the same sex and species solely in respect 
of reproduction. " ... In cases where "the males have acquired their 
present structure, not from being better fitted to survive in the struggle 
for existence, but from having gained an advantage over other males, 
and from having transmitted this advantage to their male offspring 
alone, sexual selection must have come into action." . . . " A slight 
degree of variability, leading to some advantage, however slight, in 
reiterated deadly contests, would suffice for the work of sexual selec- 
tion." ... So too, on the other hand, the females "have, by a long 
selection of the more attractive males, added to their beauty or other 
attractive qualities. " . . . " If any man can in a short time give 
elegant carriage and beauty to his bantams, according to his standard of 
beauty, I can see no reason to doubt that female birds, by selecting 
during thousands of generations the most melodious or beautiful males, 
according to their standard of beauty, might produce a marked effect. ' ' 
. . . " To sum up on the means through which, as far as we can 
judge, sexual selection has led to the development of secondary 
sexual characters. It has been shown that the largest number of 
vigorous offspring will be reared from the pairing of the strongest and 
best-armed males, victorious in contests over other males, with the 
most vigorous and best-nourished females, which are the first to 
breed in the spring. If such females select the more attractive, and 
at the same time vigorous males, they will rear a larger number of 
offspring than the retarded females, which must pair with the less 
vigorous and less attractive males. So it will be if the more vigor- 
ous males select the more attractive, and at the same time healthy 
and vigorous females; and this will especially hold goto'd if the male 
defends the female, and aids in providing food for the young. The 
advantage thus gained by the more vigorous pairs in rearing a larger 
number of offspring, has apparently sufficed to render sexual selec- 
tion efficient." Another sentence from Darwin's first statement of 
his position must, however, be added. " I would not wish," he says 
in "The Origin of Species," "to attribute all such sexual differences 
to this agency; for we see peculiarities arising and becoming attached 
to the male sex in our domestic animals, which we can not believe to 
be either useful to the males in battle or attractive to the females." 
Had Darwin seen another interpretation of the facts, he would thus 
doubtless have given it frank recognition. 



THE SEXES AND SEXUAL SELECTION. 



IV. Criticisms of Darwin's Explanation.— The above expla- 
nation may be summed up in a single sentence, — a casual variation, 
advantageous to its possesser (usually a male) in courtship and repro- 
duction, becomes established and perfected by the success it entails. 
Sexual selection is thus only a special case of the more general pro- 
cess of natural selection, with this difference, that the female for the 
most part takes the place of the general environment in the picking 
and choosing which is supposed to work out the perfection of the 
species. 

The more serious objections which have been hitherto urged 
against this hypothesis, apart altogether from criticism of special cases, 
may be grouped in four grades; (i) Some, who allow great import- 
ance to both natural and sexual selection, are dissatisfied with the 
adequacy of Darwin's analysis, and seek some deeper basis for the 
variations so largely confined to the male sex. The position occupied 
by Brooks will be sketched below. (2) Others would explain the 
facts on the more general theory of natural selection, allowing com- 
paratively little import to the alleged sexual selection exercised by 
the female. Wallace has on this basis criticised Darwin's theory. 
(3) Different from either of the above is the position occupied by 
St. George Mivart, who attaches comparatively little importance to 
either natural or sexual selection. (4) We have to recognize contri- 
butions, such as those of Mantegazza, which suggest the organic or 
constitutional origin of the variations in question. It is this con- 
structive rather than destructive line of criticism which we shall our- 
selves seek to develop. 

{a) 'Wallace's Objection. — It is more convenient to begin with 
Wallace's criticism, which precedes that of Brooks's in chronological 
order. This is the more helpful in clearing the ground, since the two 
theories of Wallace and Darwin are strikingly and, at first sight, 
irreconcilably opposed. According to Darwin, the gayness of male 
birds is due to selection on the part of the females; according to 
Wallace, the soberness of female birds is due to natural selection, 
which has eliminated those which per.'^isted to the death in being gay. 
He points out that conspicuousness during incubation would be dan- 
gerous and fatal; the more conspicuous have, he thinks, been picked 
off their nests by hawks, foxes, and the like, and hence only the 
sober-colored females now remain. Darwin starts from inconspicuous 
forms, and derives gorgeous males by sexual selection; Wallace starts 
from conspicuous forms, and derives the sober females by natural 
selection; the former trusts to the preservation of beauty, the latter to 
its extinction. In 1773, the Hon. Daines Barrington, a naturalist still 
remembered as the correspondent of Gilbert White, suggested that 



lO THE EVOLUTION OF SEX. 

singing-birds were small, and hen-birds mute for safety's sake. This 
suggestion Wallace has repeated and elaborated in reference especially 
to birds and insects. The female butterfly, exposed to danger during 
egg-laying, is frequently dull and inconspicuous compared with her 
mate. The original brightness has been forfeited by the sex as a 
ransom for life. Female birds in open nests are similarly, in many 
cases, colored like their surroundings; while in those of birds where 
the nests are domed or covered, the plumage is gay in both sexes. 
At the same time, Wallace allows original importance to sexual selec- 
tion on both sides in evolving bright colors and the like. We need 
not repeat Darwin's reply to Wallace's objections, as the reader will 
at once recognize considerable force in each position.* 

{U) Brooks has called attention to the sexual differences in lizards, 
where the females do not incubate; or in fishes, where the females are 
even less exposed to danger than the males; or in domesticated birds, 
where, though all danger is removed, the males are still the more 
conspicuous and diversified sex. "The fact, too, that many structures, 
which are not at all conspicuous, are confined, like gay plumage, to 
male birds, also indicates the existence of an explanation more 
fundamental than the one proposed by Wallace, and the latter 
explanation gives no reason why the females of allied species should 
often be exactly alike when the males are very different." To the 
explanation which Brooks proposes we must therefore pass. 
L According to Darwin, Brooks says, the greater modification of the 
males is due to their struggling with rivals, and to their selection by 
the females, but " I do not believe that this goes to the root of the 
matter." The study of domesticated pigeons, for instance, shows 
that ' ' something within the animal determines that the male should 
lead and the female follow in the evolution of new breeds. The same 
is true in other domesticated animals, where, from the nature of the 
circumstances, it is inadmissible to explain this with Darwin, by 

* Since the above was written, Mr. Wallace's book on "Darwinism" [No. 
115 and No. 116 of The Humboldt Library'] has been published, in which the 
author proceeds yet fiirther in his destructive criticism of Darwin's sexual selec- 
tion. The phenomena of male ornament are discussed, and summed up as 
being "due to the general laws of growth and development," and such that 
it is "unnecessary to call to our aid so hypothetical a cause as the cumulative 
action of female preference." Or again, "if ornament is the natural product 
and direct outcome of superabundant health and vigor, then no other mode 
of selection is needed to account for the presence of such ornament." These 
conclusions are not only important in relation to Darwin's theory, but obviously 
open up the possibility of interpreting not only these as the "natural product 
^nd direct outcome of constitutional conditions" (see chap, xxi.), but many 
other features also. This consideration, however, is fraught with serious con- 
sequences to Mr. Wallace's main thesis. 



THE SEXES AND SEXUAL SELECTION. u 



supposing that the male is more exposed than the female to the 
action of selection, whether natural or sexual. Darwin concludes, 
indeed, that the male is more variable than the female, but he gives no 
satisfactory reason why female variations should be less apt than male 
variations to become hereditary, or, in other words, why the right of 
entail is so much restricted to the male sex. Darwin merely attributes 
this to the greater eagerness of the males, which, "in almost all 
animals, have stronger passions than the females." The theory 
which Brooks maintains is bound up with an hypothesis of heredity 
differing considerably from that held by Darwin. He supposes that 
the cells of the body give off gemmules, chiefly during change of 
function or of environment, and that ' ' the male reproductive cell has 
gradually acquired, as its especial and distinctive function, a peculiar 
power to gather and store up these gemmules." The female 
reproductive cells keep up the general constancy of the species, the 
male cells transmit variations. " A division of physiological labor has 
arisen during the evolution ofhfe, and the functions of the reproductive 
elements have become specialized in different directions. " " The 
male cell became adapted for storing up gemmules" (the results of 
variations in the body), "and at the same time gradually lost its 
unnecessary and useless power to transmit hereditary characteristics." 
' ' We thus look to the cells of the male body for the origin of most of 
the variations through which the species has attained to its present 
organization." The males are the more variable, but, more than that, 
their variations are much more likely to be transmitted. ' ' We are 
thus able to understand the great difference in the males of allied 
species, the difference between the adult male and the female or young, 
and the great diversity and variability of secondary male characters; 
and we should expect to find, what actually is the case, that among 
the higher animals, when the sexes have long been separated, the 
males are more variable than the females." The contrast between 
Darwin and Brooks may now be summed up again in a sentence. 
Darwin says the males are more diversified and richer in secondary 
sexual characters, chiefly because of the sexual selection exercised 
alike in courtship and in battle. Brooks admits sexual selection, but 
finds an explanation of the greater diversity of the males in his theory 
that it is the peculiar function of the male elements to transmit 
variations, as opposed to the constant tradition of structure kept up by 
the egg-cells or ova. In other words, the females may choose, yet the 
males lead; nay more, they must lead, for male variations have by 
hypothesis most likelihood of being transmitted! 

Full consideration of this hypothesis would involve much 
discussion of the problems of inheritance, which will form the subject 



12 THE EVOLUTION OF SEX. 

of a forthcoming volume; but the general conclusion of the naturally 
greater variability of the males will be stated in a different light toward 
the close of the following chapter. It will there be shown that the 
' ' something within the animal ' ' which determines the preponderance 
of male variability may be stated in simpler terms than are involved in 
Brooks's theory of heredity. To refer preponderant male variability 
back to a power, ascribed to the male reproductive cells, of collecting 
and storing up assumed gemmules, is at best but a halfway analysis. 

Both the above critics are at one with Darwin on essential points. 
Though Wallace would explain by natural selection what Darwin 
explained by sexual selection, he does not deny the importance of the 
latter in many cases. Brooks, again, emphasizes a deeper factor, 
without doubting the general truth of Darwin's account of the process. 
Different from both these positions is that occupied by (f) St. George 
Mivart, who looks for some deeper reason than either Darwin or 
Wallace suggests. The entire theory of sexual selection appears to 
him an unverified hypothesis, only acquiring plausibility when 
supported by quite a series of subsidiary suppositions. He submits 
a number of detailed criticisms; but his chief contention is that the 
beauty of males, and other secondary sexual characters, are not the 
indirect results of a long process of external selection, but the direct 
expressions of an internal force. 

The vague suggestions of Mantegazza and others are only of 
importance as indications of progress toward a fundamental explana- 
tion. An obvious objection to the theory of sexual selection, that 
has been urged by many, is that, while it may in part account for 
the persistence and progress of secondary characters after they 
attained a certain degree of development, it does not account for their 
preservation when weak or inconspicuous; in short, the theory may 
account for the perfecting but not for the origin of the characters. It 
may be enough to account for the length and the trimmings of the 
living garment, but what we wish to know is the secret of the loom. 
Darwin's account of the evolution of the eyes on the feathers of the 
argus-pheasant is indeed ingenious and interesting; but, whatever its 
probability, it is more important to ask what the predominant 
brightness of males means as a general fact in physiology. It is of 
interest, then, to notice the hints thrown out by Mantegazza, Wallace, 
and others, directly associating decorativeness with superfluous repro- 
ductive material, and the putting on of wedding-robes with the 
general excitement of the sexually mature organism. From this 
record of the discussion, it is time, however, to turn to a more 
constructive mode of treatment. 



THE SEXES AND SEXUAL SELECTION. 



13 



SUMMARY. 

I., 11. — The existence of male and female animals is a commonplace of 
observation. They differ in primary and in secondary sexual characters, of 
which illustrations are given, chiefly from Darwin. 

III. — Darwin's hypothesis of sexual selection assumes the preservation and 
perfection of variations, advantageous in courtship or in battles with rivals. 

IV.— Wallace maintains that the females have been protectively retarded 
by natural selection; Brooks, that the males predominate in power of trans- 
mitting variations, and are therefore more divergent; while Mivart demands a 
deeper analysis than is afforded by either sexual or natural selection,— such 
a physiological rationale being hinted at. 



LITERATURE. 

Brooks, W. K. — The Law of Heredity: A Study of the Cause of Variation and 

the Origin of Living Organisms. Baltimore, 1883. 
Darwin, C— On the Origin of Species by Means of Natural Selection; or, 

The Preservation of Favored Races in the Struggle for Life. [No. 58 and 

No. 59 of The Humboldt Library, 1 
The Descent of Man, and Selection in Relation to Sex. [Nos. 74, 75, 76, 

77 of The Humboldt Library.'] 
MrvART, St. George. — Lessons from Nature. London, 1876. 
Wallace, A. R.— Contributions to the Theory of Natural Selection. London, 

1871. 

Darwinism: An Exposition of the Theory of Natural Selection, with 

Some of its Applications. [No. 115 and No. 116 of The Humboldt Library.] 



M 



THE EVOLUTION OF SEX. 



CHAPTER II. 

THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 

I. To gain a firmer and broader foundation on which to base a 
theory of the differences between the sexes, it is necessary to take 
another review of the facts of the case. Instead of considering the 
differences as they are expressed in the successive classes of animals, 
it will be more convenient to arrange them for themselves, according 
as they affect habit, size, length of life, and the hke. The review 
must again be merely representative, without any attempt at com- 
pleteness. 




Fig. 5. — Male and Female Coccus Insects. 

a^ part of a cactus-plant with the excrescences due to 

coccus insects; b, male; c, female. 

II. General Habit. — Let us begin with an extreme yet weH 
known case. The female cochineal-insect, laden with reserve prod- 
ucts in the form of the well-known pigment, spends much of its life 
like a mere quiescent gall on the cactus-plant. The male, on the other 
hand, in his adult state is agile, restless, and short-lived. Now this 
is no mere curiosity of the entomologist, but in reality a vivid emblem 
of what is an average truth throughout the world of animals — the 
preponderating passivity of the females, the predominant activity of 
the males. These coccus insects are the martyrs of their respective 
sexes. Take another illustration, again somewhat extreme. There is 
a troublesome threadworm {Heterodera schactii) infesting the turnip- 
plant, which parallels in more ways than one the contrast of the coccus 
insects. The adult male is agile, and like many another threadworm; 



THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 15 



the adult female, however, is quiescent, and bloated like a drawn-out 
lemon. It may be asked, however, is not this merely the natural 
nemesis of parasitism ? The life-history answers this objection. The 
two sexes are at first ahke — agile, and resembling most threadworms; 
they become parasitic, and lose both activity and nematode form; but 
the interesting fact is, further, that the male recovers himself while 
the female remains a victim. In other insect and worm types the 
same story, in less accented characters, may be^ distinctly read. In 
many crustaceans, again, the females only are parasitic; and while this 
is in part explained by their habit of seeking shelter for egg-laying 
purposes, it also expresses the constitutional bias of the sex. The 
insect-order of bee-parasites {Strepsiptera) is remark- 
able for the completely passive and even larval char- 
acter of the blind parasitic females, while the adult 
males are free, winged, and short-lived. Throughout 
the class of insects there are numerous illustrations 
of the excellence of the males over the females, 
alike in muscular power and sensory acuteness. The 
diverse series of efforts by which the males of so 
many different animals, from cicadas to birds, sustain 
the love-chorus, affords another set of illustrations of 
preeminent masculine activity. 

Without multiplying instances, a review of the 
animal kingdom, or a perusal of Darwin's pages, 
will amply confirm the conclusion that on an average 
the females incline to passivity, the males to activity. 
In higher animals, it is true that the contrast shows 
rather in many little ways than in any one striking ^^^^^^j,c,^6^^^^^^^^^_ 
difference of habit, but even in the human species ^P-='»;'^-^^--';,^^^^'=^;;j 
the contrast is recoenized. Every one will admit that attached just above the 

UIC LUiiLiaoi. 10 1V.V, g, J (irigm of the long egg- 

strenuous spasmodic bursts of activity characterize sacs (« of the female. 

t^ 1 1 * '1* J r roin v^l3us. 

men, especially in youth, and among the less civilized 

races; while patient continuance, with less violent expenditure of 

energy, is as generally associated with the work of women. 

For completeness of argument, two other facts, which will after- 
wards claim full discussion, may here be simply mentioned, {a) At 
the very threshold of sex-difference, we find that a little active cell or 
spore unable to develop of itself, unites in fatigue with a larger more 
qtiiescent individual. Here, at the very first, is the contrast between 
male and female, {b) The same antithesis is seen, when we con- 
trast as we shall afterwards do in detail, the actively motile, minute, 
male element of most animals and many plants with the larger pas- 
sively quiescent female cell or ovum. 




THE EVOLUTION OF SEX. 



It is possible tliat the reader may urge as a difficulty against the 
above contrast the exceedingly familiar case of the male bees or 
' ' drones. ' ' It must be frankly allowed that exceptions do indeed 
occur, though usually in conditions which afford a key to the 
abnormality. Thus it will be allowed that the "drones" are in a 
peculiar position as male members of a very complex society, in 
which what is practically a third sex is represented by the great body 

of ' ' workers. ' ' They are no more fair 
examples of the natural average of 
males than the hard-driven wives of 
the lazy Kaffir are of the normal func- 
tions of women. Nor is the exceptioa 
even here a real one, for the drone, 
although passive as compared with 
the unsexed workers, is active when 
compared with the extraordinarily 
passive queen. 

To the above contrast of general 
habit two other items may be added, 
on which accurate observation is still 
unfortunately very restricted. In some 
cases the body-temperature, which is 
an index to the pitch of the life, is 
distinctly lower in the females, as has- 
been noted in cases so widely separate 
as the human species, insects, and 
Fig. 7.— Both sexes of a Flea— the Jigger or plants. In many cascs, furthermore, 
Chigoe (5...»/.y/« /.«....«.): the female ^^^ longevity of the females is much 

much swollen with eggs. — p rom i^euckart. o J 

greater. Such a fact as that women 
pay lower insurance premiums than do men is often popularly- 
accounted for by their greater immunity from accident; but the 
greater normal longevity on which the actuary calculates, has, as we 
begin to see, a far deeper and constitutional explanation. 

III. Size. — Among the higher animals, there are curious 
alternations in the preponderance of one sex over another in size. 
Thus among mammals and birds the males are in most cases the 
larger; the same is true of lizards; but in snakes the females prepon- 
derate. In fishes the males are on an average smaller, sometimes 
very markedly so, even to the extent of not being half as large as their 
mates. Below the line, among backboneless animals, there is much 
greater constancy of predominance in favor of the females. Thus 
among insects the more active males are generally smaller, and often 
very markedly; of spiders the same is true, and the males being often 




THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 17 



very diminutive are forced to task their agility to the utmost in making 
advances to their unamiable mates. So again, crustacean males are 
often smaller than the females; and in many parasitic species what 
have been well called ' ' pigmy ' ' males illustrate the contrast in an 
almost ludicrous degree. 




Fig. 8.— Male (c). Worker (/<), and Queen (■;) Ant 
From Chmnbers s Encyc, after Lubbock. 

Two cases from aberrant worm-types exhibit very vividly this same 
antithesis of size. Among the common rotifers, the males are almost 
always very different from the females, and much smaller. Sometimes 
they seem to have dwindled out of existence altogether, for only the 
females are known. In other cases, 
though present, they entirely fail to 
accomplish their proper function of 
fertilization, and, as parthenogenesis 
obtains, are not only minute, but 
useless. In a curious green marine 
worm, BoncUia, the male remains 
like a remote ancestor of the female. 
It lives parasitically on or within the 
latter, and is microscopic in size, 
measuring in fact only about one 
hundredth part of the length of its 
host and mate. Somewhat similar 
to the case of Bonellia is that of a 
viviparous coccus insect {Lecanium 
hesperiduni), where the males are 
very degenerate, small, blind, and 
wingless. In spite of this condition, we should indeed think because 
of it, they are very male; for even the larva;, while still within the 
mother, have been shown to contain fully developed spermatozoa. 




Fig. 9. — Relative sizes of a male and a female 
Rotifer {Hydatina senta). — From Leunis. 



THE EVOLUTION OF SEX. 



It would be unfair to argue from such an extreme case as that of 
Bonellia alone, but there is no doubt that up to the level of 
amphibians at least the females are generally the larger. This, then, 
must be taken in connection with the conclusion of the previous 
paragraph. A sluggish conservative habit of body tends to an 
increase of size; lavish expenditure of energy keeps down the 
accumulation of storage. Corroborative evidence will be afterwards 
forthcoming, as we contrast {a) the large and small spores which mark 
the beginnings of sex-differences, or (U) the relatively large female cell 

or Q^g with the microscopic male celL 
or spermatozoon. 

Apparent exceptions occur, it is 
true, among the higher animals. la 
I birds and mammals the males are 
usually rather larger than the females. 
This difference consists especially in 
larger bones and muscles. The 
apparent exception is in part the 
natural result of the increased stress 
of external activities which are thrown 
upon the shoulders of the males when 
their mates are incapacitated by incu- 
bation and pregnancy. Furthermore, 
w6 must recognize the strengthening 
influence of the combats between 
males, and the effect produced on 
the accumulative constitution of the 
females by the increased maternal 
sacrifice characteristic of the highest 
animals. 
■ While it is easy to point to the 
general physiological import of large size and the reverse, physiology 
is not yet far enough advanced to afford firm foothold in dealing with 
the details of secondary sexual characters. It is only possible to point 
out the path which will eventually lead us to their complete rationale. 
This path will appear less vague if reverted to after some of the 
succeeding chapters have been grasped. The point of view is simple 
enough. The agility of males is not a special adaptation to enable 
that sex to exercise its functions with relation to the other, but is a 
natural characteristic of the constitutional activity of maleness ; and the 
small size of many male fishes is not an advantage at all, but simply 
again the result of the contrast between the more vegetative growth of 
the female and the costly activity of the male. So, brilliancy of color, 




Fig. io. — Figure of the female Bonellia (from 

Adas of Naples Aquarium), with its parasitic 

pigmy male enlarged. 

IV. Other Characters.- 



THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 19 

exuberance of hair and feathers, activity of scent-glands, and even the 
development of weapons, are not and can not be (except teleologically) 
explained by sexual selection, but in origin and continued development 
are outcrops of a male as opposed to a female constitution. To sum 
up the position in a paradox, all secondary sexual characters are at 
bottom primary, and are expressions of the same general habit of body 
(or to use the medical term, diathesis) as that which results in the 
production of male elements in the one case, or female elements in 
the other.* 
< Three well-known facts must be recalled to the reader's mind at 

this point; and firstly, that in a great number of cases the secondary 
sexual characters make their appearance step by step with sexual 
maturity itself When the animal — be it a bird or insect — becomes 
emphatically masculine, then it is that these minor outcrops are 
exhibited. Thus the male bird-of-paradise, eventually so resplendent, 
is usually in its youth comparatively dull and female-like in its coloring 
and plumage. Very often, too, whether in the wedding- robe of male 
fishes or in the scent-glands of mammals, the character rises and wanes 
in the same rhythm as that of the reproductive periods. It is 
impossible not to regard at least many of the secondary sexual 
characters as part and parcel of the sexual diathesis, — as expressions 
for the most part of exuberant maleness. Secondly, when the 
reproductive organs are removed by castration, the secondary sexual 
characters tend to remain undeveloped. Thus, as Darwin notes, stags 
never renew their antlers after castration, though normally of course 
they renew them each breeding season. The reindeer, where the 
horns occur on the females as well, is an interesting exception to the 
rule, for after castration the male still renews the growth. This, how- 
ever, merely indicates that the originally sexual characters have 
become organized into the general life of the body. In sheep, 
antelopes, oxen, &c., castration modifies or reduces the horns; and the 
same is true of odoriferous glands. The parasitic crustacean Sacculina 
has been shown by Delage to effect a partial castration of the crabs to 
which it fixes itself, and the same has been observed by Giard in other 
cases. In two such cases an approximation to the feniale form of 
appendage has been observed. Lastly, in aged females, which have 
ceased to be functional in reproduction, the minor peculiarities of their 
sex often disappear, and they become liker males, both in structure 
and habits, — witness the familiar case of " crowing hens." J 

* That Mr. Wallace has adopted the same explanation of the different sexual 
characters in his new book "Darwinism," [No. 115 and No. 116 of The Humboldt 
Library'] has been already pointed out (see p. 10, note). 



20 THE EVOLUTION OF SEX. 

From the presupposition, then, of the intimate connection between 
the sexuahty and the secondary characters (which is indeed every- 
where allowed), it is possible to advance a step further. Thus in 
regard to color, that the male is usually brighter than the female is an 
acknowledged fact. But pigments of many kinds are physiologically 
regarded as of the nature of waste products. Such, for instance, is 
the guanin, so abundant on the skin of fishes and some other animals. 
Abundance of such pigments, and richness of variety in related series, 
point to preeminent activity of chemical processes in the animals which 
possess them. Technically expressed, abundant pigments are express- 
ions of intense metabolism. But predominant activity has been already 
seen to be characteristic of the male sex; these bright colors, then, 
are often natural to maleness. In a literal sense animals put on beauty 
for ashes, and the males more so because they are males, and not 
primarily for any other reason whatever. We are well aware that, in 
spite of the researches of Krukenberg, Sorby, Macmunn, and others, 
our knowledge of the physiology of many of the pigments is still very 
scanty. Yet in many cases, alike among plants and animals, pigments 
are expressions of disruptive process, and are of the nature of waste 
products; and this general fact is at present sufficient for our conten- 
tion, that bright coloring or rich pigmenting is commonly a natural 
expression of the male constitution. For the red pigment so abundant 
in the female cochineal-insect, which appears to be of the nature of a 
reserve and not a waste product, and for similar occurrences, due 
exception must be made. 

In the same way the skin-eruptions of male fishes at the spawning 
season seem more pathological than decorative, and may be directly con- 
nected with the se.xual excitement. One instance of the way in which 
the reproductive maturity is known to effect a by no means obviously 
related result may be given. Every field-naturalist knows that the 
male stickleback builds a nest among the weeds, and that he weaves 
the material together by mucous threads secreted from the kidneys. 
The little animal is also known to have strong passions ; it is polygamous 
in relation to its mates, and most pugnacious in relation to its rivals. 
Professor Mobius has shown that the male reproductive organs (or 
testes) become very large at the breeding season, and that they press 
in an abnormal way upon the kidneys. This encroachment produces 
a pathological condition in the kidneys, and the result is the formation 
of a mucous secretion, somewhat similar to what occurs in renal dis- 
ease in higher forms. To free itself from the irritant pressure of this 
secretion, the male rubs itself against external objects, — most con- 
veniently upon its nest. Thus the curious weaving instinct does not 
demand or find rationale in the cumulative action of natural selection 



THE SEXES, AND CRmCISM OE SEXUAL SELECTION. 21 



upon an inexplicable variation, and is traced back to a pathological and 
mechanical origin in the emphatic maleness of the organism. The line 
oi variation being thus given, it is of course conceivable that natural 
selection ma)' have accelerated it. 

So too, though again the physiological details are scanty, the 
superabundant growth of hair and feathers may be interpreted, in some 
measure through getting rid of waste products, for we shall see later 
how local katabolism favors cell-multiplication. Combs, wattles, and 
skin-excrescences point to a predominance of circulation in the skin 
of the feverish males, whose temperatures are known in some cases to 
be decidedly higher than those of the females. Even skeletal weapons, 
like antlers, may be similarly interpreted; while the exaggerated 
activity of the scent-glands is another expedient for excreting >vaste. 

In regard to horns, feathers, and the like, in association with vigor- 
ous circulation, two sentences from Rolph may be quoted : "The 
exceedingly abundant circulation, which periodically occurs in the at 
first soft frontal protuberances of stags, admits and conditions the 
colossal development of horn and delicate ensheathing velvet. ... In 
the same way the rich flow of blood in the feather-papillae con- 
ditions the immense growth of the feathers, . . . and the same is true 
of hairs, spines, and teeth." 

Some of the even subtler differences between the sexes are of 
interest in illustrating the general antithesis. Thus in the love-lights 
of the Italian glow-insect {Luciold) the color is said to be identical 
in the two sexes, and the intensity is much the same. That of the 
female, however, who is in other respects rather male-like in her 
amatory emotions, is more restricted. It is interesting further to 
notice that the rhythm of the light in the male is more rapid and the 
flashes are briefer, while that of the female is longer and the flashes 
more distant and tremulous. This illustration may thus serve, in con- 
clusion, as a literally illumined index of the contrasted physiology of 
the sexes. 

V. Sexual Selection: its Limit as an Explanation. — We 
are now in a better position to criticise Mr. Darwin' s theory. On his 
view, males are stronger, handsomer, or more emotional, because 
ancestral forms happened to become so in a slight degree. In other 
words, the reward of breeding success gradually perpetuated and per- 
fected a casual advantage. According to the present view, males are 
stronger, handsomer, or more emotional, simply because they are 
njales, — that is, of more active physiological habit than their mates. 
In phraseology which will presently become more intelligible and con- 
crete, the males live at a loss, are more katabolic, — disruptive changes 
tending to preponderate in the sum of changes in their living matter or 



THE EVOLUTION OF SEX. 



protoplasm. The females, on the other hand, live at a profit, are more 
anabolic, — constructive processes predominating in their life, whence 
indeed the capacity of bearing offspring. 

No one can dispute that the nutritive, vegetative, or self- regarding 
processes within the plant or animal are opposed to the reproductive, 
multiplying, species-regarding processes, as income to expenditure, or 
as building up to breaking down. But within the ordinary nutritive 
or vegetative functions of the body, there is necessarily a continuous 
antithesis between two sets of processes, — constructive and destructive 
metabolism. The contrast between these two processes is seen through- 
out Nature, whether in the alternating phases of cell-life, or of activity 
and repose, or in the great antithesis between growth and reproduc- 
tion; and it is this same contrast which we recognize as the funda- 
mental difference between male and female. The proof of this will run 
through the work, but our fundamental thesis may at once be roughly 
enunciated in a diagrammatic expression (which in its present form we 
owe to our friend Mr. W. E. Fothergill): — 




Nutrit on. Reproductionj 



\ 




lAnabolisffl. Katatelism. Female. Male. 



Here the sum-total of the functions are divided into nutritive and 
reproductive, the former into anabolic and katabolic processes, the 
latter into male and female activities, — so far with all physiologists, 
without exception or dispute. * Our special theory lies, however, in 
suggesting the parallelism of the two sets of processes, — the male 
reproduction is associated with preponderating katabolism, and the 
female with relative anabolism. In terms of this thesis, therefore, both 

* The reader whose physiological studies have not been so recent as to 
familiarize him with that conception of all physiological processes as finding 
their ultimate expression in the metabolism (anabolism and katabolism) of pro- 
toplasm, will easily place himself in a position to check our argument (often 
indeed, we trust to carry our interpretation of sex into still further detail) by 
starting from the exposition of this doctrine in Dr. Michael Foster's article, 
"Physiology," in the Encyclopedia Britannica, or with Dr. Burdon Sanderson's 
Presidential Address to Section D, British Association, 1889. The essential 
conception will, however, become clearer as we proceed (see pp. 84, 117). 



THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 23 

primary and secondary sexual characters express the fundamental 
physiological bias characteristic of either sex. Sexual selection 
resembles artificial selection, but the female takes the place of the 
human breeder; it resembles natural selection, but the selective females 
and the combative males represent a role filled in the larger case by 
the fostering or eliminating action of the environment. As a special 
case of natural selection, Darwin's minor theory is open to the objec- 
tion of being teleological, that is, of accounting for structures in terms 
of a final advantage. It is quite open to the logical critic to urge, as a 
few have done, that the structures to be explained have to be accounted 
for before, as well as after, the stage when they were developed enough 
to be useful. The origin — or, in other words, the fundamental 
physiological import — of the structures, must be explained before we 
have a complete or adequate theory of organic evolution. 

Apart from this logical insufficiency, the theory of sexual selection 
is open to many minor objections, with some of which Darwin himself 
dealt, as is mentioned in the preceding historical chapter. One 
detailed objection which seems serious may also be urged. The 
evolution of colored markings by selective preference carries with it the 
postulate of a certain level of aesthetic taste and critical power in the 
female, and this not only very high and very scrupulous as to details, 
but remaining permanent as a standard of fashion from generation to 
generation, — large assumptions all, and scarcely verifiable in human 
experience. Yet we can not suppose that Mr. Darwin considered the 
human female as peculiarly undeveloped. It is true, doubtless, that 
both insects and birds have so far and increasingly become educated in 
such sensitiveness; but when we consider the complexity of the mark- 
ings of the male bird or insect, and the slow gradations from one stage 
of perfection to another, it seems difficult to credit birds or butterflies 
with a degree of aesthetic development exhibited by no human being 
without both special aesthetic acuteness and special training. More- 
over, the butterfly, which is supposed to possess this extraordinary 
development of psychological subtlety, will fly naively to a piece of 
white paper on the ground, and is attracted by the primary aesthetic 
stimulus of an old-feshioned wall-paper, not to speak of the gaudy and 
monotonous brightness of some of our garden flowers. Thus we have 
the further difficulty, that we must suppose the female butterfly to 
have a double standard of taste, one for the flowers which she and her 
mate both visit, the other for the far more complex coloring and mark- 
ings of the males. And even among birds, if we take those 
unmistakable hints of real awakening of the aesthetic sense which are 
exhibited by the Australian bowerbird or by the common jackdaw in 
its fondness for bright objects, how very rude is this taste comparedl 



24 THE EVOLUTION OF SEX. 

with the critical examination of infinitesimal variations of plumage on 
which Darwin relies. Is not, therefore, his essential supposition too 
glaringly anthropomorphic ? 

Again, the most beautiful males are often extremely combative; 
and on the conventional view this is a mere coincidence, yet a most 
unfortunate one for Mr. Darwin's view. Battle thus constantly decides 
the question of pairing, and in cases where, by hypothesis, the female 
should have most choice, she has simply to yield to the victor. On 
our view, however, combative energy and sexual beauty rise pari passu 
with male katabolism. 

Or again, in the Aineas group of the genus Papilio Darwin notes 
how there are frequent gradations in the amount of difference between 
the sexes. Sometimes the sexes are alike dull, where we should have 
to suppose the esthetic perception must somehow have been lost or 
inhibited; sometimes the females are dull and the males splendid, — 
for Darwin, an example of the result of sexual aesthetic perception, 
this of an exquisitely subtle kind however, and without proportionate 
cerebral enlargement. In a third set of cases, both sexes are splendid, 
which would suggest logically that the male in turn had acquired a 
taste for splendor. But such cases, which usually need more 
or less cumbrous additional hypothesis of inheritance and so on to 
explain them, are intelligible enough if we regard them as illustrations 
of increasing katabolism throughout a series of species. The third set 
may be supposed to be more male or katabolic than the first, while the 
second set are midway ; although it may be freely granted a knowledge 
of the habits, size, &c., of the particular species, would be necessary to 
verify the legitimacy of this interpretation in this particular case.* 

It is necessary once more to turn to the contrast between the 
positions of Darwin and Wallace. According to Darwin, sexual 
selection, for love's sake, has accelerated the males into gay coloring; 
according to Wallace, natural selection, for safety's sake, has retarded 
the females (birds or butterflies) and kept them inconspicuously plain. 
It is no longer difficult to establish a compromise. The true view 
seems to be that both sexes have differentiated toward their respective 
goals, but the males faster, because so katabolic; the limits are 
constantly being fixed by natural selection in Wallace's cases, and as 
constantly increased by sexual selection in Darwin's. There is, in 
fact, no reason why both should not be admitted as minor factors; but 
the greater part of the explanation is to be found in the view above 

* For a discussion of the progressive development of coloring and markings, 
whether in butterflies or mammals, the reader may be referred to the works 
of Professor Eimer, and especially to his forthcoming work on Lepidoptera. 
Reference should also be made to Weismann's "Studies in the Theory ot 
Descent," for a discussion of the markings of caterpillars and butterflies. 



THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 25 

stated, namely, in the physiological constitution of males and females 
themselves. In short, the present position allows some truth in both 
these conclusions, but regards gay coloring as the expression of the 
predominantly katabolic or male sex, and quiet plainness as equally 
natural to the predominantly anabohc females. On this view, too, we 
are able to restate part of the position emphasized by Brooks. The 
greater variability of the males is indeed natural, if they be the more 
katabolic sex. In preponderant katabolism, the combinations and 
permutations of molecules which constitute variation, are necessarily 
more probable than in the quiescent, passive, or anabolic females. 
No special theory of heredity is required, — the males transmit the 
majority of variations, because they have most to transmit. 

At a later stage something more will be said of natural selection, 
and its limits as an explanation of facts. But it is here desirable to 
emphasize that just as we admit the importance of sexual selection as a 
minor accelerant in the differentiation of the sexes, so we are bound to 
recognize that natural selection is also continually in operation as a 
check to a divergence of the sexes which would otherwise tend to 
become extreme. If this retarding influence of natural selection on the 
evolutionary process were not continually present, we should find 
cases like Bonellia and the rotifers much commoner than they are 
among animals. But it is an error to exaggerate this limiting action 
into an explanation of the process itself It should also be noted that 
both the retarding action of natural selection and the accelerant action 
of sexual selection become of increasing importance as we ascend the 
series. And thus, indeed, we are impelled toward a heresy which, as 
we shall see later, has bearings against the theory of natural selection, 
which overpass the limits of our present theme. 

Postscript. — Dr. T. W. Fulton, Naturalist to the Scottish 
Fishery Board, has been good enough to furnish us with some of his 
results on the size and numerical proportions of male and female fishes, 
(i) The females are usually considerably more numerous than the 
males, and never less numerous except in the angler and the catfish. 
The proportions of females to males among flatfishes ranges from about 
i-i in the flounder to about 12-1 in the long rough dab. Among 
"round" fishes the same proportion varies from about 3-2 in the cod 
to 9-2 in the common gurnard. (2) The female is longer and larger 
among all the flatfishes, sometimes by as much as 30 per cent. In 
cod, haddock, angler, and catfish, the males are larger, while in the 
whiting the females are slighdy larger, and in the common gurnard 
decidedly so. The subject is being worked up by the above-named 
naturalist, and can not fail to yield very valuable results. 



26 THE EVOLUTION OF SEX. 



SUMMARY. 

I., II., III. — A broader basis must be sought from which to understand the 
differences between the sexes. A general survey shows that the males are 
more active in habit, the females more passive; that the males tend to be 
larger and to live longer. 

IV. — The close association of secondary sexual characters with the repro- 
ductive function is shown in the period or in the periodicity of their develop- 
ment, in the effects of castration, in the peculiarities of aged females, &c. Richer 
pigmentation, and other male characteristics, are to be interpreted as expressions 
of the katabolic predominance in the constitution of males, as opposed to the 
anabolic preponderance of the females. 

V. — Sexual selection, as an explanation of secondary sexual characters, is 
limited, by being teleological rather than aetiological, does not account for origins 
nor incipient stages, postulates subtle aesthetic sensitiveness, and is beset by 
numerous minor difficulties. Yet the opposed positions of Darwin and Wallace 
both emphasize indubitable facts; while the criticisms of Mivart, the theory of 
Brooks, and the suggestions of Rolph, Mantegazza, and others, lead on toward 
a deeper analysis. The general conclusion reached recognizes sexual selection 
(so far with Darwin) as a minor accelerant, natural selection (so far with Wallace) 
as a retarding "brake," on the differentiation of sexual characters, wWch essen- 
tially find a constitutional or organizmal origin in the katabolic or anabolic 
diathesis which preponderates in males and females respectively. 



LITERATURE. 

Brooks, Darwin, Mivart, Wallace. — As before. 

EiMER, G. H.T. — Die Enstehung derArten aufGrund vonVererben erworbener 
Eigenschaften, nach den Gesetzen organischen Wachsens. Jena, 1888. 

Geddes, p. — Articles "Reproduction," "Sex," "Variation and Selection," 
Encylc. Brit. Also on the ' ' Theory of Growth, Reproduction, Sex, and 
Heredity," Proc. Roy. Soc. Edin. 1885-6. 

RoLPH, W. H. — Biologische Probleme. Leipzig, 1884. 

Weismann, a.— Studies in the Theory of Descent (Meldola's Translation). 
London, 1880-82. 

Wallace, A. R. — Darwinism: An Exposition of the Theory of Natural Selec- 
tion, wdth Some of its Applications. [No. 115 and No. 116 of The Humboldt 
Library. '\ 



THE DETERMINATION OF SEX. 



27 



CHAPTER III. 

THE DETERMINATION OF SEX. 

{Hypotheses and Observations.) 

CO far the differences between the sexes as observed in adult forms. 
Attention must now be turned to the origin of sex itself in the 
individual organism. The historic beginning of sex will be discussed 
at a later stage; the present problem concerns the factors which 
determine whether any given organism will develop into a male or 
into a female. 

I. The Period at •which the Sex is Determined. — Every 
organism, whether male or female, develops from a fertiUzed egg-cell, 
apart of course from the occurrence of a sexual and parthenogenetic 
reproduction. This material, which in one case develops into a male, 
in another into a female, is, so far as our experience can go, always 
the same; and when the sex of the organism is absolutely decided 
is a question to which no general answer can be given. In the higher 
animals (birds and mammals) it is possible at quite an early date in 
embryonic life to tell whether the young organism will turn into a 
male or a female, though in the very earliest stages it is impossible to 
determine whether the rudiment of the reproductive organs is going 
to become a testis or an ovary. But in lower vertebrates, such as 
frogs, the period of embryonic indifference is greatly prolonged; and 
it seems certain that a hatched tadpole, even after a tendency toward, 
say maleness, has actually arisen, may in certain conditions have this 
altered in the opposite direction. Among invertebrates, the sexual 
. organs are often late in acquiring definite predominance in favor of 
either sex, — that is, the period of undecided indifference is, as one 
would expect, usually much longer. 

The factors which are influential in determining sex are numer- 
ous, and come into play at different periods, so that it is quite pos- 
sible for a germ-cell to have its future fate more than once changed. 
The constitution of the mother, the nutrition of the ova, the constitu- 
tion of the father, the state of the male element when fertilization 
occurs, the embryonic nutrition, and even the larval environment in 
some cases, — these and yet other factors have all to be considered. 

Some observations by Laulanie as to the embryonic organs are of 
interest in this connection. He distinguishes both in birds and mam- 
mals three stages in the individual development of the reproductive 
organs. These he calls (i) Germiparity, (2) Hermaphroditism, (3) 



28 THE EVOLUTION OF SEX. 

Differentiated Unisexuality; and regards them as parallel to the stages 
of historic evolution. Even for the first stage, however, when the 
elements are still very primitive, he would not allow the accuracy 
of the terms neutrality or indifference. The elements in both sexes 
are almost similar, but yet their future fate has been decided. 

Sutton has also emphasized his conviction that in the individual 
development a state of embryonic hermaphroditism obtains, and 
maintains that one set of elements predominates over the other in 
the establishment of the normal unisexual state. Ploss and others 
take up a similar position in regard to an early hermaphrodite state. 
It can only be concluded that the higher the organism is in the 
series the earlier is its sexual fate sealed; and that it is only in 
lower vertebrates, and among backbonelesss animals, that we can 
speak of prolonged neutrality of sex, or embr)'onic hermaphroditism. 

II. Answers to the Question — 'What Determines Sex? — 
To the question what settles whether an organism shall develop into 
a male or into a female, many and varied answers have been given. 
At the beginning of the last century, the theories of sex were esti- 
mated at so many as five hundred, and they have gone on increas- 
ing. It is evident that even an enumeration of these is not pos- 
sible, nor is it indeed desirable. As in so many other cases, our ideas 
respecting the determination of sex have been looked at in three 
different ways. For the theologian, it was enough to say that " God 
made male and female." In the period of academic metaphysics, 
still so far from ended, it was natural to refer to " inherent proper- 
ties of maleness and femaleness' ' ; and it is still a popular ' ' explana- 
tion ' ' to invoke undefined ' ' natural tendencies ' ' to account for the 
production of males or females. This mode of treatment, it need not 
be said, is being abandoned by biologists. It is recognized that the 
problem is one for scientific analysis; thus the constitution, age, nutri- 
tion, and environment of the parents must be especially considered. 
These investigations, which are mainly restricted to observation and 
statistics, will be first noticed; the more experimental researches, and 

\ the general conclusions, will be discussed in the next chapter. That 
the final physiological explanation is, and must be, in terms of pro- 
toplasmic metabolism, we must again, however, remind the reader 
(see p. 22, note). 

III. The theory that there are two kinds of ova, respectively 
destined to develop into males or females, is more than a mere beg- 
ging of the question. The constitution of the ovum is undoubtedly 
a fact of primal importance, but we must also recognize that what is 
virtually decided at this early stage may be counteracted by later 
influences of an opposite character. The hypothesis of two kinds of 



THE DETERMINATION OF SEX. 29 



ova was advanced, for example, by B. S. Schultze; but as the grounds 
for his views are not admitted as correct, only its existence need be 
noticed till more observations are forthcoming. 

IV. Numerous authors have attached great importance to the 
process of fertilization as a detern^jnant of the sex. 

One of the most crude positions has been that of Canestrini, who 
ascribed the determination of sex to the number of sperms entering 
the ovum. The more sperms the greater the tendency to male offspring. 
It has, however, been shown by Fol, Pfliiger, Hertwig, and others, 
that "polyspermy," or the entrance of more than one sperm, is 
extremely rare, — is, in fact, generally impossible; and when it does in 
rare conditions occur, indicates a pathological condition of the egg- 
cell, and tends to produce abnormalities. Pfliiger dilijted the seminal 
fluid of male frogs, and found that no change resulted in the normal 
numerical proportion of the sexes. The case of drones, furthermore, 
where males are known to arise from unfertilized ova, is a familiar 
example, exactly counter to Canestrini' s proposition, which may in 
fact be dismissed as wholly untenable. 

V. Time of Fertilization. — With greater weight various 
authorities have insisted upon the time of fertilization. Thus accord- 
ing to Thury (1863), followed by Diising (1883), an ovum fertilized 
soon after liberation tends to produce a female, while an older ovum 
will rather develop into a male. As a practical breeder, Thury claimed 
to determine the sex of cattle upon this principle; Cornaz and Knight 
have both practically confirmed this; while Girou has pointed out that 
female flowers fertilized as soon as they were able to receive pollen 
tended to produce female offspring. Hertwig has also shown that the 
internal phenomena of fertilization vary somewhat with the age of the 
ovum at the time. Hensen is inclined to accept the general accuracy 
of Thury' s conclusion, but extends it to the male element as well. 
' ' A very favorable condition in both ovum and sperm will probably 
lead to the formation of a female." "According to its condition, a 
sperm may either insufficiently corroborate the favorable state of the 
ovum, or constitutionally strengthen an ovum less satisfactorily con- 
ditioned." 

VI. — Age of Parents. — Hofacker (1823) and Sadler (1830) inde- 
pendendy published a body of statistics, each including about 2,000 
births, in favor of the generalization that when the male parent is the 
older the offspring are preponderatingly male; while if the parents be 
of the same age, or a fortiori if the male parent be the younger, female 
offspring appear in increasing majority. This conclusion, generally 
known as Hofacker' s and Sadler's law, has received both confirmation 
and perplexing contradiction. It has been confirmed by Gohlert, 



30 



THE EVOLUTION OF SEX. 



Boulenger, Legoyt, and others, and by some breeders of stock and 
birds, but is denied by other practical authorities, and directly contra- 
dicted by the recent statistics of Stieda, from Alsace-Lorraine, and of 
Berner, from Scandinavia. 

Summary of Statistics bearing on Relative Number of Males and Females. 



Obsirver. 


Number of 
BirdiB. 


Locality. 


FatlHT 

oKilT. 

Proportion 
ot Maloa to 
100 FemalcH 


Kuther of 
eciutil a^^c. 
Proportimi 
of Males to 
100 Females. 


Fatlier 

younger. 

Proportion 

of iVIates to 

100 Females 


Average 
Proportloli 
of Males to 
100 Females. 


Remarks. 




Hofacker 


1,996 


Tiibingeii 


II7.8 


92.0 


90.6 


I07-5 






Sadler 


2,068 


England 


121. 4 


94.8 


86.5 


114,7 






Gohlert 


4,584 




108.2 


93-3 


82.6 


I05-3 






Legoyt 


52,311 


Paris 


104.49 


102.14 


97-5 


102.97 






Boulenger 


6,006 


Calais 


109.98 


107.92 


101.63 


107.09 






Noirot 


4,000 


Dijon 


99-7 




116.0 


103-05 






Breslau 


8,084 


Zurich 


I03-9 


103. 1 


117. 6 


186.6 






Stieda 


100,590 


Alsace- 
Lorraine 


105.03 




108.39 


106.27 


Contradictory 




Berner 


267,946 


Sweden 


104.61 


106.23 


^07-45 


106.0 


Contradictory 
(see text.) 





The above table (in its upper part taken mainly from Hensen, 
after CEsterlen) shows vividly how much the results of Stieda and 
Berner conflict with the law of Hofacker and Sadler. In regard to 
Bemer's statistics, it ought to be further noted that the figures quoted 
refer to cases where the father or mother is only from one to ten 
years the older. If the father be more than ten years older, the male 
majority is 103.54; if the mother be more than ten years older, the 
proportion is 104.10 again, against Hofacker's and Sadler's conclusion. 
Compared with the above human statistics, Schlechter's results in 
regard to horses also militate against the alleged law. 

In regard to plants, various naturalists have drawn attention to the 
influence of age upon sex. The following observations are quoted by 
Heyer: In Leontarus domestica, according to Rumpf, the female plant 



THE DETERMINATION OE SEX. 31 

may bear male blossoms before its proper female flowers. In Morus 
nigra, and in other cases, according to Miller, male flowers may be 
borne first, and afterwards fruit. Treviranus observed that the first 
flowers of beech, chestnut, and other trees are male. Clausen gives 
similar examples; and Hoffman notes that in the horse-chestnut, and 
several other cases, male flowers appear first, and afterwards 
hermaphrodites or females. 

Most of the results in regard to the influence of age are, however, 
extremely unsatisfactory and conflicting. This is evident from the 
above statistics. The law of Hofacker and Sadler can not be regarded 
as in any sense established. In fact, as Hensen remarks, unless 
statistics are enormously large they prove very littie. The number of 
other factors besides parental age which may operate in any case is 
evidently great, — health, nutrition, frequency of sexual intercourse, 
abstinence after the birth of a male, and the like, all reduce the 
feasibility of the statistical method. At present, at any rate, we are 
not justified in ascribing much importance to the relative age of the 
parent except as a secondary factor, influential doubtless in relation to 
nutrition. 

VII. Comparative Vigor. — The best known, and probably still 
more influential, theory is that of ' ' comparative vigor. ' ' As 
elaborated by Girou and others, this hypothesis connects the sex of 
the offspring with that of the more vigorous parent. It can not be 
said, however, that facts bear out the case. Thus consumptive 
mothers produce a great excess of daughters, while Girou's theory 
-would lead us to expect the opposite. We require, in fact, to have 
"vigor" analyzed out into its component factors, and in so .'oing we 
shall afterwards find not only facts but reasons in fa\'cr of the 
conclusion, in part included in the above theory, that highly 
nourished females tend to produce female offspring. That form of 
the hypothesis which refers the determination of sex to ' ' genital 
superiority, " or to " relative ardency, ' ' can hardly be seriously 
considered. In this connection it has been maintained that in 
"marriages of love," after a short bethrothal, female offspring 
predominate; and a number of other interesting facts ol a like nature 
are suggested. Some skepticism as to the practicability of such 
inductions is, however, inevitable. 

VIII. Starkweather's Law of Sex.— Closely allied to the 
theory of comparative vigor is that elaborately worked out by Stark- 
weather, which is suggestive enough to deser\'e separate summary. 
He starts from a discussion of the alleged superiority of either sex. 
Few maintain that the sexes are essentially equal, still fewer that the 
females excel; the general bias of authority has been in favor of the 



32 THE EVOLUTION OF SEX. 

males. From the earliest ages philosophers have contended that 
woman is but an undeveloped man; Darwin's theory of sexual 
selection presupposes a superiority and an entail in the male line; for 
Spencer, the development of woman is early arrested by procreative 
functions. In short, Darwin's man is as it were an evolved woman, 
and Spencer's woman an arrested man. 

This notion of the superiority of males has formed the basis of 
many theories of sex. As a good illustration of this opinion, a few 
sentences may be quoted from Richarz : ' ' The sex is not a quality 
transmitted from the parents, but has its basis in the degree of 
organization attained by the offspring. The male sex represents to a 
certain extent a higher grade of organization or development in the 
embryo. This is attained when the reproductive efficiency of the 
mother is specially well developed, and the resulting male offspring 
more or less resembles the mother. But if the maternal reproductive 
power be weak, the ovum does not attain to maleness, and the 
resulting female more or less resembles the father." Thus Hough 
thinks males are born when the maternal system is at its best; more 
females at periods of growth, reparation, or disease. Tiedman and 
others regard female offspring as arrested in the original state; while 
Velpau conversely regards females as degenerate from primitive 
maleness. 

Reacting from such speculations as to superiority of either sex, 
Starkweather firmly maintains that ' ' neither se.\ is physically the 
superior, but both are essentially equal in a physiological sense." 
This is true in the average, but yet in each pair a greater or less 
degree of superiority on one side or other must usually be conceded. 
Granting this. Starkweather states, as his chief conclusion, that "sex 
is determined by the superior parent, ' ' also that ' ' the superior parent 
produces the opposite sex. ' ' Referring the reader to the Encydopisdia 
Briiannica article " Sex," for some critical notes, it is enough here to 
notice that, just like ' ' comparative vigor, ' ' sex ' ' superiority ' ' has 
little more than verbal simplicity to recommend it, since it lumps a 
great variety of factors under a common name. Yet, in justice to its 
author, we may admit that it is the algebraic sum of these which he 
aims at expressing. 

IX. Darwin's Position. — Neither in regard to the origin of sex, 
nor its determination in individual cases, did Darwin see further than 
his contemporaries. He refers to the current theories of the influence 
of age, period of impregnation, and the like; and further contributes a 
great body of statistics on the numerical proportions of the sexes, 
and the supposed influence of polygamy. "There is reason," he 
says, "to suspect that in some cases man has by selection indirectly 



THE DETERMINATION OF SEX. 



33 



influenced his own sex-producing powers." He falls back upon the 
unanalyzed ' ' belief that the tendency to produce either sex would be 
inherited like almost every other peculiarity, — for instance, that of 
producing twins." "In no case, as far as we can see, would an 
inherited tendency to produce both sexes in equal numbers, or to 
produce one sex in excess, be a direct advantage or disadvantage to 
certain individuals more than to others; . . . and therefore a 
tendency of this kind could not be gained through natural selection." 
' ' I formerly thought that when a tendency to produce the two sexes 
in equal numbers was advantageous to the species, it would follow from 
natural selection, but I now see that the whole problem is so intricate 
that it is safer to leave its solution for the future. ' ' Any other hints 
that Darwin threw out have been so well elaborated by Diising's work 
on the advantageous self-regulation of the sex-proportions, that 
reference to the latter is more profitable. 

X. Dusing on the Proportions of the Sexes, and the 
Regulation of these. — In an important work, Dusing has recently 
treated the whole subject with some synthetic result. He recognizes 
that the fates or factors determining the sex are manifold, and operate 
at different periods. Much is determined by the condition of the 
reproductive elements, that is, by the constitution and habits of the 
parents; much depends also on the period of fertilization; while again 
the nutrition of the embryo may be of moment. Diising has collected 
a great body of facts, from both plants and animals, in favor of his 
conclusions; but the copious summary of his work, given in the 
article ' ' Sex ' ' already referred to, need not here be repeated, while 
some of his experimental results will be included in the next chapter. 

Diising's memoir is very important, however, for this special 
reason, that he analyzes what may be termed the mechanism by which 
the proportion of the sexes is regulated. Instead of vaguely referring 
the whole matter to natural selection, he shows in detail how the 
numbers are in a sense self-regulating, how there is always produced 
a majority of the sex that is wanted. That is to say, if one sex be in the 
decided minority, or under conditions which come to the same thing, 
then a majority of that sex will be produced. If there be, for instance, 
a great majority of males, there is the greater likelihood of the ova 
being fertilized early, but that means a probable preponderance of 
female offspring, and thus the balance is restored. It would be rash 
to say that in every case he makes out his contention, but his general 
argument that disturbances in the proportion of the sexes bring about 
their own compensation is carefully and convincingly worked out. 

XI. Sex of Twins. — It sometimes happens among many 
different classes of animals that from one ovum two organisms 



34 'J'ti^ EVOLUTION OF SEX. 



develop. We have then a case of ' ' true ' ' twins, as opposed to cases 
where multiple offspring do not arise from one ovum. Such ' ' true ' ' 
twins seem to occur not uncommonly in the human species, and are 
either most markedly similar to one another or strongly dissimilar. 
The import of this is one of the minor problems of heredity, and can 
not be here discussed, but we have to note the general fact, which 
holds without exception in the human species, that ' ' true ' ' twins are 
of the same sex. 

From a very early date an exception to this rule has been known 
in regard to cattle, and applies to some other organisms as well. 
From the careful researches of Spiegelberg and others, it appears that 
in cattle {a) the twins may be both female and then both normal, or 
{!}) that the sexes may be different and normal, or (c) that both may 
be males, in which case one always exhibits the peculiar abnormality 
known as a " free-martin. ' ' The internal organs are male, but the 
external accessory organs are female, and there are also rudimentary 
female ducts. No theory has yet explained the facts of this case. 

It is now necessary, with Diising for transition, to pass from the 
historical mode of treatment to something more constructive. Leav- 
ing mere hypotheses behind, as well as theories based on insufficient 
statistics, an induction from experimental evidence will be built up in. 
the following chapter. 



THE DETERMINATION OF SEX. 35 

SUMMARY. 

I. The epoch at which the sex is finally determined is variable in different 
animals, and diverse factors operate at successive epochs. 

II. Theological and metaphysical theories of sex have preceded the.scientific ; 
observation and statistics have been resorted to before experiment ; and over 
500 theories in all have been set forth. 

III-VI. That there are two kinds of ova is still for the most part an assumption ; 
that the entrance of more than one spermatozoon normally occurs, and is a 
determining factor, is erroneous. Thury's emphasis on the age of the ovum 
when fertilized is probably justified ; while Hensen extends this notion to the 
male element as well. The age of the parents is probably only of secondary 
import, and the law of Hofacker and Sadler is not confirmed. 

VII-VIII. Theories of " comparative vigor " and the like must be dismissed; 
while Starkweather's theory of the relative superiority of either sex, and of 
the influence of this on the sex of the offspring, requires fiirther analysis. 

IX-X. Darwin's position contains nothing novel, and has been superseded 
by Diising's synthetic treatment and explanation of the self-regulating 
numerical proportion of the sexes. 

X. From this point, after a note on the similar sex of " true " twins, we pass 
to the experimental data and constructive treatment. 



LITERATURE. 

Berner. — Hj. Om Kjonsdannelsens Aarsager, En biologisk Studie (with 

numerous references). Christiania, 1883. 
Darwin, C— The Descent of Man, Chap. VIII. Nos. 74, 75, 76, 77. 

The Variation of Animals and Plants under Domestication. Lond. 

DiJsiNG, C. — Die Regulierung des Geschlechtsverhaltnisses bei der 

Vermehrung der Menschen, Thiere, und Pflanzen. Jena, 1884 ; or, Jen. 

Zeitsch. f. Naturw., XVII., 1883. 
Geddes, p.— As before. 
Hensen, V.— Physiologie der Zeugung. Hermann's Handbuch der 

Physiologie, Bd. VI., pp. 304, with references to PIoss, Schultze, &c. 

Leipzig, 1 881. 
His, W.— Theorien der geschlechtlichen Zeugung. Arch. f. Anthropologie. 

Bde. IV.-VI. 
Hofacker.— Ueber die Eigenschaften, welche sich bei Menschen und Thieren 

auf die Nachkommen vererben. Tubingen, 1828. 
Laulanie, F.— Comptes Rendus, CI,, pp. 593-5- 1885. 
RoLPH, W. H.— As before. 

Roth, E.— Die Thatsachen der Verebung (historical). Berlin, 1885. 
Ppluger, E.— Ueber die das Geschlecht bestimmenden Ursachen und die 

Geschlechts-verhaltnisse der Frosche. Arch. f. d. ges. Physiol. XXIX. 
Sadler.— The Law of Population. London, 1830. 
Schlechter.— Ueber die Ursachen welche das Geschlecht bestimmen. Rev. 

f. Tierheilkunde. Wien, 1884. Biol. Centralblt., IV., pp. 627-9. 
Starkweather.— The Law of Sex. London, 1883. 
Stieda.— Das Sexual Verhaltniss bei Geborenen. Strasburg, 1875. 
Sutton, J. B.— General Pathology. London, 1886. 

Thurv.— Ueber das Gesetz der Erzeugung der Geschlechter. Leipzig, 1863. 
Wappceus.— Allgemeine Bevolkerungs-Statistik. Leipzig, 1861. 



36 THE EVOLUTION OF SEX. 

CHAPTER IV. 

THE DETERMINATION OF SEX. 

(^Experiment and Rationale?) 

Influence of Nutrition. — Throughout Nature the influence 

of food is undoubtedly one of the most important environmental 
factors. To Claude Bernard, indeed, the whole problem of evolution 
was very much a question of variations of nutrition. ' ' L' evolution, 
c'est r ensemble constant de ces alternatives de la nutrition; c'est 
la nutrition consideree dans sa realite, embrassee d'un coup d'oeil i 
traverse le temps." It is fitting that we should begin our survey 
of the factors known to influence sex with the fundamental function 
of nutrition. 

{a) The Case of Tadpoles. — Not a few investigaters who have 
passed from statistics and hypothesis to experiment and induction, 
have found their material in tadpoles, where the sex seems to remain 
for a comparatively long period indeterminate. If we take the 
verdict of Yung, who has had most experience with these forms, 
tadpoles pass through a hermaphrodite stage, in common, according 
to other authorities, with most animals. During this phase external 
influences, and especially food, decide their fate as regards sex, though 
the hermaphroditism, as we shall afterwards see, sometimes per- 
sists in adult life. It is fair, however, to notice that Pfliiger gives 
a somewhat different account of the actual facts, distinguishing among 
tadpoles three varieties — (a) distinct males, ib) distinct females, and 
(c) hermaphrodites. In the last, testes, or male organs, develop 
round primitive ovaries, and if the tadpoles are to become males the 
inclosed female organs are absorbed. 

Adopting the view stated by Yung, we shall simply state the 
striking results of one series of observations. When the tadpoles 
were left to themselves, the percentage of females was rather in the 
majority. In three lots the proportions of females to males were 
as follows: 54-46; 61-39; and 56-44. The average number of 
females was thus about 57 in the hundred. In the first brood, by 
feeding one set with beef, Yung raised the percentage of females 
from 54 to 78; in the second, with fish, the percentage rose from 
61 to 81; while in the third set, when the especially nutritious flesh 
of frogs was supplied, the percentage rose from 56 to 92. That is 
to say, in the last case the result of high feeding was that there were 
92 females to 8 males. From the experience and carefulness of the 
observer, these striking results are entitled to great weight. 



THE DETiIRMINATION OF SEX. 



37 



(b) The Case of Bees. — The three kinds of inmates in a beehive 
are known to every one as queens, workers, and drones; or, as 
fertile females, imperfect females, and males. What are the factors 
determining the differences between these three forms ? In the first 
place, it is believed that the eggs which give rise to drones are not 
fertilized, while those that develop into queens and workers have 
the normal history. But what fate rules the destiny of the two latter, 
determining whether a given ovum will turn out the possible mother 
of a new generation, or remain at the lower level of a non-fertile 
working female ? It seems certain 
that the fate mainly lies in the quan- 
tity and quality of the food. Royal 
diet, and plenty of it, develops the 
reproductive organs of the fiiture 
queens; sparser and plainer food re- 
tards the sexuality of the future workers, 
in which reproductive organs do not 
develop. Up to a certain point, the 
nurse-bees can determine the future des- 
tiny of their charge by changing the 
diet, and this in some cases is certainly 
done. If a larva on the way to become 
a worker receive by chance some f,g. x^.- The Queen (a), Worker (o, and 

crumbs from the royal superfluity, the Drone (b) of the Common Hive-bee. 

reproductive function may develop, and what are called ' ' fertile 
workers," to a certain degree above the average abortiveness, 
result; or, by direct intention, a worker grub may be reared into a 
queen-bee. 

The following table, after a recent analysis by A. von Planta, shows 
the differences of diet as far as solids are concerned. For queens 
69.38 per cent, for drones 72.75 per cent, and for workers 71.63 
per cent is water. 




Solids. 


Queens. 


Drones. 
I to 4 days. 


Drones. 
After 4 days. 


Workers. 


Nitrogenous ... ... 

Fatty 

Glucose ... ... 

Ashes 


45-14 

13-55 

20.39 

4.06 


55-91 
11.90 

9-57 


4-74 

38-49 

2.02 


51.21 

6.84 

27-65 



From the above, it is seen that the queen larvae get a quantity of 
fatty material double that given to the workers. The drones at first 
receive a large percentage of nitrogenous material, but this soon 



-.8 THE EVOLUTION OE SEX. 



falls below the share which workers and queens obtain. The fatty 
material, at first large, soon falls to about a third of that given to 
the queens. Hence the percentage of glucose, except at first, is so- 
much larger than in the other two cases. 

It is not necessary, however, to go into details to see the import- 
ance of the main point- — that differences of nutrition, in great 
part at least, determine the all-important distinctions between the 
development and retardation of femaleness. Nor are there many 
facts more significant than the simple and well-known one, that 
within the first eight days of larval life, the addition of food will 
determine the striking structural and functional differences between 
worker and queen. 

Eimer has drawn attention to the interesting correlation exhibited 
in the fact that a larva destined to become a worker, but converted 
into a queen, attains with the increased sexuality all the little structural 
and psychological differences which otherwise distinguish a queen. 
Regarding fertilization as a sort of nutrition, he considers drones, 
workers, and queens, as three terms of a series, and the same view 
is suggested by Rolph. Eimer recalls some interesting corroborations 
from humble-bees. There the queen-mother, awakened from her 
winter sleep by the spring sun, makes a nest, collects food, and lays 
her first brood. These are not too abundantly supplied with nourish- 
ment, the queen having much upon her shoulders; they develop 
into small females, workers in a sense, but yet fe-dle, though only 
to the extent of producing drones. By-and-by a second brood of 
workers is born; these have the advantage of the existence of elder 
sisters, are more abundantly nourished, and develop into large 
females. Still, like the first brood, they produce drones, though 
occasionally females. Finally, with the advantage of two previous 
broods of small and large females, the future queens are born. The 
above facts not only afford an interesting corroboration of the 
influence of nutrition upon sexuality, but are of importance as sug- 
gesting the origin of the more highly specialized society of the hive- 
bee. 

(c) Von Siebold's Experiments. — With a somewhat different pur- 
pose than that at present pursued, Von Siebold made a series ot 
careful observations on a species of wasps, A^emahis vcntricosus. These 
afford, as Rolph has noted, some valuable results in regard to the 
determination of sex. In this wasp, the fertilized ova, unlike those 
of hive-bees, develop into males as well as females; while the unfer- 
tilized or parthenogenetic eggs may produce females in small per- 
centage. From spring onwards, as warmth and food both increased, 



THE DETE^iJMINATION OF SEX. 



39 



Von Siebold estimated the percentages of males and females in broods 
of larvae reared from fertilized ova. The results of a series of obser- 
vations may be condensed in a table : — 



End of Larval Period. 
(Pupation.) 


Percentage of 
Females. 


No. of 
Females. 


No. of 
Males. 


15th June . . . 

tt : 

August . . . 
End of August 
September . . . 


14 

77 
269 
340 
500 
100 


19 

66 
579 


136 

66 
215 



As Rolph remarks, the results are not altogether satisfactory foi 
the present purpose, ' ' but this much is clear, that the percentage 
of females increases from spring to August, and then diminishes. We 
may conclude, without scruple, that the production of females from 
fertilized ova increases with the temperature and with the food-supply 
(^Assimilations kisiung), and decreases as these diminish." 

From the work of Rolph, which is full ot suggestiveness which 
the author unfortunately did not live to elaborate, we shall quote 
another paragraph summing up further experiments of Von 
Siebold : — 

"Not less instructive," he says, "are the experiments with 
mifertilized ova (_see Table). 



No. of 
Experiments. 


Duration of Embryonic 
and Larval State. 


Sex. 


II 
12 
13 
14 
15 
16 

17 


21 days. 

19 " 

18 " 
17 " 

17 " 

18 " 
24 " 


All Males. 

All Males. 
493 Males. 2 Females. 
265 " 2 

374 " 8 
168 " I 
I " ... 



' ' This table shows the same general result as before. The more 
abundant the metabolism {Stoffwechsel) and the nutrition, the greater 
tendency to the production of females, which at the beginning and 
at the end are wholly absent. In the above series of experiments, 
they only appear when the metabolism and the nutrition were so 
abundant that the entire development of the young wasps only 
occupied eighteen or fewer days up to the period of pupation." 
The peculiarity in this last case, if the experiments were correct, is 



_^o THE EVOLUTION OF SEX. 

that in parthenogenesis, where the production of males is the ncnnal 
condition, favorable environmental influences appear to introduce 
females. 




Fig. 13. — Two forms of a common Plant-louse or Aphis. 

This figure equally well illustrates three different things: 
a winged male and a wingless female: a winged and a 
wingless parthenogenetic female; a winged sexual female; 
and an ordinary parthenogenetic female. — From Kessler. 

(d) The Case of Aphides. — One of the most familiar illustrations 
of the influence of nutrition upon sex is found in the history of the 
plant-lice or aphides, which is indeed full of other suggestions in 
regard to the whole theory of sex and reproduction. Details in 
regard to these plant-lice, which multiply so rapidly upon our rose- 
bushes, fruit-trees, and the like, differ somewhat in the various species, 
but the general facts are recognized to be as follows. During the sum- 
mer months, with favorable temperature and abundant food, the aphides 
produce parthenogenetically generation after generation of females. 
The advent of autumn, however, with its attendant cold and scarcity 
of food, brings about the birth of males, and the consequent recur- 
rence of strictly sexual reproduction. In the artificial environment of 
a greenhouse, equivalent to a perpetual summer of warmth and 
abundant food, the parthenogenetic succession of females has been 
experimentally observed for four years, — it seems, in fact, to continue 
until lowering of the temperature and diminution of the food at once 
reintroduce males and sexual reproduction. 

(e) Butierjiies and Moths. — Still keeping to insects, we may note 
Mrs. Treat's interesting experiment, that if caterpillars were shut up 
and starved before entering the chrysalis state the resultant butterflies 
or moths were males, while others of the same brood highly nourished 
came out females. Gentry too has shown for moths, that innutritious 
or diseased food produced males, and suggests this as a partial expla- 
nation of the excess of male insects in autumn, although we suspect 
that temperature is in this instance probably more important. 



THE DETERMINATION OF SEX. 41 

(/) Crustaceans. — In support of the same contention, Rolph has 
drawn attention to the following among other facts. One of the brine- 
shrimps {Artemia salina) resembles not a few crustaceans in the local 
and periodic scarcity or absence of males, associated of course with 
parthenogenesis. At Marseilles, Rolph says, this artemia lives in 
especially favorable conditions, as its large size plainly indicates; there 
it produces only females. Where the conditions of existence are less 
prosperous, it produces males as well. ' ' A certain maximum of 
abundance and optimum of vital conditions in parthenogenetic ani- 
mals — daphnids and aphides, Apus, Branchipus, Artemia, and 
numerous other crustaceans — produce females; while less favorable 
conditions are associated with the production of males. ' ' In regard, 
however, to water-fleas (daphnids), it is fair to notice that Rolph' s 
conclusions do not quite consist with Weismann's, who, with unique 
experience in regard to these curious little animals, is disinclined to 
allow the direct influence of temperature and nutrition in the matter. 

(^g) Mainmals. — When we pass to higher animals, the difficulties 
of proving the influence of nutrition upon sex are much greater. Yet 
there are decisive observations which go to increase the cumulative 
evidence. Thus an important experiment was long ago made by 
Girou, who divided a flock of three hundred ewes into equal parts, of 
which one half were extremely well fed and served by two young rams, 
while the others were served by two mature rams and kept poorly 
fed. The proportion of ewe-lambs in the two cases was respectively 
sixty and forty per cent. In spite of the combination of two factors, 
the experiment is certainly a cogent one. Diising brings forward 
further evidence in favor of the same conclusion, — noting, for 
instance, that it is usually the heavier ewes which bring forth ewe- 
lambs. He emphasizes the fact that the females, having a more 
serious reproductive sacrifice, are more dependent on variations of 
nutrition than males. Even in birds, as Stolzmann points out, there 
is a much greater flow of blood to the ovaries than to testes,— the 
demands are greater, and the consequences therefore more serious 
if these are not fulfilled. 

(Ji) In the human species, lastly, the influence of nutrition, though 
hard to estimate, ,is more than hinted at. Ploss may be mentioned 
as an authority who has emphasized this factor in homo. Statistics 
seem to show that after an epidemic or a war the male births are 
in a greater majority than is usually the case. Diising also points 
out that females with small placenta and little menstruation bear more 
boys, and contends that the number of males varies with the harvests 
and prices. In towns, and in prosperous families, there seem to be 



42 



THE El'OLUTION OF SEX. 



more females, while males are more numerous in the country and 
among the poor. 

(?) Determination of Sex in Plants. — It is at present extremely 
difficult to come to any very satisfactory conclusion in regard to the 
influence of nutrition upon the sex of plants. The whole subject, as far 
as its literature is concerned, has been recently discussed by Heyer, 
but his survey is by no means a sanguine one. His conclusions, in fact, 
seem to land him in a skepticism as to all modification of the organism 
by environmental influences, which we should of course be far from 
sharing. It must be admitted that the experiments of Girou (1823), 
Haberlandt (1869), and others, yielded no certain result; while the 
conclusions of some others, are conflicting enough to justify not 
indeed Heyer' s despair, but his present caution. Still a few inves- 
tigations, especially those of Meehan (1878), which are essentially 
corroborated by Diising (1883), go to show, for some cases, that 
abundant moisture and nourishment do tend to produce females. 




Fig. 14. — Male and Female Flowers of Pink Campion {Lychnis diuirtu^. 

Some of Meehan' s points are extremely instructive. Thus old branches 
of conifers, overgrown and shaded by younger ones, produce only 
male inflorescence. Various botanists, quoted by Heyer, confirm one 
another in the observation that prothallia of ferns grown in unfavor- 
able nutritive conditions produce only antheridia (male organs), and 
no archegonia or female organs. 

The botanical evidence, though by no means very strong, cer- 
tainly corroborates the general result that good nourishment pro- 
duces a preponderance of females. The contrast of the sexes in our 
common diaecious plants is here very instructive. Taking, for 
instance, the dog-mercury {Mercurialis perennis) of any shady dell, 
or the day-lychnis (Z,. diurna), so often hardly less abundant on its 
sunnier slopes, experiments are still certainly wanting with regard to 
given plants as to what circumstances originally determined their 



THE DETERMINATION OF SEX. 



43 



sexual differences; but the fact of superior constitutional vegetative- 
ness in the females is here so peculiarly obvious that it can hardly 
fail to arouse a strong impression that more or less advantageously 
nutritive conditions, whether of the embryo or of the seedling, are 
sufficient to account for the differences of sex. 

II. Influence of Temperature. — In this connection not a few 
-writers have referred to an observation by Knight, which, from its 
comparatively ancient date, perhaps deserves to be recorded in his 
own words, if only to show the necessity of caution in such matters. 
A watermelon was grown in a heated glasshouse, where the tem- 
perature sometimes rose on warm days to iio° Fahr. "The plant 
grew with equal health and luxuriance, and afforded a most abundant 
blossom; but all its flowers were male. This result did not in any 
degree surprise me, for I had many years previously succeeded, by 
long continued very low temperature, in making cucumber-plants 
produce female flowers only; and I entertain but little doubt that 
the same fruit-stalks might be made, in this and the preceding species, 
to support either male or female flowers in obedience to external 
causes. ' ' 

This experiment was obviously more sanguine than satisfactory. 
Heyer justly points out that of the watermelon only a single plant 
was taken. Furthermore, he says, the watermelon in nature usually 
bears only female flowers on the apices of the older twigs, and may 
bear only a minimum number of these. Knight's observations on 
cucumbers are also open to serious objections, and were too scanty to 
prove anything. 

Meehan finds that the male plants of the hazel grow more actively 
in heat than the female; and Ascherson has made the interesting 
observation that the water-soldier {Stratiotes aloides) bears only female 
flowers north of 52° lat, and from 50° southwards only male ones. 

In the human species, Diising and others have noted that more 
males are born during the colder months; and Schlechter has reached 
the same results from observations upon horses. The temperature 
of the time, not of birth but of sex-determination, must of course be 
noted; nor must it be forgotten that temperature may have its influence 
indirectly through the nutritive functions. 

III. Summary of Factors. — If we now sum up the case, it 
must first be recognized that a number of factors cooperate in the 
determination of sex; but that the most important of these, with 
increasing penetration ci analysis, may be more and more resolved 
into plus or minus nutrition, operating upon parent, sex-elements, 
embryo, and in some cases larvae. 

{a) Starting with the parent organisms themselves, we find this 



44 'ftiE EVOLUTION OF SEX. 



general conclusion most probable — that adverse circumstances, 
especially of nutrition, but also including age and the like, tend to 
the production of males, the reverse conditions favoring females. 

(b) As to the reproductive elements, a highly nourished ovum, 
compared with one less favorably conditioned, in every probability will 
tend to a female rather than to a male development. Fertilization, 
when the ovum is fresh and vigorous, before waste has begun to set 
in, will corroborate the same tendency. 

{/) Then if we accept Sutton's opinion as to the transitory hermaph- 
rodite period in most animals, from which the transition to unisex- 
uality is effected by the hypertrophy of the female side or preponder- 
ance of the male in respective cases, the vast importance of early 
environmental influences must be allowed. The longer the period of 
sexual indifference (though this term be an objectionable one) con- 
tinues, the more important must be those outside factors, whether 
directly operative or indirectly through the parent. Here again, 
then, favorable conditions of nutrition, temperature, and the like, tend 
toward the production of females, the reverse increase the probability 
of male preponderance. 

The general conclusion, then, more or less clearly grasped by 
numerous investigaters, is that favorable nutritive conditions tend to 
produce females, and unfavorable conditions males. 

IV. Let us express this, however, in more precise language. 
Such conditions as deficient or abnormal food, high temperature, 
deficient light, moisture, and the like, are obviously such as would 
tend to produce a preponderance of waste over repair, — a katabolio 
habit of body, — and these conditions tend to result in the production 
of males. Similarly, the opposed set of factors, such as abundant 
and rich nutrition, abundant light and moisture, favor constructive 
processes, — that is, make for an ajiabolic habit, — and these condi- 
tions result in the production o{ females. With some element oi 
uncertainty, we may also include the influence of the age and physio- 
logical prime of either sex, and of the period of fertilization. But 
the general conclusion is tolerably secure — that in the determination 
of sex, influences inducing katabolism tend to result in production oi 
males, as those favoring anabolism similarly increase the probability 
of females. 

V. This is not all, however: the above conclusion is indeed valu- 
able, but it acquires a deeper significance when we take it in connec- 
tion with the result of a previous chapter. There it was seen, as the 
conclusion of an independent induction, that the males were forms 
of smaller size, more active habit, higher temperature, shorter life, 
&c. ; and that the females were the larger, more passive, vegetative. 



THE DETERMINATION OF SEX. 45 

and conser\'ative forms. Theories of ' ' inherent ' ' maleness or female- 
ness v/ere rejected, since practically merely verbal; more accurately, 
however, they have been interpreted and replaced by a more material 
conception, which finds the bias of the whole life, the resultant of its 
total activities, to be a predominance of the protoplasmic processess 
either on the side of disruption or construction. This conclusion has 
still to receive cumulative proof; but one large piece of evidence is 
now forthcoming, — that, namely, of the present chapter. If influ- 
ences favoring katabolism make for the production of males, and if 
anabolic conditions favor females, then we are strengthened in our 
previous conclusion — that the male is the outcome of predominant 
katabolism, and the female of equally emphatic anabolism. 

VI. Weismann's Theory of Heredity. — In thinking of the 
environment as a factor determining the sex, it is impossible to ignore 
that such facts as we have noted above have some bearing upon the 
problem of heredity. Much of the recent progress in the elucidation 
of the facts of inheritance has been due to Weismann, who, in his 
theory of the continuity of the germ-plasma, has restated the very- 
important and fundamental conception of a continuity between the 
reproductive elements of one generation and those of the next. To 
this restatement we shall afterwards have to refer: it is with another 
position, not peculiar to but emphasized by the same authority, that 
we have here to do, namely, with his denial of the inheritance of indi- 
vidually acquired characters. Any new character exhibited by an 
organism may arise in one of two ways, which it is easy enough to 
distinguish theoretically: it may be an outcrop of some property 
inherent in the fertilized egg-cell, — that is, it may have a constitu- 
tional or germinal origin; but, on the other hand, it may be impressed 
upon the individual organism by the environment, or acquired in the 
course of its functioning, — that is, it may have a functional or environ- 
mental origin. Thus an increase of calcareous matter in an animal 
might well be wholly of constitutional origin; but a change to a new 
diet, or to a new medium, might be followed by modifications arising, 
in one sense, from without. But all such functional and environmental 
variations are, according to Weismann, restricted to the individual 
organism; they are not transmissible. 

And why not ? This denial of the inheritance of dints from with- 
out, and of acquired habits other than constitutional, can be no 
mere optimism on Weismann's part. It is, he maintains, a scientific 
skepticism, based on the one hand on the absence of data demonstrat- 
ing what we may still call the current belief; and on the other hand on 
the improbability of changes produced as above explained reacting 
from the "body" on the reproductive cells. If such a reaction do 



46 THE EVOLUTION OE SEX. 

not occur, Weismann's position is secure; and though in a system 
saturated with alcohol, or transferred to a new climate, the repro- 
ductive cells may vary along with the body, no modification of nerve 
or muscle can, as such, be transmitted in inheritance. In short, the 
reproductive protoplasm must be in a sense insulated, and leads a 
charmed life away from external disturbance. 

This view, supported as it is by many authorities, is obviously of 
the utmost importance, both for the general theory of evolution, and 
for such practical problems as those of the pathologist and the teacher. 
Its full consideration is here impossible, involving matter enough for a 
special treatise on heredity. The difficulty of any yea or nay lies in the 
relative scarcity of experimental data, in the divergence of opinion as 
to the pathological evidence, and very largely in the difficulty of 
applying our logical or anatomical distinctions to the intricate facts of 
Nature. Thus the distinction between "acquired," and germinal or 
constitutional, is easily made on paperj but is difficult in actual 
practice; nor is the line between a variation of the reproductive cells 
along with the body, and one produced by the body, readily drawn in 
concrete cases. 

One criticism is suggested by the present chapter, — the assumed 
insulation or separateness of the reproductive elements from the 
general life of the body. How far is this real ? In view of the 
genuine unity of the organism, a charmed life of one of the systems 
seems to some a ' ' veritable physiological miracle' ' ; and therefore we 
point to such a case as Yung's tadpoles, where an outside influence of 
nutrition saturated through the organism and did affect the repro- 
ductive elements, — not indeed to the degree of altering any structural 
feature of the species, but yet to the extent of altering the natural 
numerical proportions of the sexes. 



• I 



THE DETERMINATION OF SEX. 47 



SUMMARY. , 

I. Nutrition is one of the most important factors in determining sex. In 
illustration, note (a) the experiments of Yung, which raised the percentage of 
lemales from 56 to 92 by good feeding ; (d) the case of bees, where the 
differences between queen and worker well illustrate the enormous results of a 
slight nutritive advantage ; also the case of humble-bees, with three successive 
broods increasing in nutritive prosperity and in femaleness ; (c) Von Siebold's 
experiments with a wasp, which showed most females in favorable conditions ; 
(rf) Aphides, in prosperity of summer, yield a succession of parthenogenetic 
females, in cold and scarcity of autumn males return ; (e) starved caterpillars of 
moths and butterflies become males ; (/) Rolph's observations on crustaceans ; 
(g-) also the facts noted by Girou, Diising and others, on the influence of good 
nourishment of mammalian mothers in favoring female offspring ; (K) the hints 
of the same results in the human species ; (?) the various observations in regard 
to plants which favor the same general conclusion. 

II. As to the influence of temperature, favorable conditions again tend to 
femaleness of offspring, extremes to males. 

III. These factors are now added up, — (a) the nutrition, age, &c., of 
parents ; (6) the condition of the sex-elements ; (c) the environment of 
embryo. 

IV. The generalization is thus reached, — anabolic conditions favor 
preponderance of females, katabolic conditions tend to produce males. 

V. But females have been already seen to be more anabolic, and males 
more katabolic. This view of sex is therefore confirmed. 

VI. How does Weismann explain the determination of sex, which illustrates 
an outside influence penetrating to the reproductive cells ? 



LITERATURE. 

See works mentioned in Chapter III., especially those of Diising, Geddes 
(article "Sex," Ency. Brit.), Hensen, and Sutton ; also those of Eimer, Geddes, 
and Rolph in Chapter II. 
DiJsiNG, C. — As before ; also. Die experimentelle Priifung der Theorie von der 

Regulirung des Geschlechtsverhaltnisses. Jen. Zeitschr. f. Naturwiss. 

XrV., Supplement, 1885. 
Heyer, F. — Untersuchungen iiber das Verhaltniss des Geschlechtes bq 

einhausigen und zweihausigen Pflanzen, unter Beriicksichtigung des 

Geschlechtsverhaltnisses bei den Thieren und den Menschen, Ber. 

landvnrthschafU. Inst. Halle. V. 1884, pp. 1-152. 
Meehan, T.— Relation of Heat to the Sexes of Flowers. Proc. Acad. Nat 

Science, Philadelphia (1884), pp. 111-117- 
Semper, C— The Natural Conditions of Existence as they Affect Animal Life. 

Intemat. Science Series, London, 1881. 



48 THE EVOLUTION OF SEX. 



Thompson, J. A. — Synthetic Summary of the Influence of the Environment 

upon the Organism. Proc. Roy. Phys. Soc. Edin., IX. (1888), pp. 446-499. 

(Supplementary to Semper's work, with bibliography.) 
The History and Theory of Heredity. Proc. Roy. Soc. Edin., 1889, pp. 

91-116, with bibliography. 
Wkismann, a. — Die Continuitat des Keimplasmes als Grundlage einer 

Theorie der Vererbung, Jena, 1885 ; and numerous other papers, now 

translated, in i vol. — Essays upon Heredity and Kindred Biological 

Problems, authorized translation, edited by E. B. Poulton, S. Schonland, 

and A. E. Shipley, 8 vo. Oxford, 1889. 
WiLCKENS, M. — Untersuchungen iiber das Geschlechtsverhaltniss und die 

Ursachen der Geschlechtsbildung in Haustieren. Biol. Centralblt. VI. 

(1886), pp. 503-510; Landworth, J. B., XV., pp. 607-610. 
Yung, E.— Contributions ^ 1 'Histoire de ITnfluence des milieux Physiques sur 

les Etres Vivants. Arch. Z06I. Exper., VII. (1878), pp. 251-282 ; (1883), pp. 

31-55 ; Arch. Sci. Phys. Nat, XIV. (1885), pp. 502-522, &c., &c. 



if I 



BOOK II. 



ANALYSIS OF SEX- ORGANS, TISSUES, 
CELLS. 



SEXUAL ORGANS AND TISSUES. 51 

CHAPTER V. 

SEXUAL ORGANS AND TISSUES. 

Tt is the object of this portion of the book to continue the analysis 
of sexual characters, but now in a deeper way, reviewing suc- 
cessively the organs, tissues, and cells concerned in sexual repro- 
duction. The essential and auxiliary organs of the two sexes, the 
frequent combination of these in hermaphrodite plants and animals, 
the sex-cells both male and female, will be discussed in order. This 
survey will be for the most part structural or morphological; the 
special physiology of sexual union and of fertilization will be discussed 
at a later stage. 

I. Essential Sexual Organs of Animals. — It is now a well 
established fact that among the ciliated infusorians, which swarm 
especially in stagnant waters, a process occurs which can not but be 
described as in part sexual reproduction. Two individuals, to all 
appearance alike be it noted, become temporarily associated, and 
interchange some of the elements of their accessory nuclear bodies. 
This process of fertilization is essential to the continued vigor of the 
species, and will be afterwards described at length. Such a very 
simple form of sexual union differs from what occurs in higher 
animals, in two conspicuous respects, — {a) the organisms are 
apparently quite similar in form and structure; (i5) they are unicellular, 
and thus there is no distinction between ' ' body ' ' and reproductive 
cells. What is fertilized by the mutual exchange in those infusorians 
is, roughly speaking, the entire animal, for the whole is but a unit-mass 
of living matter. 

Among the protozoa, however, loose colonies of cells occur, which 
bridge the gulf between unicellular and multicellular animals. In these 
we find the first indications of the afterwards conspicuous difference 
between ' ' body ' ' and reproductive cells. From these loose colonies 
certain of the units are set adrift, and meeting with others more or less 
like themselves fuse to form a double cell, virtually a fertilized ovum, 
from which by continuous division a fresh colony is then developed. 
In these transition-forms there are thus reproductive cells of slight 
distinctness, but as yet obviously no sexual organs. 

When we pass to the sponges, we find colonies consistmg 01 
myriads of cells, among which there is a considerable division or 
labor. An outer layer (or ectoderm) usually consisting of much 
subordinated cells, an inner layer (or endoderm) of predominantly 



52 



THE EVOLUTION OF SEX. 



active and well-nourished cells, a middle layer of heterogeneous con- 
stituents, can always be distinguished. Every average infusorian is as 
good as its neighbors, so far as reproduction of new individuals by 
division is concerned; in the colonial protozoa, the units that are set 
adrift are very little different from their fellows that remain behind; but 
this ceases to be true when we pass to colonies v/here considerable 
division of labor has been established. It is certainly true that even a 
tiny fragment of sponge, cut off from the larger mass, may, if it con- 
tain sufficient samples of the body, and if the conditions be favorable, 
reproduce a new individual. Cultivaters of bath-sponges habitually 
take advantage of this fact. But the sponge starts its new colonies for 




Fig. 15. — Volvox, a loose colony of cells, with some set apcirt for reproduction. — After Kirchner. 



itself usually in quite a diiferent way, namely, by the process of sexual 
reproduction. Among the cells of the middle stratum of the sponge- 
body certain well-nourished passive cells appear. These are the ova, 
at first very like, but eventually well marked from the other 
constituent units of the layer. Besides these there are other celk, 
either in the same sponge or in another, which exhibit very different 
characters. Instead of growing large and rich in reserve material like 
the egg-cells or ova, they divide repeatedly into clusters of infinitesimal 
cells, and form in so doing the male elements or spermatozoa. The 
male and female cells meet one another, they form a fertilized ovum; 
the result is continued division of the latter till a new sponge is built 
up. Here, then, there are special reproductive cells, quite distinct 



SEXUAL ORGANS AND TISSUES. 



53 



from those of the "body"; and here, furthermore, these reproductive 
cells are markedly contrasted as male and female elements. As yet, 
however, there are no sexual organs. 

Passing to the next class, the stinging animals or coelenterates, we 
find in one of the simplest and most familiar of these, the common 
fresh-water hydra, a good illustration of primitive sexual organs. As 
in sponges, a cut-off fragment of the body, if sufficient samples of the 
different component cells are included, is able to reconstitute the 
whole. But no one body-cell has of course any such power; this is 
possible for the fertilized ovum alone. Now this ovum occurs, not 
anywhere within a given layer as in sponges, but always near one spot 
on the body. Toward the base of the tube a protuberance of outer- 
layer cells is developed. This forms a rudimentary ovary, or female 
organ. It has this peculiarity, not however unique, that while the 
organ consists of not a few cells, only one of these becomes an ovum. 
A similar protrusion, or more than one, often at the same time and on 
the same animal, may be recognized further up the tube, nearer the 
tentacles of the hydra. Of somewhat smaller size, such a superior 
protuberance consists of numerous small cells, most of which, multiply- 
ino- by division, form male elements or spermatozoa. We have here 
the simplest possible male organ or testis. 

More elaborate organs occur in the other ccelenterates, complicated 
however by two interesting facts, which will be afterwards discussed. 
(a) Many of the ccelenterates are well known to form elaborate 
colonies, — zoophytes, Portuguese men-of-war, and the like. In these, 
division of labor frequently goes further than the setting apart of 
special organs. Entire individuals become reproductive "persons" 
(as they are technically called), in contrast to the nutritive persons of 
the colony, {b) In some of those reproductive individuals, a curious 
phenomenon, known as migration of cells, has been observed by 
Weismann and others. The reproductive cells, arising in various 
parts of the body, have been shown to migrate in some cases to 
another part, where they find final lodgment in more or less definite 
oro-ans. This occurrence is intimately associated with ' ' alternations of 
generation, " and will be afterwards discussed under that heading. 

It is in no wise the purpose of the present work to describe the 
details respecting the ovaries and testes, as they occur in the various 
classes of animals. • It is enough for our purpose to have emphasized 
the fact of their gradual differentiation, and to note that they are al- 
most always developed in association with the middle layer of the 
body, and usually occupy a posterior position on the wall of the body- 
cavity. The details will be found in any standard work on com- 
parative anatomy, — very conveniently, for example, in Prof Jeffrey 
Bell's "Comparative Anatomy and Physiology," London, 1885, 



54 



THE EVOLUTION OF SEX. 



II. Associated Ducts. — It is only in a few animals, like hydra 
and its allies, that the ovaries and testes are external organs, which 
have simply to burst to liberate their contents. They are usually of 
course internal, and thus arises the necessity of some means of com- 
munication with the outside world. In the simplest cases, the male 
elements find their way out to the surrounding medium without any 
specialized mode of exit. They there meet, by chance combined with 
physical attraction at short range, with the ova, which in the simplest 
cases again have found their way out in an equally primitive fashion. 
Thus in the enigmatical parasitic mesozoa (orthonectids, &c.), libera- 
tion of the germs may occur by perforation or by rupture of the 
excessively simple bodies. In some of the marine worms (for example, 
Polygor'dius), the liberation of the ova at least is accompanied by the 
fatal rupture of the mother organism, a vivid instance of reproductive 
sacrifice. Even in some of the common nereids, the same uneconom- 
ical mode of liberation by rupture appears to occur. The forcible 
rupture may be referred to pressure of the relatively large mass of 
growing cells which the ovaries often present. 

As high up as backboned animals, the absence of ducts may be 
traced. Thus among the seasquirts or tunicates, the reproductive 
organs are frequently ductless, and the same thing is true of some 
fishes. The sex-cells burst into the body-cavity, and thence find their 
way to the exterior by apertures. In most cases, where ducts are 
absent, fertilization of the ova is external, but this is not necessarily so. 
In sponges, for instance, fertilization is almost always internal. Male 
elements are washed in by the water-currents, find their way to the 
ova, and fertilize them in sitzi. Almost without exception, embryo 
sponges, not ova, make their way to the exterior. In the higher 
animals, where definite ducts are present, alike for the inward passage 
of spermatozoa and the exit of ova or embryos, it ought further to be 
noticed that the ovaries can hardly ever be said to be in direct con- 
nection with their ducts. The ova usually burst from the ovary into 
the body-cavity, whence they are more or less immediately caught up 
by or forced into the canals, by which they pass outwards. With the 
testes it is different, for if ducts be present, they are in direct connec- 
tion with the organs. 

It is enough to state that in the great majority of cases ducts are 
associated with the essential organs. Those of the male serve for the 
exit of the spermatozoa, and may be terminally modified as 
intromittent organs. Those of the female serve either solely for the 
emission of unfertihzed eggs, or for the reception of spermatozoa, and 
the subsequent exit of fertilized ova or growing embryos. In some 
worm-types, and in all vertebrates, from amphibians onwards, the 



SEXUAL ORGANS AND TISSUES. 55 

reproductive ducts are also in various degrees associated with excre- 
tory functions. For an account of the origin of the ducts in higher 
animals, the reader must be referred to the embryological textbooks of 
Balfour and Hertwig, or most conveniently of Haddon. Similarly for 
such modifications as that of the female duct into oviduct and uterus, 
reference must be made to the larger anatomical works of Gegenbaur 
and Wiedersheim, or for a briefer account to Parker' s translation and 
edition of Wiedersheim' s smaller textbook, and to Prof Jeffrey Bell's 
work already mentioned. 

III. Yelk-Glands. — As we shall afterwards see, the ovum is 
often furnished with a large quantity of nutrient material. This 
serves as the food-capital for the growing embryo or young larva. 
It is obtained in various ways — from the vascular fluid, from the 
sacrifice of adjacent cells, or from special organs known as yelk-glands 
or vitellaria. The yelk-glands, as they occur for instance in some 
of the lower worms (turbellarians, flukes, tapeworms,) are of some 
general interest. They represent, as Graff has shown, a degenerate 
portion of the ovary, in which the cells have become even more 
highly nutritive than ova. ' ' The origin of the yelk-gland, ' ' Gegen- 
baur says, "is probably to be found in the division of labor of a 
primitively very large ovary." In more technical language, yelk- 
glands are hypertophied or hyperanabolic portions of the ovary. 
Apart from this nutritive capital, the egg is often equipped with 
envelopes or shells of some sort, which may be furnished by special 
organs, or by the sacrifice of surrounding cells, or by the walls of 
the ducts as the eggs pass out. 

IV. Organs Auxiliary to Impregnation. — In most animals 
in which internal fertilization of the ova occurs, there are in both 
sexes special structures auxiliary to the function of impregnation. 
Thus the end of the male canal is commonly modified into an intro- 
mittent tube or penis, through which the male elements flow into 
the female duct. In the crustaceans some of the external appendages 
are often modified, as in the crayfish, to serve this purpose, and the 
same is the case with minute structures on the posterior abdomen 
of many insects. Sometimes, as in the snail {Helix), which may 
be taken as an extreme type of reproductive specialization, separate 
oro-ans are present, in which the spermatozoa are compacted into 
masses or packets, known as spermatophores. In most cuttlefishes, 
these pass from the male ducts to one of the "arms," which thus 
laden is occasionally set free bodily into the mantie-ca\'ity of the 
female, where it was of old mistaken for a worm, and called Hcdo- 
cotvlus. So in some spiders, the palps near the mouth recei\-e th 
male elements and transfer them to the female. Special storing 



e 



^6 THE EVOLUTION OF SEX. 

receptacles and secreting glands are also very frequently in ashocia- 
tion with the male ducts, and there is a long list of curious modifi- 
cations utilized in the process of copulation. Thus, male frogs have 
their swollen thumbs, and gristly fishes their ' ' claspers, ' ' which are 
modified parts of the hind limbs, and are inserted into the cloaca of 
the female. The common snails eject a limy dart {spiculum amoris), 
which appears to be a preliminary excitant to copulation. 

So too, in the female sex, the termination of the duct may be 
modified for reception of the male intromittent organ, and special 
receptacles may be present for storing the spermatozoa. Where 
a single fertilization occurs, as in the queen-bee, previous to a long- 
continued egg-laying period, the importance of a storing organ is 
obvious. As the female is usually more or less passive during copula- 
tion, the adaptations for this purpose are less numerous than in the 
males. It is interesting to notice, that among amphibians, where 
the male often takes upon himself distinctly maternal duties, one 
case is known where the female seems more active than the male 
during copulation. 

V. Egg-laying Organs. — Cases where the ova simply pass 
out into the water, or on to the land, are of course associated with 
the absence of any special organs. In a great many animals, how- 
ever, more care is taken, and auxiliary structures are present. One 
of the simplest of useful developments is exhibited by glands, the 
viscid secretion of which moors the ova, and keeps them from being 
set wholly adrift. In insects, where it is specially important that 
the eggs should be well concealed, or buried in conveniently nutri- 
tive material, hints of the ancestral abdominal appendages remain 
as ' ' ovipositors. ' ' Throughout the series a great variety of structures 
occur in this connection. 

VI. Brooding and Young-feeding Organs. — From very 
lowly animals onwards, structures are present which are utilized in 
the protection of the young in their helpless stages. The repro- 
ductive buds of some coelenterates become true nurseries; in one at 
least of the marine worms [Spirorbis spirilluni), a tentacle serves as 
a brood-pouch; various adaptations, such as tents of spines, or cavities 
in the skin, are utilized in echinoderms. The young shelter under 
the hard cuticle, or among the appendages of crustaceans, in the 
gills of bivalves, and a cuttlefish has been seen with the eggs in 
its mouth. Among the higher animals, the brood-pouch of Appendi- 
cularia (one of the very lowest Chordatd), the pockets of not a few 
fishes, the cavities on the back of the Surinam toad, the pouches 
of marsupials, are only a few instances amid a crowd. Sometimes, 
especially in fishes and amphibians, — for example, the seahorse, with 



SEXUAL ORGANS AXD TISSUES. 57 

its breast-pouch, and Rhinoderma darwinii, with its enlarged croaking 
sacs, — it is the male which undertakes the brooding office. When 
the young are born alive, the internal female ducts become developed 
in this connection to form uteri. The ovary appears to serve as a 
womb in the genus Girardinus among fishes, but it is usually the 
median portion of the female duct which has this function. In pla- 
cental mammals, where the young are born at an advanced stage, 
and where the maternal sacrifice is at its maximum, the uterine 
adaptations become more important and complex. The organs of 
lactation will be afterwards discussed. 



5S 



THE EVOU'TIOX OF SEX. 



SUMMARY. 

The gradual differentiation of essential sexual organs in animals, — isolated 
cells, aggregated tissues, definite organs. 

Associated male and female ducts for the liberation of male elements, fertili- 
zation, exit of ova, or birth of embryos. 

Yelk-glands, &c., for nourishment and equipment of the ova. Vitellaria have 
been interpreted as degenerate ovaries. 

Illustrations of organs auxiliary to impregnation. In the male — penis, storing 
sacs, spermatophore-making organs, "claspers." Curiosities, such as the hecto- 
cotylus of cuttlefishes, and the Cupid's dart of snails. Adaptations in the female 
are less frequent, but storing receptacles for the male elements are common. 

Egg-laying organs : — frequency of ovipositors. 

Brood-pouches and the like are widely present in most classes of animals. 



LITERATURE. 

Balfour, F. M. — A Treatise on Comparative Embryology. 2 vols. London, 

1881. 
Bell, F. Jeffrey. — Comparative Anatomy and Physiology. London, 1885. 
Glaus, C. — Elementary Textbook of Zoology, trans, by A. Sedgwick. 2 vols. 

London, 1885. 
Geddes, p. — Op. cit. 
Gegenbaur, C. — Elements of Comparative Anatomy, trans, by Prof. Jeffrey 

Bell. London, 1878. 
Haddon, a. C. — An Introduction to the Study of Embryology. London, 1887. 
Hensen, V. — Op. cit. 
Hertwig, O. — Lehrbuch der Entwicklungsgeschichte des Menschen und der 

Wirbeltheire. Jena, 1888. 



SEXUAL ORGANS AND TISSUES. 59 

Hatchett Jackson's (W.) Edition of RoUeston's Forms of Animal Life. 
Oxford, 18SS. 

Huxley, T. H. — Anatomy of Vertebrate and Invertebrate Animals. London, 
1871 and 1877. 

Sachs, J. — Textbook of Botany, Edited by Prof. Vines. Second edition. 
Oxford, 1882. And similar works. 

Lectures on the Physiology of Plants, trans, by Prof. Marshall Ward. 

Cambridge, 18S7. 

VrNES, S. H. — Vegetable Reproduction (Ency. Brit.). Lectures on the Physiol- 
ogy of Plants. Cambridge, 1886. 

WiKDERSHEiM, R. — Elements of the Comparative Anatomy of Vertebrates, 
trans, by Prof. W. N. Parker. London, 1886. Also unabridged work. 



5o I'lIE E VOL UTION OF SEX. 



CHAPTER VI. 
HERMAPHRODITISM. 

I. When an organism combines within itself the production of 
both male and female elements, it is said to be bisexual or hermaph- 
rodite. This is the case with most flowers, and with many lower 
animals, — such, for instance, as earthworms and snails. It is not 
desirable to extend the term, as is sometimes done, to cases like 
ciliated infusorians, where sex itself is only incipient. Undoubtedly 
in those Protozoa recent researches have distinguished what in loose 
analogy may be called male and female nuclear elements, but this 
primitive condition is rather a state antecedent to sex than a union 
of sexes in one organism. 

In most phanerogams, as every one knows, male and female 
organs occur on different leaves (stamens and carpels) of each flower. ' 
The flower as a whole, or the entire plant, may then be called her- 
maphrodite. But as the male and female organs are restricted to 
different leaves, each leaf by itself unisexual, when compared, for 
instance, with the prothallus of a fern, which bears on the same small 
expansion both male and female organs. When stamens and carpels 
unite together, as in orchids, a more intimate hermaphroditism is 
obviously developed. So with animals. While the general definition 
of hermaphroditism, as the union of the two sexes in one organism, 
is plain enough, the union is exhibited in a great variety of ways and 
degrees. Of these it is necessary first to take account. 

II. Embryonic Hermaphroditism. — Some animals are her- 
maphrodite in their young stages, but unisexual in adult life. Allusion 
has already been made to the case of tadpoles, where the bisexuality 
of youth occasionally lingers into adult life. According to some, 
most higher animals pass through a stage of embryonic hermaphro- 
ditism, but decisive proof of this is wanting. 

The research of Laulanie may now be referred to at greater length. 
As the result of observation on the development of the reproductive 
organs in the higher vertebrates, and especially in birds, he seeks to 
establish a strict parallelism between the individual, and what he 
believes to have been the racial history. In the chick, he distinguishes 
three male stages in the development — (i) germiparity, (2) her- 
maphroditism, (3) differentiated unisexuality. These he regards as 
recapitulating the great steps of the historic evolution, (i) For 
the first period of "germiparity," — -from the fourth to the sixth 



HERMAPHRODITISM. 6 1 

day, — the designation, sexual neutrality, or indifference, is inappro- 
priate, since the "cortical ovules" of the germinal epithelium have 
from the first the precise morphological significance of female elements 
or ova. In the female, they proceed by multiplication to forrn the 
ovary; in the male, they degenerate. (2) The period of hermaph- 
roditism begins with the seventh day. In the male, the male ovules, 
from which the sperms are afterwards developed, appear in the central 
tissue; but at the same time cortical or female ovules may be seen 
persisting. Similarly, in the developing ovary of the female, the 
central or medullary portion, stricdy separated by a partition of con- 
nective tissues from the egg-forming layer, contains a large number 
of medullary or male ovules. (3) This hermaphroditism is of short 
duration. The cortical or female ovules disappear from the testes by 
the eighth or ninth day; and the medullary or male ovules have by 
the tenth day disappeared from the ovary. In regard to mammals, 
Laulanie affirms, allowing some peculiarities, that the same three 
stages of germiparity, hermaphroditism, and unisexuality occur. 

Ploss has already been referred to as another invesdgater who 
maintains the existence of embryonic hermaphroditism. Such also 
is the view held by Professor Sutton, who concludes that both sets 
of organs are equally developed up to a definite period, and empha- 
sizes the consequent necessity for the hypertrophy of one sexual 
rudiment over the other. Only thus can unisexuality be established. 
It ought, perhaps, to be noted that hypertrophy is hardly a term. 
strictly applicable to predominance of male over female organs, 
since, in our contention, the whole nature of male organs or elements 
is the physiological reverse of abundant nutrition. 

III. Casual or Abnormal Hermaphroditism. — In many species 
which are normally unisexual, a casual hermaphrodite form occasionally 
presents itseU". The embryonic equilibrium or bisexuality — one of 
the two must in a variable degree exist — is retained as an abnormality 
into adult life. Even as far up in the organic series as birds and 
mammals, such casual and yet true hermaphrodites occur. In most 
cases, at least, the result is sterility. Among amphibians, which 
abound in reproductive peculiarides, hermaphrodidsm exceptionally 
occurs, apart from the one case (see below) where it is known to be 
constant. The common frog, so much dissected in our laboratories, 
has supplied several good illustrations. Thus Marshall notes that 
the testes may be associated with genuine ova, or an ovary may occur 
on one side and a testis with an anterior ovarian portion upon the 
other. Bourne gives a case of a frog with the ovary well developed 
on the right side, and opposite this an ovary anteriorly replaced by 
testis. One of the toads {Pelobates ficscus) seems to be frequently- 



THE EVOLUTION OF SEX. 



hermaphrodite, the male being furnished with a rudimentary ovary 
in front of the testes. A similar hermaphroditism is not at all infre- 
quent in cod, herring, mackerel, and many other fishes; while slightly 
lower down in the series it occurs in the hagfish {Myxine). Some- 
times a fish is male on one side, female on the other, or male anteriorly 
and female posteriorly. Sir J. W. Simpson, in a learned article on 
the subject, has distinguished cases of true hermaphroditism according 
to the position of the organs, into lateral, transverse, and vertical, or 
double. Among invertebrates the same has been occasionally 
observed, — especially among butterflies, where striking differences 
in the coloring of the wings on the two sides have in some cases 
been found to correspond to an internal coexistence of ovary and 
testis. The same has been observed in a lobster, and is probably 
commoner than the recorded cases warrant one in asserting. As low 
down as ccelenterates, casual hermaphroditism may occur, as F. E. 
Schulze showed in one of the medusoids. 

IV. Partial Hermaphroditism. — An organism may be said to 
be truly hermaphrodite when both male and female organs are present, 
or when, without there being separate organs, both male and female 
elements are produced. It is then both anatomically and physio- 
logically hermaphrodite; and of this, as we shall see, there are 
abundant illustrations among lower animals. Snail, earthworm, and 
leech are examples of this hermaphroditism, in varying degrees oi 
intimacy. 

But, as we have just noticed, a species normally unisexual may 
occasionally exhibit hermaphrodite individuals. In these only one oi 
the double essential organs may be functional, or both may be sterile. 
Whether physiologically or not, such animals are anatomically 
hermaphrodite. Both kinds of essential organs are at least present. 

To those must now be added a further series of cases to which the 
term partial hermaphroditism seems most applicable. Only one kind 
of sexual organ, ovary or testis, is developed; but while one sex pre- 
ponderates, there are more or less emphatic hints of the other. As 
the reproductive organs are to be regarded as the most important, but 
not by any means the sole expression of the fundamental sex-differ- 
ences, it is impossible to separate partial hermaphroditism by any hard 
and fast line from the above, and from the next set of cases (para- 
graphs III and V). Almost all cases of partial hermaphroditism 
occur as exceptions, though a few are constant. 

In the higher animals, partial hermaphroditism is usually expressed 
in the nature of the reproductive ducts. In this connection the 
structural resemblance of the male and female organs must be once 
more emphasized. Even the Greeks had their vague and fanciful 



HER3!APHR0D/riSJr. 63 



theories of what we now call the homology of the reproductive organs 
and ducts in the two sexes. Through the labors of the anatomists of 
Cuvier's school, such as his fellow-worker Geoffroy St. Hilaire, and 
yet more through more recent embryological discoveries, there is now 
both clearness and certainty as to the main facts. The reproductive 
organs proper, the ducts, and the external parts, are developed upon 
the same plan in male and female. Thus, except in the lowest verte- 
brates, what serves as an oviduct in the female is equally present in 
the embryo male, and persists in the adult as a more or less function- 
less rudiment. In the same way, what serves as the duct for the 
sperms {vas deferens) in the male, is equally present in the embryo 
female, and persists in the adult as a rudiment, or is diverted to some 
other purpose. This is a perfectly normal occurrence, dependent 
upon the embryological history of the ducts in question. It is neces- 
sary, however, to reahze both the primitive resemblance and the 
fundamental unity of the two sets of organs, in order to understand 
how partial hermaphroditism is so frequent, and also to distinguish 
it from "spurious hermaphroditism," where a merely superficial 
abnormality or even injury of the ducts in one sex produces a 
resemblance to those of the other. 

We have already mentioned that in the case of twin calves two 
females may occur, and both are then normal; or two normal twin 
calves may be born of opposite sexes; but in the third place, if both be 
males, one of these* very generally exhibits the peculiar phenomena of 
what is called a "free-martin." In the commonest form of this, 
partial hermaphroditism is well illustrated. The essential organs are 
male, but there is a rudimentary uterus and vagina, and the external 
organs are further those of a female. 

It is necessary to note that a simulation of even this partial 
hermaphroditism may result from malformation or rudimentary 
development of the external organs. On this subject we may quote 
an acknowledged authority, alike in anatomical and embryological 
matters. "From the fact," Prof O. Hertwig remarks, "that the 
external sexual organs are originally of uniform structure in the two 
sexes, we can understand the fact that, in a disturbance of the normal 
development, forms arise in which it is extremely difficult to decide 
whether we have to deal with male or female external organs. These 
cases, in earlier times, were falsely interpreted as hermaphroditism. 
They' may have a double origin. Either they are referable to the feet 
that in the female sex the development may proceed along the same 
path as in the male, or to this, that in the male the normal develop- 
ment may come at an early stage to a standstill, and lead to the 
formation of structures which resemble the female parts." In the firet 



64 THE EVOLUTION OF SEX. 

ca.-^e, he goes on to say, there may be a simulation of a penis, and the 
ovaries may even be shifted so as to produce an appearance like that 
of the testes within their scrotal sac. In the second case, the pro- 
cesses of coalescence which give rise to the penis may not occur, only 
a rudimentary organ is formed, and there may even be an inhibition of 
the usual descent of the testes into their sacs. 

Of this superficial hermaphroditism, really not hermaphroditism at 
all, there are numerous cases among mammals. But there remain a 
large number of recorded instances, where the anatomy of the ducts 
was predominantly that of the sex opposite to that indicated by the 
essential organs, and where the combination of the two sexes was also 
expressed in external configuration and even in habit. Amphibians 
again furnish some interesting examples. Attached to the anterior 
end of the testis in various species of toad i^Bufd), there is an organ 
known as " Bidder's," which has contents like young ova. These do 
not, however, get past the early stages, and the organ is quite 
different from the more than rudimentary ovary which occurs con- 
stantly in the males of Bufo cinerezis and some other species. The 
two may, in fact, occur together. In the common frog, dissecters 
have also recorded several cases of hermaphroditism expressed in the 
ducts. Lastly, it is perhaps not going too far to include here some 
reference to the curious " fatty bodies " which occur in all amphibians 
at the apex of the reproductive organs in both sexes, These appear 
to nourish the ovary and testis, especially during hybernation, and 
may perhaps be associated with similar lymphoid structures in fishes 
and reptiles. Prof Milnes Marshall was of opinion that the fatty 
bodies have resulted from the degeneration of the anterior part of the 
reproductive gland while still in an indifferent state; but Mr. Giles 
has recendy traced the history of these bodies, and shown them to 
result from the degeneration of the anterior set of excretory tubules, 
the pronephros. 

Leaving the ducts out of account, we may arrange the import- 
ant phenomena of hermaphrodidsm in amphibians in a series as 
follows : — 

((7) Embo'onic hermaphroditism, demonstrated as of normal occurrence in 
frog-tadpoles. 

{b) Partial hermaphroditism, expressed in Bidder's organ in male toads; 
(also e.xpressed in various states of the ducts'). 

{c) True adult hermaphroditism, — normal in some species oi Bufo: casual 
in frogs, &c. 

Well-developed ovary, rudimentary ovary or Bidder's organ, and 
"fatty bodies," may be taken as illustrating the normal and the 
pathological preponderance of anabolic processes. Amphibians, every 



HERMAPHRODITISM. 65 

one will admit, are for the most part animals of distinctly sluggish 
habit; the natural characteristics of the male sex may be said to be 
to some extent handicapped, and curious instances are known where 
the more external functions of the two sexes are strangely inverted. 
The male obstetric frog is not alone in taking charge of the ova, and 
the female of one of the efts behaves in copulation like a male. 

The list need not be further followed; it is enough to note the 
very wide occurrence of partial hermaphroditism. In many cases, 
however, this takes an interesting form, by expressing itself in the 
external characters. Forms occur in which the minor peculiarities 
of the two sexes — coloring, decorations, weapons, and the like — ■ 
appear blended together, or in which the secondary sexual characters 
are at variance with the internal organs. In most cases, one is safe 
in saying that there is no true internal hermaphroditism in any degree. 
Arrest of maturity or puberty," cessation of the reproductive functions, 
removal or disease of the essential organs, and the like, may alter the 
secondary sexual characters from female toward male, or, less fre- 
quently, z'ice versa. A female deer may develop a horn, or a hen 
a spur, and in such cases the ovaries are generally found to be 
diseased. The prettiest cases of superficial hermaphroditism occur 
among insects, especially among moths and butterflies, where it often 
happens that the wings on one side are those of the male, on the 
other those of the female. Only the external features have been 
observ'ed in most cases; but it has been shown by dissection that such 
superficial blending may exist along with internal unisexuality, or, in 
a few cases, with genuine internal hermaphroditism. A beautiful case 
of intimate blending of superficial sex-characters was lately shown to 
us by Mr. W. de V. Kane, of Kingstown. A specimen of butterfly 
{Euckloe euphenoidcs) showed the anterior half of the fore-wings and 
part of the hind-wings with the characteristic white ground of the 
female, while in the posterior half of the fore-wings and on most of 
the hind-wings the characteristic sulphur of the male prevailed. In 
other minor ways the characteristics of the two sexes, which are well 
marked, were intimately blended. Similar cases are on record. 

V. Normal Adult Hermaphroditism. — This is rare among 
the higher animals, but common among the lower. On the threshold 
of the vertebrate series we find it indeed constant among the tunicata; 
but above these it is only known to occur normally in two genera of 
fishes, and in one genus of amphibians. " A testis is constantly found 
imbedded in the wall of the Gvary in Chrysophrys and Serranus, and 
the last-named fish is said to be self-impregnating." In some species 
of male toad (for example, Bufo cincreus) a somewhat rudimentary 
ovary is always present in front of the testes. All other cases among 



66 THE ErOLUTION OF SEX. 

vertebrates are either casual (par. Ill) or partial (par. IV). Among" 
invertebrates, true hermaphroditism is, however, of frequent normal 
occurrence; for example, in sponges, coelenterates, worm-types, and 
mollusks. It is necessary to take a brief survey of some of these. 

1. Sponges. — As already mentioned, the sex-cells of sponges 
start simply among the other components of the middle layer {jncso- 
glced) of the body. It is at least possible that in a7iy sponge they 
may develop either into ova or into sperms, or into both, within the 
same organism, according to nutritive and other conditions. The 
facts, however, are these. Many sponges are only known in a uni- 
sexual state, while others are genuinely hermaphrodite. But among 
the latter it is not uncommon to find (for example, in Sycandra 
raphanus) that the production of one set of elements preponderates 
over the other, and thus we have hermaphrodites with a distinctly 
male or female bias. In other words, they are verging toward unisex- 
uality. It does happen in fact (for example, in Oscarella lobidayis) that 
a species normally hermaphrodite may exhibit unisexual forms. It is 
possible, of course, in such cases one set of sexual elements may have 
been wholly discharged, or may even have been overlooked in obser- 
vation; but there is no improbabihty against the supposition that a 
preponderance of favorable nutritive conditions might induce a form 
normally hermaphrodite to become wholly female. This, as we have 
seen above, is what some believe to take place in the individual history 
of higher forms. 

2. C(xle7iteraies. — The members of this class are higher, in having 
the production of the sex-cells more restricted to definite regions, 
tissues, organs, or even "persons." The highly active Ctenophores, 
like Berbe, are all hermaphrodite, and that very closely. On one 
side of the meridional branches of the alimentary canal ova arise, on 

. the other side spermatozoa. Among sea-anemones and corals the 
hermaphrodite condition appears in a number of cases, but is some- 
times obscured, corresponding to different physiological rhythms in 
the life of the organism. The genus Corallhim (the red coral of com- 
merce) is peculiarly instructive. The whole colony may be unisexual, 
or one branch of the colony, or only certain individuals on a branch, 
while genuine hermaphroditism of individual polyps also occurs. 
Among hydrozoa (zoophytes, swimming-bells, jellyfish), hermaphro- 
ditism is a rare exception, or, we may almost say reversion. The 
common hydra, which is a somewhat degenerate type, is hermaph- 
rodite, though at the same time individuals may be found with 
only ovary or only testes. Eleuiheria is also hermaphrodite, and 
" abortive ova occur in the male of Gonothyrea loveni." Sometimes 
a colony is hermaphrodite (^Dicoryne), but the stems and individuals 



HERMAPHRODITISM. 67 



unisexual. Sometimes a stem is hermaphrodite, but the individuals 
vmisexual (certain sertularians). Among jellyfishes the genus Chrysa- 
ora is known to be hermaphrodite. 

3. " Worms." — The condition of the sexual organs ^-aiies 
enormously among the diverse types lumped together under the title 
of ' ' worms " or " Vermes. ' ' In the lowly turbellarians, all the 
genera are hermaphrodite except two; but, as in many other cases, the 
organs do not reach maturity at the same time, the male preceding. 
In the related trematodes or flukes, hermaphroditism again obtains, 
with one exception, or perhaps two. The certain exception is the 
curious parasite Bilharzia, where the male carries the female about 
with him in a " gynaecophoric canal," formed of folds of skin. In the 




Fig. 16. — Bilharzia, a parasitic trematode, in which the male carries 
the female in a special fold of skin called the "gynaecophoric canal." 
After Leuckart. 

adjacent class of cestodes or tapeworms, all the members are hermaph- 
rodite. These three classes are doubtless related; but it seems 
plausible to connect the retention of hermaphroditism with the 
degeneracy of parasitism, and also with the rich yet at the same time 
stimulating nutrition, which may favor the retention of double 
sexuality. The utility of the hermaphrodite state, if the eggs of these 
animals are to be fertilized and the species maintained, can hardly be 
doubted, but this does not explain the facts. It is important to 
notice too, that self-fertihzation — that is, union of the eggs and sperms 
of the same organism — has been proved to occur in several 
trematodes, and seems to be almost universal in cestodes. This may 
be one of the conditions of the degeneracy of these parasites, for 
frequent as hermaphroditism is among plants and animals, self- 
fertilization is extremely rare. 

Hermaphroditism is rare among the free-Hving nemerteans, but 
constant in the semiparasitic leeches. The only exception to separate- 
ness of the sexes among threadworms or nematodes is the very 
curious case of the genus Angiostomum. Here, in an organism which 
is anatomically a female, the reproductive organ starts with produc- 
ino- spermatozoa, which fertilize the subsequent ova. The animal is 



68 THE EVOLUTION OF SEX. 

thus physiologically hermaphrodite, and at the same time self-impreg- 
nating. Approaching the higher annelid worms, we find the prim- 
iti\'e Protodrilus hermaphrodite; the earthworms are constantly so, but 
all their marine relatives have the sexes separate. The genus Sagitta, 
which stands by itself, is hermaphrodite; the same condition is known 
as a rarity among the ancient brachiopods {^Lingida), but is frequent 
among the colonial Polyzoa. 

4. Echhiodermata. — The members of all the echinoderm class, 
except one brittle star {^Amphiura squamata) and one genus of holo- 
thurians (Syiiapta), have the sexes separate. 

5. Arthropods. — Among crustaceans, hermaphroditism is a rare 
exception, though it occurs in the majority of the fixed quiescent 
acorn-shells and barnacles (^Cirripedid). There it is associated with 
the presence of small males, which Darwin called " complemental. " 
In the Cymothoidcz {Isopods), we have a curious occurrence, some- 
what like that of Angiostomum above noticed. ' ' The sexual organ 
of the young animal is male, of the old, female in function." In 
such cases, one must remember the antithesis between the body 
proper and the reproductive cells. In youth the demands of the 
body during growth are greater; there is no anabolic surplus to 
gpare, all goes to increase the body. When mature size is reached, 
and both growth and activities lessened, there is more likelihood 
of anabolic preponderance in the reproductive, as opposed to the 
vegetative, system. 

Myriopods and insects have always separate sexes, excluding of 
course abnormal hermaphroditism among the latter. An exception 
among arachnids, otherwise unisexual, is found in the degenerate 
water-bears or sloth-animalcules (^Tardigrada). 

6. MoUusks. — Most bivalves are of separate sexes, but exceptions 
often occur, — for example, in common species of oyster, cockle, 
clam, &c. In the case of the oyster, the familiar species Ostrea edulis 
is hermaphrodite, and a neighboring species apparently unisexual. In 
both cases the organs are the same, but in O. edulis the same inti- 
mate recesses of the reproductive organ produce at one time ova, at 
another time sperms. 

The snails, or gasteropods, are divided into two great groups, 
according to the twisting of their nerves. The one group {Strepto- 
neurd) have the sexes separate; the members of the other series 
{Ejithyneurd) are hermaphrodite. 

The sea-butterflies, or pteropods, are hermaphrodite, but the ele- 
phant' s-tooth shells {Scaphopods) are unisexual. So in cuttlefishes 
(^Cephalopods) the sexes are separate. 



HERMAPHRODITISM. ' 69 

VI. Degrees of Normal Hermaphroditism. — From what 
has been already said, it is evident that hermaphroditism may be 
more or less intimate. As an entire plant, an Arum is hermaph- 
rodite, with female flowers on the better nourished lower portion, 
and male flowers above. This may be paralleled by the red coral, 
which is sometimes female as regards one branch, and male as 
regards another. If we keep, however, to hermaphrodite individuals, 
it is evident that an orchid, with stamens and carpels united, is more 
closely hermaphrodite than a buttercup flower. So in a leech, with 
the ovaries far forward, and independent of the long row of testes, the 
hermaphroditism is less intimate than in a tunicate, where the testes 
and ovary may form one mass, the male cells spreading over the 
surface of the ovary. In the same way, the organ of a scallop, 
which exhibits more or less distinct male and female portions, is in 
a state of less intimate anatomical hermaphroditism than the oyster, 
where the same c£eca of the same organ fulfill both functions at 
different times. 

This last caution must be kept in view throughout. If the her- 
maphroditism be very intimate, — that is, if the seats of the ovum- and 
sperm-production be very close to one another, — it is not to be 
expected that the development of the two kinds of cells will go on 
simultaneously. Such would, indeed, be a physiological impossibility. 
Antagonistic protoplasmic rhythms may rapidly alternate, but can not 
coexist. Whether the hermaphroditism be anatomically intimate or 
not, there is throughout, in varying degrees, a tendency to periodicity 
in the production of male and female elements. Such a want of "time- 
keeping ' ' between the sexes is called, in botanical language, dichog- 
amy, and is one of the conditions which render self-fertilization rarely 
possible. Both in plants and in animals, the male function has in the 
majority of cases the precedence. Thus ' ' protandrous dichogamy' ' 
(stamens taking the lead) is very much commoner than ' ' protogynous 
dichogamy," where the carpels are first of all matured. This agrees 
with the curious cases of Angiostomum and Cymothoida; already men- 
tioned, where the organ was first male and then female, and indeed 
with at least most cases among closely hermaphrodite animals. Where 
the male organs are situated in one part of the body, and the female 
organs in another, there is less reason against the production of 
sperms going on at the same time as the production of ova. The 
very physiological conditions which first determined the position of 
the ovaries here and the testes there, may remain to render it possible 
for the two opposing functions to go on at the same time. 

The common snail {Helix) is not only easily dissected, but in the 
complexity of its arrangements is full of interest. Here not only are 



THE EVOLUTION OF SEX. 



ova and sperms produced within the compass of one small organ, but 
each little corner of the organ shows female cells forming on the walls 
and male cells in the center. It has been justly suggested by Platner 
that the outer cells are the better nourished: they therefore naturally 
become developed into anabolic ova. 

VII. Self-fertilization. — We have noted above that though 
male and female organs be present in the same organism, they tend 
to become mature at different times, and that the more the closer the 
seats of formation of the two kinds of elements. It is equally neces- 
sary to emphasize that though both male and female elements may be 
produced in the same plant or animal, it is probably exceptional for 
the ovule to be penetrated by a pollen-cell from the same flower, and 
it is certainly rare for an animal to fertilize its own ova. 

It is believed by breeders of higher animals that " close-breeding" 
beyond a certain point is dangerous to the welfare of the breed. The 
offspring tend to be abnormal or unhealthy. In view of this, the 
rarity of self-fertilization among hermaphrodites has been explained 
in terms of the disadvantage of the process. In reality, however, this 
is putting the cart before the horse. In hermaphrodities, we take it 
that the two kinds of sexual elements mature and are liberated at 
different times, not because of any reaction of the disadvantageousness 
of self-fertilization on the health of the species, but simply because 
the simultaneous coexistence of opposite physiological processes is in 
varying degrees prohibited. More technically, dichogamy is not a 
subsequent result of the disadvantage of self-fertilization, but cross- 
fertilization is the subsequent result of increasing dichogamy. 

Self-fertilization does, however, occur as an exception among 
animals, — thus, in all probability, in the exceptional fish Serranus; 
certainly in many parasitic flukes or trematodes; "commonly, if not 
universally," in tapeworms or cestodes; also in the curious thread- 
worm Angiostoimcm, and probably in ctenophores, and in some other 
cases. In regard to some cases — for example, among hermaphrodite 
bivalves (where the sperms are usually wafted in with the water) — it 
is impossible as yet to say whether self-impregnation does or does not 
occur. Some curious, but not very reliable, observations are on 
record in regard to self-impregnation in casually hermaphrodite 
insects. 

Arguing from the bad effects of close breeding among higher 
animals, Darwin and others have called attention to the numerous 
contrivances among plants which are said to render self-fertilization 
impossible. It must again be said that this survival of a very old way 
of explaining facts — in terms of their final advantage — is not really 
a causal explanation at all. It has been pointed out that in some cases 



HERMAPHRODITISM. 



the pollen of a given flower is quite inoperative on the ovule of the 
same flower, or has the result of producing weakly offspring. Then 
there are a great variety of mechanical devices, as the result of which 
it is more or less physically impossible for the pollen of the stamens 
to reach the stigmas of the flower, or even to be dusted upon them 
by the unconscious agency of the intruding insects. Moreover, as 
among animals, so among plants, it is common for the male organs to 
become mature before the carpels are ready, or, in rarer cases, for the 
reverse to occur. 

There is no doubt that cross-fertilization ver)^ generally occurs, 
and it is physiologically probable that this is a considerable advantage, 
though less among plants (which are so very "female," that is, vege- 
tative) than among animals. But there is an increasing impression 
that both the occurrence of cross-fertilization, and the necessity of it 
among higher plants, have been exaggerated by the extreme Darwinian 
school. One of the most thoughtful and observant of American botan- 
ists, Mr. T. Meehan, has raised a vigorous protest against the prevalent 
view. In the Yucca, or Adam's needle, which is regarded as cross- 
fertilized by insects, he showed by experiment that there was in each 
flower ' ' no abhorrence of its own pollen. " " Even when fertilized at 
all by in.sects, I am sure the fertilization is from the pollen of the same 
flower. ' ' 

Then as to mechanical contrivances, he says, "we are told that 
iris, campanula, dandelion, oxeye daisy, the garden-pea, lobelia, clover, 
and many others, are so arranged that they can not fertilize themselves 
without insect aid. I have inclosed flowers of all these named in fine 
gauze bags, and they produced seeds just as well as those exposed." 

We can not here enter into a full statement of Meehan' s careful 
observations, but his three main propositions well deserve statement 
and due consideration: — 

1. Cross-fertilization by insect agency does not exist nearly to the 
extent claimed for it. 

2. Where it does exist, there is no evidence that it is of any 
material benefit to the race, but to the contrary. 

3. Difficulties in self-fertilization result from physiological disturb- 
ances that have no relation to the general welfare of plants as. 

species. 

VIII. Complemental Males. — When Mr. Darwin was inves- 
tigating barnacles and acorn-shells, in preparation for his monograph 
on the group, he discovered the remarkable fact that some of the 
hermaphrodite individuals carried minute males concealed under their 
shells. These he regarded as advantageous accessory forms, insuring 
cross-fertilization in the hermaphrodites which harbor them. The 



72 



THE EVOLUTION OF SEX. 



g^eat majority of the cirripedes are hermaphrodite; but among the 
barnacles proper — the stalked forms, which are nearer the ancestral 
type — separate sexes sometimes occur. On the females of a few of 
these, pigmy males, like those found upon hermaphrodites, also occur. 
These pigmy males, whether on females or hermaphrodites, are not 
only dwarfish, but are very often degenerate, sometimes wanting 
(according to Darwin ) both alimentary canal and thoracic legs. Some 
of them, in fa-^, are little more than parasitic testes. 




Fig. 17. — Myzostomata: A hermaphrodite (i), and a 
pigmy male {2). — From Nansen. 



(i) The original state of affairs in this case was probably the 
ordinary crustacean condition of separate sexes. (2) The males, as 
in some of the "water-fleas" or copepods, tend to be smaller, — 
smaller, indeed, to a vanishing-point, — while the females became 
more and more sluggish, and settled down. (3) In the genera Alcippe 
and Ciyptophialus , in the species Ibla cunmiingii and Scalpelluvt 
ornaium, we find true females, with attached pigmy males, often 
several, leading a shabby existence as parasites. (4) In other species 
oi Scalpellum and Ibla the same pigmy males occur, but attached, as 



HERMAPHRODITISM. 73 



we have noted, to hermaphrodites, which in these forms have replaced 
the true females. (5) Lastly, in many genera, like Pollicipes, only 
hermaphrodites occur. 

What Darwin did for the cirripedes, Graff" has done for another 
very curious set of animals, the Mysostotnata. These are degenerate 
chaetopods or bristle-footed worms, which occur as outside parasites 
on sea-lihes (crinoids), on the arms of which they make curious 
galls. The majority are hermaphrodite, but again some species have 
the sexes separate, and again in a few cases complemental males 
have been demonstrated. If the hermaphrodite condition was here 
primitive, it persists in the majority of cases; thus, Myzostoma glabrum 
is hermaphrodite, with a minute complemental male; M. cysticolum has 
the sexes distinct, but the female is just euierging from (or approach- 
ing) hermaphroditism, for it includes rudimentary testes; in M. tenuis- 
pinum, inflator, murrayii, there are separate sexes, with the females 
predominating in size. One conclusion, at least, is vividly suggested 
by these curious facts — the tendency of the male form to become 
reduced to a vanishing-point. 

IX. Conditions of Hermaphroditism. — In looking back over 
the cases where normal hermaphroditism occurs, a few general conclu- 
sions are readily drawn. Thus Claus points out that hermaphrodit- 
ism finds most abundant expression in sluggish and fixed animals. 
Flatworms, leeches, earthworms, tardigrades, land-snails, &c. , well 
illustrate the first of these; and among sponges, sea-anemones, corals, 
polyzoa, bivalves, &c. , we find frequent illustration of the associa- 
tion of fixedness and hermaphroditism. Most of the tunicates are 
also fixed, and all are hermaphrodite. Claus notes further how in 
flukes and tapeworms hermaphroditism is associated with isolated 
habits of life. The isolation, however, is only sometimes true, for 
flukes may occur near one another in great numbers; and as many 
as ninety tapeworms {Bothriocepkalus) have been known to occur at 
one time in a single host. 

Simon has gone further, in insisting on the real connection between 
quiescent and parasitic habit and the hermaphrodite condition. In 
flukes and tapeworms, leeches, Myzostmnata, and some cirripedes, 
we find the association of hermaphroditism with a more or less inti- 
mate parasitic habit. It will be remembered, too, that the hagfish, 
in which hermaphroditism is common, is also to a large extent a 
parasite. But what Simon points out is, that organisms on which great 
demands are made, especially in the way of muscular exertion, can 
not afford to be hermaphrodite; while a plethora of nutrition, as in 
parasitism, tends to make the persistence of the double state pos- 
sible. He gives numerous illustrations of this very reasonable con- 



74 THE EVOLL'TIOX OF SKX. 

tention. For it seems plausible that, with more available material 
for internal dififerentiation, such should actually occur. But it is pos- 
sible to venture still further. 

A sluggish habit is usually associated with a large surplus of nutri- 
tive material, and at the same time very frequently with an accumula- 
tion of waste products. Parasitism means not only abundant but 
rich and stimulating nutrition. Conditions which combine these two 
factors will tend to secure the persistence of primitive hermaphroditism, 
or even to develop it from a previously attained unisexual state. It 
must be noted, however, that exceptions occur, which it is at present 
difficult to explain. The ctenophores are all hermaphrodite, yet very 
active. So, too, are not a few tunicates; while the brachiopods are 
extremely passive, but not Sfiecially characterized by hermaphroditism. 

X. Origin of Hermaphroditism. — There can be very little 
doubt that hermaphroditism was the primitive state among multicel- 
lular animals, at least after the dififerentiation of sex-elements had been 
accomplished. In alternating rhythms, eggs and sperms were produced. 
The organism was alternately male and female. Of this primitive 
hermaphroditism, there is probably more or less of a recapitulation 
in the life-history of all animals. Gegenbaur states the ccmmon opin- 
ion in the following cautious and terse words : ' ' The hermaphrodite 
stage is the lower, and the condition of distinct sexes has been denved 
from it." Unisexual "dififerentiation, by the reduction of one kind 
of sexual apparatus, takes place at very different stages in the develop- 
ment of the organism, and often when the sexual organs have attained 
a very high degree of dififerentiation." The first structural stage in 
the separation would probably be the restriction of areas, in which the 
formation of two kinds of cells still went on at dififerent times in one 
organism. In different individuals the opposite tendencies we have 
already spoken of more and more predominated, till unisexuality 
evolved out of hermaphroditism. 

We may, in brief, suggest as the three probable grades in the 
history: — {a) The liberation of unindividuated sex-elements; ib) the 
formation of two di-\'erse kinds of sex-elements, incipiently male or 
female, at the same time or at dififerent periods, according to nutritive 
and other conditions; (c) the unisexual outcome, where the production 
of one set of elements has preponderated over that of the other. 

As at present existing, hermaphroditism may he interpreted as a 
persistence of the primitive state, or as a reversion to it. Individual 
cases must be judged by themselves, and the history of each must be 
taken into account. But where the hermaphroditism is manifestly 
exceptional, there can be seldom any question in regarding it as a 
reversion. The reversion would generally occur on the female side, 



HERMAPHRODITISM. 



for on a priori physiological grounds, it is, as Simon remarks, more 
readily intelligible that a female should produce sperms than that a 
male should produce ova. In this connection it is interesting to 
notice how Brock, in regard to the development of the reproductive 
organs of snails, maintains that they are laid down and developed on 
the female type, and only become secondarily hermaphrodite. Purely 
female forms still occasionally occur, which he interprets as exaggera- 
tions of the side normally preponderant. So in hermaphrodite bony 
fishes, the same author has shown that the preponderance is distinctly 
female. 

Hermaphroditism is associated in some cases (for example, Polyzoa) 
with the occurrence of parthenogenesis in allied forms; and it 
may be noted, as will become clear afterwards, that for a female to 
become hermaphrodite is a sort of step toward parthenogenesis. It 
means that certain cells of the reproductive organs are able to divide 
of themselves, — to form, however, not an embryo, but a bundle of 
sperms. 

The general conclusion, then, is that hermaphroditism is the 
primitive condition, and that the cases now existing either indicate 
persistence or reversion. 



76 THE EVOLUTION OF SEX. 



SUMMARY. 

I. Hermaphroditism is the union of the two sexual functions in one 
organism. This occurs, however, in varying degrees. 

II. Embrj'onic hermaphroditism is probably a general fact with even 
unisexual animals. It is certain in some cases. 

III. Casual or abnormal hermaphroditism is not infrequent. 

IV. Partial hermaphroditism (not involving the essential organs) is 
exceedingly common. 

V. Normal adult hermaphroditism ; review of its occurrence. 

VI. True hermaphroditism occurs in many degrees of intimacy. 

VII. Self-fertilization is a rare exception among animals ; commoner in 
plants. 

VIII. " Complemental males" — pigmies attached to hermaphrodites — - 
occur in two groups. 

IX. The conditions of hermaphroditism, in part, involve a surplus of 
material. 

X. Hermaphroditism is primitive ; the unisexual state is a subsequent 
differentiation. The present cases of normal hermaphroditism imply either 
persistence or reversion. 



LITERATURE. 

See already cited works of 
Gegenbaur, Hknsen, Hertwig, Hatchett Jackson and Rolleston, 

passhn. 
Bourne. — On Certain Abnormalities in the Common Frog. i. The Occurrence 

of an Ovotestis. Quart. Journ. Micr. Sci., XXIV. 
Brock. — Morph. Jahrb., IV. Beitriige zur Anatomic und Histologic der 

Geschlechtsorgane der Knochenfische. 
Giles. — Quart. Journ. Micf. Sci. 1888. 
Laulanie, F. — Comptes Rendus, CI. (1885), pp. 393-5. 
Marshall, A. Milnes. — On Certain Abnormal Conditions of the Reproductive 

Organs in the Frog. Journ. Anat. Physiol., XVIII., pp. 121-44. 
Meehan, T. — On Self-Fertilization and Cross-Fertilization in Flowers. Penn. 

Monthly, VII. (1876), pp. 834-43. 
Pfluger, E. — Archiv. ges. Physiol., XXIX. 
Simpson, J. Y. — Todd's Cyclopedia of Anatomy and Physiology. Art. 

Hermaphroditism, pp. 684-738 (1836-9). 
Spengel. — Arb. Wiirzburg, III., 1876. Ueber d. Urogenital System der 

Amphibian. 

Zwitterbildung bei Amphibien. Biol. Centrlbl., IV., 8, cf 9. 

Sutton, J. B. — Hypertrophy and its Value in Evolution. Proc. Z06I. Soc., 

London, 1888, pp. 432. 
General Pathology. London, 1886. 



THE ULTIMATE SEX-ELEMENTS. 77 



CHAPTER VII. 

THE ULTIMATE SEX-ELEMENTS. 

(^Geyieral and Historical.') 

TN our analysis of sex-characters we have followed the general course 
of biological history. We first passed from the form and habit 
of a male or female organism to the structure and functions of 
the sexual organs. In discussing hermaphroditism, we had occa- 
sion to refer to a third step of biological analysis — that which 
involves an investigation of the properties of the tissues. Now it is 





Fig. 18. — Mammalian ovum, showing nucleolus {a), nucleus {&), 
yelk (c), external porous zone or zona pellucida (d), and follicular 
cells {e). — From Hertwig, after Waldeyer. 

necessary to penetrate deeper, namely, to the sex-cells. After these 
have been considered, not in themselves, but finally and fundamentally 
in terms of the changes in the protoplasm that make them what they 
are, then we shall be in a better position to reascend to some of the 
problems of reproduction. 

I. The Ovum-theory. — It is now a commonplace of observa- 
tion and established fact that all organisms, reproduced in the ordinary 
way, start in life as single cells. We see insects laying their ova upon 
plants, or fishes shedding them in the water, and may watch how 
these cells, provided they be fertilized, give rise eventually to the 



78 



THE EVOLUTION OF SEX. 



adult organisms. Conveniently in the ordinary frog-spawn from the 
ditch, we can read what was so long a riddle — how development pro- 
ceeds by successive cell-divisions and by arrangement of the multiple 
results. Readily seen in many instances, it is true of all cases of 
ordinary sexual reproduction, that the organism starts from the union 
of two sex-cells, or that it is with the division of a fertihzed ovum that 
development begins. 

This profound fact, technically known as the "ovum-theory," has 
been not unjustly called by Agassiz "the greatest discovery in the 
natural sciences of modern times." We shall the better realize the 
magnitude of the difference which its recognition has introduced into 
biology if we briefly review the history. 

II. The History of Embryology, Evolution, and Epigen- 
esis. — The development of the chick, so much studied in embryo- 
logical laboratories today, was the subject of inquiry two thousand 
years ago in Greece. Some of the conspicuous marvels of reproduc- 
tion and development were persistently but fruitlessly speculated about 
throughout centuries. It was only during the scientific renaissance of 
the seventeenth century that the inquiry became more keen and 
sanguine, and began to rely to some extent at least on genuine 
observation. 

(a) Harvey (1651), with the aid of magnifying glasses {per- 
specillcB), demonstrated in the fowl's (fg^ the connection between the 
cicatricida of the yelk and the rudiments of the chick, and also 
observed some of the stages of uterine life in mammals. More import- 
ant, however, were his far-sighted general conclusions: (i) that every 
animal was produced from an ovum {ovum esse primordium commune 
mnnibus animalibus') ; and (2) that the organs arose by new formation 
(epigenesis), not from the mere expansion of some invisible preforma- 
tion. In this generalization, without however any abandonment 01 
the hypothesis of spontaneous generation of germs, he strove, as he 
said, to follow his master Aristotle, and was in so doing as far ahead 
of his contemporaries as a strong genius usually is. Before Harvey, 
the observational method had indeed begun. Thus, as Allen Thom- 
son notes, Volcher Coiter of Groningen (1573), along with Aldrovan- 
dus of Bologna, had watched the incubated egg in its marvelous 
progress from day to day. Fabricius ab Aquapendente (1621) had 
also studied the changes in the incubated egg, and the stages of the 
mammalian foetus. In keenness of vision, Harvey was far ahead 
of any of these. 

(S) Malpighi (1672), using a microscope with phenomenal skill, 
traced the embryo back into the recesses of the cicatricula or rudiment, 
but again missed a magnificent discovery, and supposed the rudiments 



THE ULTIMATE SEX-ELEMENTS. -jg 

to have preexisted in the egg. In 1677, Leeuwenhock was led by 
Hamm to the discovery of the spermatozoa; and this was followed 
up, though not to much profit, by Vallisneri and others. Steno, too, 
in 1664, had given the ovary its present designation; and De Graaf 
had interpreted the vesicles of this organ, which now bear his name, 
as for the most part equivalent to the ova which he had discovered 
in the oviduct. Needham (1667), Swammerdam (1685), and J. van 
Heme, also contributed items of information not then appreciated in 
their real relations. 

(c) The Theory of Preformation. — Ovists and Aiiimalculists. — 
In the early part of the eighteenth century, the embryological 
observations of investigaters, like Boerhaave, were summed up in the 
conception that development was merely an expansion or unfolding 
of a preexistent or preformed rudiment within the egg. Harvey had 
indeed striven for an opposite conclusion ; but his view was negatived, 
as we have seen, by Malpighi's failure to trace the embryo beyond the 
rudiments of the cicatricula. 

The notion of a preformed rudiment, thus suggested by Boerhaave, 
Malpighi, and others, rapidly became the prevalent theory. In so 
far as it emphasizes one side of the facts, it is bound in modified form 
so to remain. Leibnitz, Malebranche, and others, found it to fit in 
better with their cosmic conceptions than the older view of Aristotle 
had done, and welcomed it accordingly. 

The positions occupied by the physiologist Haller well illustrate 
the alterations of opinion. As Allen Thomson points out in his 
article ' ' Embryology, ' ' in the Encydopa:dia Britannica, ' ' Haller was 
originally educated as a believer in the doctrine of ' preformation ' by 
his teacher Boerhaave, but was soon led to abandon that view in favor 
of ' epigenesis ' or new formation. But some years later, and after 
having been engaged in observing the phenomena of development in 
the incubated egg, he again changed his views, and during the 
remainder of his life was a keen opponent of the system of epigenesis, 
and a defender and exponent of the theory of 'evolution,' as it was 
then named." 

The preformation theory found more and more definite expression 
in the works of Bonnet, Buffon, and others. It is now necessary to 
sum up its main propositions. 

The germ, whether egg-cell or seed, was believed to be a minia- 
ture model of the aduh. ' ' Preformed ' ' in all transparency lay within 
the egg the fliture organs, only requiring to be unfolded. Bonnet 
affirmed that before fertilization there lay within the fowl's ovum an 
excessively minute but complete chick. They compared the germ 
to a complex bud, which hides within its hull the floral organs 01 



8o 



THE EVOLUTION OF SEX. 



llie future. Harvey has said ' ' the first concrement of the future 
body grows, gradually divides, and is distinguished into parts ; not 
all at once, but some produced after the others, each emerging in its 
order." Very different was Haller's first and la^t utterance, "There 
is no becoming ; no part of the body is made from another ; all are 
created at once." This was obviously a short and easy method with 
embryology, if the organism was literally preformed in the germ, and 
its development simply a growth and an unfolding. 





Fig. 19. — The first stages ofdevelopment in a number of animals. A, Sprwige, Coral Earth 
worm, or Starfish. — B, Crayfish or other Arthropod. — C, Tunicate, Lancelet 
&c. — D, Frog or other Amphibian. 

1. Fertilized ovum. — 2. Segmented ovum, aball of cells, morula, or blastosphere. 
same, after further division or in section.^. The gastrula stage. 



The 



But this was not all. The germ was more than a marvelous bud- 
like miniature of the adult : it necessarily included in its turn the 
next generation, and this the next, — in short, all future generations. 
Germ within germ, in ever smaller miniature, after the fashion of an 
infinite juggler's box, was the corollary logically appended to this 
theory of preformation and unfolding, — of evolution, as it was then 
called, in a very different but more hteral sense from that in which 
we now use the word. 



THE ULTIMATE SEX-ELEMENTS. 



A side controversy of the time arose between two schools, who 
called each other "ovists" and " animalculists. " The former main- 
tained that the female germ-element was the more important, and 
only required to be as it were awakened by the male element to begin 
the process of unfolding. The animalculists, on the other hand 
asserted the claims of the sperm to be the bearer of the miniature nest 
of organism within organism, and supposed that it only required to be 
fed by the ovum to enlarge and unfold the first of the models which 
it concealed. 

(d) Wolff's Reassertian of Epigenesis. — The above ingenious 
construction was rudely shaken down, however, in 1759, when 
Caspar Friedrich Wolff showed, in his doctorial dissertation, the ' 
illegitimacy of the suppositions which lay at the root of the preforma- 
tion theory. He traced the chick back to a layer of organized par- 
ticles (the familiar cells of today), in which there was no likeness of 
the future embryo, far less adult. More than that, he followed the 
disposition of these primitive elements to the upbuilding of some of 
the important organs. He undoubtedly reached too far in his 
emphasis on the entire simplicity of the germ, and many of his details 
were mistaken; but none the less did he recall embryologists from 
speculation to take the facts as they found them, and lay the founda- 
tion of modern embryology in the fact that organization was gradu- 
ally acquired by an observable process of development. 

{e) Wolff's Successors. — The important conclusion reached by 
Wolff remained for about sixty years without effect. In 18 17, 
Christian Pander took up embryological research exactly where Wolff 
had left it, and worked out the history of the chick in more exact 
detail. In 1824, Prevost and Dumas noticed the division of the ovum 
into masses; and in the following year Purkinje discovered the nucleus 
or "germinal vesicle." Von Baer followed up his friend Pander's 
work, and in 1827 made the memorable discovery of the mammalian 
ovum, which he traced from uterus to oviduct, and then to its position 
in the ovary itself Thus, after a century and a half, De Graaf's 
endeavor was at length fulfilled. Soon afterwards, Wagner, von 
Siebold, and others, elucidated what was still hidden from von Baer — 
the real nature of the spermatozoa. Meanwhile, Bichat's analysis 
(1801) of the organism into tissues, was with improved appliances 
deepened in the casual description of ' ' cells' ' ; and an important gen- 
eralization had its forecast in 1835, when Johannes Miiller pointed out 
in the vertebrate notochord the existence of cells resembling those 
of plants. 

III. The Cell-theory. — Without continuing the history further, 
we must simply note that in 1838 Schleiden referred all vegetable 



82 



THE Kl'OLUTION OF SEX. 



tissues to the cellular type, and traced back the plant-embryo to a 
single nucleated cell; while, in the following year, Schwann boldly 
extended this conception of plant structure and development to the 
animal world, and so fully constituted the " cell-theory." The ovum, 
recognized as a cell, became a " primordium commune" in a deeper 
sense than Harvey dreamt of; the masses described by Prevost and 
Dumas were seen as the products of cell-division; and Kolliker led 
the way, now so well followed up, in tracing these cells to their results 
in the tissues of the organism. 

IV. Protoplasmic Basis. — Only one step further is it possible 
for biological analysis to penetrate, and that within the last few years 
is being persistently essayed. It is impossible to rest at the cell-theory 



^ 




/- ~--\ 








B 




^-:::-,iv:: 


....■* 


^';^r<5'^'"'v--... 


"^ Fig.l. 


"■•«1 



Fig. 20. — Ground-plan of Protoplasmic Changes. 

level. To recognize the ovum as a cell and the spermatozoan as 
another, to find the starting-point of the organism in the double 
unity formed from these two, to demonstrate the process of develop- 
ment as one of cell-multiplication and arrangement, express great but 
not final biological facts. Thus it is that of late years, what Michael 
Foster has called the ' ' protoplasmic movement ' ' has made itself felt, 
not only in study of the general functions of the body, but in the 
spvecial physiology of the reproductive cells and their history. Even 
in morphological or structural studies, attention has shifted from the 
shapes of cells to the structure of their living matter, or from the 
different forms of ovum and spermatozoan to the germinal protoplasm 
or keimplasma which they contain. On this level — in fact where 
biology has touched the bottom — morphology and physiology have 



THE ULTIMATE SEX-EI.EMENTS . 



83 



become more than ever inseparable. All the facts of structure on the 
one hand and of function on the other have both to be interpreted in 
terms of the constructive and disruptive changes in the living matter 
itself The general theory may be summarized in the accompanying 
diagram. Protoplasm is regarded as an exceedingly complex and 
unstable compound, undergoing continual molecular change or meta- 
bolism. On the one hand, more or less simple dead matter or food 
passes into life by a series of assimilative ascending changes, with each 
of which it becomes molecularly more complex and unstable. On 
the other hand, the resulting protoplasm is continually breaking down 
into more and more simple compounds, and finally into waste pro- 
ducts. The ascending, synthetic, constructive series of changes are 




Fig. 21. — Protospongia, a colonial infusorian, showing the difference 
between outer and inner cells. — From Saville Kent. 



termed " anabolic " ; and the descending, disruptive series, "katabolic." 
Both processes may be manifold ; and the predominance of a partic- 
ular series of anabolic or katabolic changes implies the specialization 
of the cell. The upper figure (a) represents the complex unstable 
protoplasm as if occupying the summit of a double flight of steps ; 
it is formed up the anabohc steps, it breaks up and descends by the 
katabolic. The lower figure (b) is a projection of the other, its con- 
vergent and divergent lines serving to represent the various special 
lines of anabolism and katabolism respectively, and the definite com- 
ponent substances ("anastates" and "katastates") which it is the task 
of the chemical physiologist to isolate and interpret (see pp, 114-7)- 



THE EVOLUTION OE SEX. 



V. Protozoa and Metazoa. — It has been emphasized above 
that every multicelhilar organism, reproduced in the ordinary way, 
starts from a fertilized ovum, from what may be fairly called a single 
cell. Sponge, butterfly, bird, and whale, start at the level of the 
simplest animals or Protozoa, which (with the exception of very loose 
colonies) remain always unicellular. The simplest organisms leave off 
where the higher plants and animals begin, that is, as unit masses 
of living matter. They correspond, in fact, to the reproductive cells 
of higher animals, and may be called, according to their predominant 
character, protova and protosperms. A fertilized ovum, as we have 
seen, proceeds by division to form a "body"; the protozoon remains, 
with few exceptions, a single cell, in which there is obviously no 
distinction between reproductive elements and entire organism. 




Fig. 22. — Ophrydium, a colonial infusorian. — From Saville Kent. 



Reference will have to be made to the Protozoa in three connec- 
tions, which may be here simply noted: — 

(a) In their chief groups, and in the stages of their life-histories, 
they express phases in the same cell-cycle which recurs in higher 
forms in the component elements of the body, and in the reproduc- 
tive cells. The contrast, in other words, between an infusorian and 
an amoeba, between the ciliated and amoeboid stage in the life-historv 
of many forms, is a forecast of the contrast between a ciliated cell and 
a white blood-corpuscle, between a mobile spermatozoon and a young 
ovum. That is to say, a predominance of the same protoplasmic pro- 
cesses is the common explanation of such similarities of form (see 
p. 114). 

{5) It is among the Protozoa that we must presently look, if we 
hope to understand the origin and import either of " male and female," 
or of fertilization (see pp. 104, 119). 



THE ULTIMATE SEX-ELEMENT. 



85 



(c) Among the loose colonies which some Protozoa form, and 
which bridge the gulf between the unicellular animals and the 
Metasoa, there is seen the beginning not only of the formation of a 
' ' body, ' ' but also the setting apart of special reproductive cells 
(see figs, on pp. 83, 84). On this point more emphasis must be laid. 
The ordinary protozoon is a single cell, and forms no body. It divides, 
indeed, and multiplies accordingly, but the products of division go 
asunder, whereas in the segmentation of the ovum they remain con- 
nected. In most Protozoa., there is continual self-recuperation ; in 
most, division occurs without any loss ; in most, there is no distinction 
between parent and offspring; in most, as there is no body, there is 
no death. Thus it is that, with one weighty caution to be afterwards 
noted, it seems justifiable to speak with Weismann and others of 
the "immortality of the Protozoa.'' In a certain sense, too, as we 
shall see, it is justifiable to speak of the immortality of the reproduc- 
tive cells in higher animals. The body dies, but the reproductive 
cells escape, before its deaths to live on, as new organisms, inclosing 
new sets of reproductive cells. Again, there is similarity between the 
Protozoa and the reproductive cell. 

But in some of the loose colonies (for example, Volvox) we see 
the beginning of the change which introduced death as a constant 
phenomenon (see fig. p. 122). The cell, which starts one of these 
colonies, divides ; the products of division, instead of going apart as 
usual, remain connected; a loose body of many cells is thus formed. 
In this cluster of cells, certain elements are in turn set apart and 
eventually adrift, as reproductive cells. They start new colonies, and 
thus we are introduced to what is constant in higher animals. The 
only marked differences are — (a) that the body of the metazoon is 
more than a loose colony of cells ; {U) that the reproductive elements 
are usually liberated from some definite region or organ; and (c) that 
they are more markedly differentiated as male and female cells. 

VI. General Origin of the Sex-cells. — Except in the lowest 
invertebrates, the sponges and ccelenterates, the reproductive elements 
almost always arise in connection with the middle layer (mesoderm or 
mesoblast) of the body. 

Neither in sponges nor in ccelenterates is there a middle layer exactly 
comparable to the mesoderm of higher animals ; the less definite middle 
stratum is now frequently termed a mesoglcca. In sponges, we already 
mentioned that the reproductive cells simply arise here and there among the 
other elements of the stratum. The ova are highly nourished mesogteal cells ; 
the primitive male-cells, which divide into numerous minute spermatozoa, are 

the reverse. 

In ccelenterates the phenomena are of much interest ; the origin of the sex- 
cells is very diverse. Some time ago considerable emphasis was laid by E. van 



86 THE EVOLUTION OF SEX. 



Beneden and others on the fact that, in certain Hydrozoa, "the ova are 
derived from the endoderm, and the sperms from the ectoderm." Thus 
Gegenbaur, accepting this, remarlvs that in such cases "the endoderm is the 
female and tlie ectoderm the male germinal layer." Such a generalization, if 
established, would be plausible enough, seeing that the inner or endoderm 
layer is the more nutritive or anabolic of the two. A controversy, however, 
soon arose, the result of which was to overthrow the generalization. In hydra, 
we have already noticed that both products arise from the ectoderm ; the same 
was shown by Ciamician to be true of Tubularia inesembryanthemum ; while in 
the Eudendrium ramosum the ova appeared to arise from the ectoderm, and 
the male elements from the endoderm, the very reverse of \'an Beneden's 
conclusion. The matter was settled, so far as the general facts are concerned, 
by Weismann, who established the fact of active migration of the elements 
from one la>-er to another. He has since been followed by other investigaters. 
(a) The sex-elements, both male and female, may appear first in the endoderm, 
whether they originate there or not, and from this inner laj'er they migrate 
to the ectoderm, where they ripen, (b) In rare cases they even ripen in the 
endoderm. (c) Very commonly the sex-cells originate in the ectoderm and 
ripen there, or they may pass thence into the endoderm and back again to 
the ectoderm, {d) In the medusa of Obelia, the ova appear to ripen partly in 
both layers. These facts, a convenient summary of which will be found in 
Hatchett Jackson's erudite edition of Rolleston's "Forms of Animal Life," 
show plainly enough how varied are the origin and history of the sex-cells in 
these forms. 

The colonial hydroids typically produce well-marked reproductive individuals 
or sexual zooids, set free as "swimming-bells" or medusoids (in a process to 
be afterwards described under "Alternation of Generations "). In these the 
reproductive elements are typically developed. But in van,'ing degrees these 
medusoids have degenerated, and are frequently not only not liberated, but lose 
their characteristic features, and become mere reproductive buds. In these 
buds the sex-cells are normally developed. But it very frequently happens that 
they arise more or less in the body of the asexual vegetative hydroid. They ripen 
early, and subsequently migrate to their proper place ; the asexual stage 
incorporating more and more of the originally separate sexual generation. 
Weismann has emphasized the value of this early ripening as an advantage to 
the race, lessening the danger of its extinction ; and this has doubtless to be 
considered, though it can hardly be regarded as a physiology of the facts. 

VII. Early Separation of Sex-cells. — Havino noted the 
general fact of mesodermic origin, and some of the interestino- 
phenomena observed in ccelenterates, we shall not further pursue the 
subject except as regards one question, the period at which the repro- 
ductive cells make their appearance. This is sometimes early, some- 
times late; and it is not yet decisively known how widely early separa- 
tion occurs, nor how far the fact is of much significance. The ques- 
tion will have to be discussed in the volume treating of heredity;, 
only a brief reference is here possible. 

In the case of a well-known fly {Chironomus), Prof Balbiani, 
unprejudiced by any theory of heredity, observed the following facts: 
Before the segmentation of the ft^-g had at all advanced, before what 



THE ULTIMATE SEX-ELEMENTS. 87 

embryologists call the blastoderm was more than hicipient, two cells 
were observed to be set apart externally. (.These had nothing what- 
ever to do with the polar globules seen in most ova at maturation.) 
The development proceeded apace, but the isolated cells took no 
share; they may be presumed to have retained intact the characters 
which they received when first divided off from the ovum. At a 
certain stage, however, the insulated cells sank inwards, took up an 
internal position, became the rudiments of the reproductive organs. 
Here, then, at an early stage, before differentiation is marked, the 
reproductive cells are set apart. They must therefore preserve much 
of the character of the parent ovum, and hand on the tradition intact 
by continuous cell-division to the next generation. 

In other words, in the preceding case, at a very early stage in the 
embryo, the future reproductive cells are distinguishable and separable 
from the body-forming cells. The latter develop in manifold variety, 
into skin and nerve, muscle and blood, gut and gland; they differ- 
entiate, and lose almost all protoplasmic likeness to the mother ovum. 
But the reproductive cells are set apart; they take no share in the 
differentiation, but remain virtually unchanged, and continue unaltered 
the protoplasmic tradition of the original ovum. After a while they, 
or their division-products rather, will be liberated as reproductive 
cells. These in a sense will be continuous with the parental germ. 
Their protoplasm will be more or less identical. The original ovum 
has certain characteristics, a 5 f; it divides, and all its cells must at 
first more or less share these characteristics; the body-cells lose them, 
the insulated reproductive cells must retain them. The ovum of the 
next generation has thus also the characteristics a b c, and must there- 
fore produce an organism essentially hke the parent. 

An early isolation of the reproductive cells, though never so, strik- 
ing as in Chironomus, has been observed in many cases, — for exam- 
ple, in other insects, in the aberrant worm-type Sagitta, in leeches, in 
threadworms or nematodes, in some Pofysoa, in some small crusta- 
ceans known as Cladocera, in the water-flea Moina, and in some 
spiders {Phalangida;), and probably in other cases. As the series is 
ascended, the reproductive organs are later in making their appear- 
ance, or at least they are only detected at a later stage; and it must 
also be pointed out that, in cases of alternation of generations, an 
entire asexual generation, or more than one, may intervene between 
one ovum and another. 

VIII. Body-cells and Reproductive Cells.— Various natural- 
ists have insisted on the contrast hinted at above, between the cells 
of the embryo which go to form the body, and those which are set 
apart as reproductive organs. 



8S THE EVOLUTION OF SEX. 

(a) As early as 1849 Owen noted that, in the developing germ, 
it was possible to distinguish between cells which became much 
changed to form the body, and cells which remained little changed 
and formed the reproductive organs. This view, as Brooks points 
out, he unfortunately afterwards departed from in his ' ' Anatomy of 
the Vertebrates." 

(b) In 1866 Haeckel connected reproduction with discontinuous 
growth, and insisted upon the material continuity between parent and 
offspring. Somewhat later, both he and Rauber drew a clear contrast 
between the somatic and reproductive elements, between the ' ' per- 
sonal ' ' and ' ' germinal ' ' portions of the embryo, or between the body 
and the sex-cells. 

(c) W. K. Brooks, in 1876 and 1877, again drew attention to 
this significant contrast. 

id) Yet more exphcil, in 1877, was the ingenious Dr. Jager, now 
better known in a very different connection, and a few of his sentences 
well deserve to be quoted. Referring to a previous paper, he writes 
as follows: "Through a great series of generations, the germinal pro- 
toplasm retains its specific properties, dividing in every reproduction 
into an ontogenetic portion, out of which the individual is built up, 
and a phylogenetic portion, which is reserved to form the reproductive 
material of the mature offspring. This reservation of the phylogenetic 
material I described as the continuity of the gei-m-pj'otoplasm. Encap- 
suled in the ontogenetic material, the phylogenetic protoplasm is 
sheltered from external influences, and retains its specific and embry- 
onic characters." 

{e) In an exceedingly clear manner, to which sufficient attention 
seems hardly to have been accorded, Galton, in 1876 and at other dates, 
as again more indirectly in his recent "Natural Inheritance," drew 
attention to the contrast between the gemmules of the ovum (stirp) 
which go to form the body, and those which, remaining undeveloped, 
form the sex-cells. " The developed part of the stirp is almost sterile," 
(that is, without influence in heredity); "it is from the undeveloped 
residue that the sexual elements are derived. 

(_/) Lastly, in 1880, Nussbaum, in an elaborate investigation on 
the differentiation of the reproductive cells, drew emphatic attention 
to some cases of their early separation, and reasserted Jager' s concep- 
tion of a continuity of germ-protoplasm. In this survey, however, 
we do not pretend to decide the difficult question of priority in the 
enunciation of this conception. Like many other generalizations, it 
appears to have arisen all but simultaneously in many minds. 

IX. ^Veismann's Theory of the Continuity of the Germ- 
protoplasm. — In some cases referred to in a foregoing paragraph, 



THE ULTIMATE SEX-ELEMENTS. 89 

it is possible to trace a direct cellular continuity, first of all, between 
the ovum and early separated and reproductive rudiments; secondly, 
between the latter and the future ova and sperms. There is not only 
cellular continuity between the ovum which gives rise to parent, 
and the ovum which gives rise to offspring, — that the cell-theory 
demands, — but there is a continuity in which the character of the 
original ovum is never lost by differentiation. In fact, there is a 
continuous chain of reproductive cells quite apart from the body-cells. 
It is in this sense that some of the authors quoted have spoken of the 
continuity of the germ-cif//f. This is certainly true for some cases. If 
it were true for all, the problems of reproduction and heredity would 
be much simpler than they at present appear to be. 




Fig. 23. — The relation between reproductive cells and the body. — The continuous chain of dotted 
cells at first represents a succession of Protozoa; further on, it represents the ova from which the 
"bodies" (undotted) are produced. At each generation a spermatozoan fertilizing the liberated ovum 
is also indicated. 

For in the present state of our knowledge we can only speak of 
the continuity of the reproductive cells, in exceptional or rather in a 
small minority of cases. Alike in the higher vertebrates and the 
lowly hydroids, the reproductive cells may appear late. After the 
differentiation of the vertebrate embryo has progressed far, or the life 
of the polyps continued for long, the germ-cells make their appearance; 
and though we know of course that they are descendants of the 
original ovum, yet we must allow, with Weismann, that in the form 
of special cells they are now for the first time to be detected. There- 
fore, Weismann says, "a continuity of gaxm-cells is now for the most 
part no longer demonstrable." 

Yet there is nothing that Weismann more strongly insists upon 
than the reality of continuity between ovum and ovum. In what does 
it consist, if a chain of ovum -like cells is only true of a minority of 
organisms? It consists, according to Weismann, in the "keimplasma" 
or germ-protoplasm. 

The germ-plasma is the distinctive part of the nucleus of the germ- 
cell. It has an extremely complex, and at the same time persistent, 
structure. It is the substance which enables the germ-cell to build 
up an organism, the architectural living matter, and the immortal 
bearer of all properties transmitted in inheritance. ' ' In every devel- 
opment," according to Weismann, "a portion of this specific germ- 



90 



THE EVOLUTION OF SEX. 



plasma, which the paternal ovum contains, is unused in the upbuilding 
of the oft'spring's body, and is reserved unchanged to form the germ- 
cells of the next generation. . . . The germ-cells no longer 
appear as products of the body, at least not in their most essential 
part — the specific germ-plasma; they appear rather as something 
opposed to the sum-total of body-cells; and the germ-cells of success- 
i\'e generations are related to one another like generations of Pro- 
tozoa. ' ' But the continuity is rarely kept up by a chain of undiffer- 
entiated reproductive cells; it depends upon the continuance and 
unchanged persistence of a minimal quantity of the original germ- 
plasma. 



THE ULTIMATE SEX-ELEMENTS. 



91 



SUMMARY. 

The progressive analysis through organism, organs, tissues, and cells, to the 
living matter itself. 

I. The Ovum-theory. — Every organism, reproduced in the ordinary way, 
arises from a fertilized egg-cell, and development proceeds by cell-division. 

II. Epigenesis and Evolution. — History of the different views taken of the 
development of the organism ; ancient speculations. The scientific renaissance. 
(<;) Harvey's prevision of the ovum-theory, and emphasis upon "epigenesis." 
(b) Observations of Malpighi and others, mostly against Harvey's view, (c) 
The theory of preformation, — of a nest of miniature models within the egg, 
only requiring to be unfolded in successive generations ; Ovists versus 
Animalculists. (rf ) WolfTs reassertion of "epigenesis," the foundation of 
modern embryology ; his exaggeration of the simplicity of the germ. («?) Wolff's 
successors. 

III. The Cell-theory.— All organisms are made up of cells, and start from 
cells. 

IV. A protoplasmic basis now being laid. The "germ-plasma" more 
important than the egg-cell. All to be explained in terms of protoplasmic 
changes. 

V. The contrast between Protozoa and Metazoa. — The making of a " body " 
as distinct from reproductive cells. 

VI. General origin of the sex-cells, indefinite in sponges, variable in 
ccelenterates, generally from the mesoderm in higher animals. 

VII. Early separation of the reproductive cells to be seen in a minority of 
cases. 

VIII. The contrast between somatic and reproductive cells, and the 
continuity of the latter; Owen, Haeckel, Rauber, Brooks, Jager, Galton, 
Nussbaum. 

IX. Weismann's theory of the continuity of the %%rca-plasma (a specific 
nuclear matter), as opposed to continuity by a chain of undifferentiated cells, 
which is known to occur only in a minority of organisms. 



LITERATURE. 

For relevant literature and further details, consult the textbooks of Balfour, 

Haddon, and Hertwig ; also, 
Geddes, p.— Encyclopaedia Britannica articles already referred to ; abo 

"Morphology," ibid. 
Hensen, V. — Op. cit. 

M'Kendrick, J. G.— Textbook of Physiology. Lond., 1888. 
Thomson, J. A.— Arts. " Cell " and "Embryology," new Edition of Chambers's 

Encyclopaedia. 

History and Theory of Heredity. Proc. Roy. Soc. Edin., 1888. 

Waldeyer, W.— Die Karyokinese, &c. Arch. Mikr. Anat., 1888. 

Weismann. — Opp. cit. 

Zoological Record, General Subjects : Cell, Oogenesis, &c., since 1886. 



92 THE EVOLUTION OF SEX. 



CHAPTER VIII. 

THE EGG-CELL OR OVUM. 

TN the preceding chapter we sketched the history of the "ovum- 
theory," which expresses the now familiar fact that every organism, 
reproduced in the ordinary way, develops from a fertilized egg-cell. 
It is now necessary to attend more carefully to the essential characters 
and history of this " primordium commune," — this common starting- 
point of life, — leaving the details, along with the other problems of 
development, to a special volume devoted to Embryology. 

I. Structure of the Ovum. — The ovum presents all the essen- 
tial features of any other animal cell. There is the cell-substance, con- 
sisting in part of genuine living matter or protoplasm; and there is 
the nucleus, or "germinal vesicle," which plays such an important 
part in the ripening, fertilizing, and subsequent division of the cell. 

The cell-substance exhibits, when highly magnified, a homogene- 
ous matri-x, traversed by a delicate network, with minute yelk-balls, 
pigment, and other granules strewn about the meshes. So much of 
it is genuine protoplasm, of course, but then there are also substances 
in process of ascent and even descent from the climax of living matter, 
and there is in more or less abundance a reserve capital of yelk- 
nutriment for the future em.bryo. Delicate observations, by the 
modern masters of microscopic technique, have detected many mar- 
vels in the egg-cell, into which Ve can not at present enter. Thus, 
within the last year, Boveri has drawn attention to a special element 
in the protoplasm, which he calls archoplasm, a substance which, as its 
name suggests, seems to have an altogether marvelous architectural 
function in relation to the changes of the nucleus in segmentation. 

When Purkinje, in 1825, discovered the nucleus of the fowl's egg, 
he could have little idea that the little ' ' vesicle ' ' to which he directed 
the attention of investigaters was in reality an intricate microcosm. 
Little more than ten years elapsed before R. Wagner began to com- 
plicate matters by the discovery of the nucleolus or germinal ' ' spot ' ' 
within the "vesicle." We now know that the nucleus has not only a 
very complex structure, but in a sense a curious internal life all its 
own. 

The nucleus, when quiescent, often lies in a little nest or chamber 
within the cell-substance, and is limited from the latter by a more or 
less distinct nuclear membrane, which disappears as the period ol 
activity begins. Inside this membrane, it is often possible to dis- 
tinguish one or more of the aforesaid nucleoli, lying in a more fluid 



THE EGG-CELL OR OVUM. 



93 



material often called the ' ' nuclear sap. ' ' About these nucleoli and 
bodies more or less like them, about the reasons for their variable 
number and form, very little that is certain can be said. Much more 
important is the essential constituent of the nucleus, a system of strands, 
coils, or loops, which stain deeply with various dyes, and are there- 
fore known as the chromatin elements. In contrast thereto, the less 
stainable and less essential constituents of the nucleus are distinguished 
as achromatin. 

The chromatin elements in the resting nucleus are oftenest 
arranged in a manifold coil, like a disordered ball of twine, while in 
other cases they appear rather as a living network. One thing 
about them seems very certain, and that is that they are in no dis- 




FlG. 24. — Animal Cell, showing the chromatin elements ot nucleus {a) in a 
long coil, and the protoplasmic network {b) round about. — From Camoy. 

order, but really preserve a very thorough definiteness. Whether the 
coil be continuous, as Van Beneden and others describe, or inter- 
rupted, as Boveri and others maintain, is subsidiary to the more strik- 
ing fact, that in the state of activity the number and disposition of 
the dislocated or loosened parts of the coil remain definite and 
orderly, and that their behavior is so like that of minute independ- 
ent individualities that any rough-and-ready account of the mechanics 
of cell-division must at once be ruled out of court. It is within the 
chromatin substance too that the germ-plasma, on which Weismann 
and others have so much insisted, has its seat. 

II. Growth of the Ovum. — When the ovum is very young, it 
very generally presents the features of an amoeboid cell. In some 
cases this phase persists for a longer time, as in the ovum of hydra, 



94 



THE EVOLUTION OF SEX. 



which in all essentials is comparable to an amceba. Even in the 
simplest animals, however, the amoeboid phase constantly shows a 
tendency to pass into greater quiescence, — to become, in fact, more 
or less encysted. So is it with ova, which though at first often 
resembling various forms of amoeboid cells, tend more or less quickly 
to pass into the encysted phase. The protoplasm no longer flows 
out in irregular ever-changing processes, but is gathered up into a 
sphere, rounded off, and surrounded by a more or less definite 
envelope. This transition from a state of relative equilibrium between 
activity and passivity to one in which passivity undoubtedly prepon- 
derates, is associated with an increase of nutriment and reserve-pro- 
ducts. The ovum feeds, becomes heavy with stored capital, becomes 
Jess active, and more encysted in consequence. 




Fig. 25. — Ovum of a Threadworm {Ascaris), showing {a) the 
chromatin elements of the nucleus, and (/') the appearance of the 
surrounding yelk. — From Camoy. 

III. Yelk. — The essential part of an egg-cell is always small, 
though even in this there are great differences. The nucleus, for 
instance, in the large eggs of amphibians, reptiles, and birds, may be 
detected with the unaided eye; while in other cases, such as sponges, 
the entire ovum is very minute. Yet every one knows that eggs vary 
enormously in size. The egg of a skate is very much larger than the 
^•g^ of a salmon ; and the eggshell of the extinct giant-bird of Mada- 
gascar (Epiornis) is big enough to hold the contents of one hundred 
and fifty hens' eggs. Similarly the contrast between the eggs 01 
ostrich and humming-bird is, as one would expect, extremely striking. 
Yet the eggs of whales are "not larger than fern-seed," and the 



THE EGG- CELL OR OVUM. 



95 



same is true for most mammals, except the very lowest. The differ- 
ences in size, when very striking, are due not so much to any marked 
disproportion in the essential parts of the ova, but to certain extrinsic 
additions. The most important of these is the yelk, which serves as 
nutritive capital for the embryo or young animal. Besides the yelk, 
we have also to take into account the frequent pigment, so familiar in 
frog-spawn, the albumen well seen in the white of birds' eggs, vari- 
ous forms of protective and attaching viscid material, and, lastly, more 
or less elaborate egg-envelopes or shells. The most important, how- 
ever, is the yelk, and in regard to its origin and disposition a little 
must be said. 




Fig. 26. The relation between the disposition of the yelk and 

the mode of segmen tation. ~ yl , diffuse yelk, for example, sponge. 
.S, polar: for example, frog.— C, central yelk; for example, cray- 
fish. Dy predominant: for example, bird. — A ', total and equal 

segmentation.— i5', total and unequal segmentation. — C, periph- 
eral segmentation.— /?', partial segmentation. 

The egg has its nutritive capital increased in three different ways : 
(a) Very generally it feeds on the nutritive elements in the general 
lymph or vascular fluid of the body. {U) At the same time, or in another 
case it avails itself of the debris of surrounding cells. In many 

instances for example, in the minute ovary of hydra, or in the ovarian 

tubes of insects — the ovum is but the surviving competitor among a 
crowd of surrounding cells, which to start with were all potential ova. 



96 THE El'OLUTION OF SEX. 



(c) In the third place, and this is the rarest form, the egg-cell 
acquires a store of food-material from a special yelk-gland, as in 
many of the lower " worms." But we have already pointed out that 
this yelk-gland is usually interpreted as a degenerative portion of the 
essential organ. 

The yelk, gained in the above ways, is more or less readily dis- 
tinguished from what is often called the formative protoplasm. Out 
of the latter the embryo is built up, while the yelk has for the most 
part only a secondary and nutritive role. We can not, of course, 
enter here into the difficult embryological question as to the extent in 
which the yelk ever shares in directly contributing to embryonic 
structures. The possibility of distinguishing between formative pro- 
toplasm and the nutritive material, depends on the quantity of the 
latter that is present, and on the way in which it is disposed, (a) When 
there is not much of it, as in the small ova of mammals and many 
invertebrates, the yelk-material is diffusely distributed. Then the 
ovum undergoes complete segmentation, {b) In the frog's ovum, on 
the other hand, there is a large proportion of yelk, which has especially 
accumulated in the lower hemisphere of the cell, while the darker half 
includes the truly formative protoplasm. In this case too the fi<g<g 
divides as a whole, but the divisions go on much more rapidly in the 
upper hemisphere, and it is there that the embryo is really formed, 
(c) A distinct mode of yelk-arrangement occurs in arthropods 
(crustaceans, insects, &c.J, where the center, not a pole, of the 
ovum is occupied by the nutritive material. In this case the forma- 
tive protoplasm divides round about the nutrient core, (d) In the 
majority of fishes, in reptiles, and in birds, the eggs show a much 
more marked polar accumulation of yelk. On the top of a large 
mass of nutritive material, the specifically lighter formative protoplasm 
lies like a tiny drop, and in those cases the division of the ovum is 
very partial, — that is, it is mainly restricted to the upper formative 
region. It is thus to be noted that the quantity of yelk present, and 
its diffuse, polar, or central arrangement, are associated with striking 
differences in the degree and symmetry of the segmentation. 

IV. Composite Ova. — We have emphasized the fact that the ovum must be 
regarded as a single cell. To this a definite but pedantic objection has been 
raised. In some parasitic flatworms there occur what have been called com- 
pound ova. A minute single cell arises, as usual, in the ovary, but in the 
course of its somewhat intricate history this becomes associated with several 
nutrient cells derived from the yelk-gland. These surround the original ovum, 
so that the whole now consists of several cells. But it must be noticed that only 
the central cell --the ovum proper — is fertilized, and that it contains all the 
formative protoplasm. Those that surround it are wholly nutritive; they event- 
ually break np, and are absorbed. 



THE EGG-CELL OR OVUM. 



97 



In other cases, especially in insects, the ovum grows rich at the expense of 
neij;hboring cells, which are sacrificed to its nutritive equipment. But it is evi- 
dent enough that a cell remains a cell, however many of its neighbors it may 
happen to absorb. 

V. Egg-envelops. — The ovum starts as a naked cell, but generally becomes 
furnished with ensheathing envelops. The exact history of the egg-membranes 
and sheaths is a very complex matter. Only the most general facts can here be 
stated. The envelops may be derived (a) from the ovum itself, {b) from 
surrounding cells, (r) from the secretion of special glands. 

{a) Just as a protozoan often exhibits distinct outer and inner zones, 
distinguished by minor physical and chemical peculiarities, so it is with the 
ovum. What are called yelk or vitelline membranes are generally produced by 
the ovum itself Furthermore, the outer protoplasm often forms a distinct firm J 
zone, known as the zona pellucida. This may be traversed by fine radiating 
pores establishing nutritive communication with the exterior, and is then known 
as the zona radiata. A special aperture or micropyle is sometimes present, 
through which the sperm enters, or nutritive supply is sustained. 

(b) The ovum; in its young stages, is very frequently seen surrounded by a 
circle of small cells, which form what is called a follicle. These may produce a 
membrane or a glairj' investment. According to some investigators (for 
example, Will), the follicular cells sometimes arise from within the ovum, as the 
result of an early activity in the nucleus. This \ iew, however, can not be said 
to be confirmed. 

(c) As the ovum ripens, and passes from the ovary into the duct, it often 
becomes surrounded by gelatinous, horny, limy, and other investments. In 
most cases, it necessarily follows that the egg has first been fertilized. The 
investments are usually referable to the activity of the walls of the oviduct or 
uterus, though sometimes there are special shell-glands, and the like. The 
chitinous cases of some insect-ova, the horny mermaids' purses of many gristly 
fishes, the more or less limy egg-envelops of reptiles, the firm limy eggshells of 
birds, so often stained with pigments, afford good illustrations of these secondary 
investments. Quite distinct are cocoons, such as those of earthworm and leech, 
which surround several eggs, and are produced from the skin of the animal. 

VI. Birds' Eggs. — The student may be fitly directed to the egg 
of the fowl, or of some other bird, for a convenient concrete illustra- 
tion of many facts. There he will see the great mass of yelk, of two 
kinds, yellow and white, and on the top of this the minute area ol 
formative protoplasm. It was on this, as it gradually revealed the 
cloudy outlines of the embryo chick, that the Greeks looked with 
naive unaided eyes. Here it was that Aldrovandus, Harvey, Mal- 
pighi, Haller, and the early embryologists, with clear vision, saw- 
almost as much as their appliances would permit. It was this which, 
in its primitive simplicity, impressed Wolff with the reality of epigen- 
esis ; and it is this that the observers of today look down upon 
through their embryoscopes, or cut sections of with their microtomes. 
Then round about all is the secondary investment of " white of &gg'^ 
or albumen ; round this a shell-membrane, between the two layers ol 
which the little air-chamber is formed ; and finally, the hard but 



gS THE El'OLUTION OF SEX. 

porous limy shell. There arises the difficult problem of the origin of 
the shell, in regard to which it is to be noted that Mr. Irvine, of 
Granton, has recently shown that fowls kept with access to no car- 
bonate, but only to other salts of lime, can still form a normal shell. 
This still consists of carbonate of lime, and is as firm as usual, demon- 
strating, like the same investigator's experiments on crabs, that animals^ 
possess no little power of changing one salt of lime into another. 
Then, in the eggs of other birds, the import of the seven or more 
pigments which produce the marvelous variety and beauty comes into 
question. Sorby has shown that they are related to the pigments of 
blood and bile ; but what they exactly mean no one yet knows. Wider 
still, the problem arises of how this coloration is so often protective; 
and whether Lucas is right in supposing that the color of the surround- 
ings can actually influence the deposition of pigment, by acting on the 
nervous system of the mother bird. Or again, there is the curious 
fact that the size of the &^% is often much out of proportion to the size 
of the bird, and the question arises as to how far this can be inter- 
preted as the result of the more or less anabolic and sluggish con- 
stitution. 

VII. — Chemistry of the Egg. — Every one knows that the eggs of birds form 
highly nutritious diet. As the egg contains nourishment for the young bird for 
a considerable time, it must, like milk, contain all the essentials of food. The 
results of a recent analysis of the fowl's egg may be taken as a sample. 

The germinal or formative disc consists chiefly of albiuninoid bodies, 
apparently of the globulin group, plus smaller quantities of lecithin and the like. 
The subtle protoplasm itself, it need hardly be said, defies analysis. 

In the yelk there are firm fats (tripalmitin, probably plus a little stearine), aed 
a fluid oil or glyceride. Fatty acids develop during hatching. A relatively 
large quantity of lime is present, probably, for the most part, as calcium 
albuminate. In the white of eggs there are true albumens, also globulins, and 
the quantity of peptones increases with the age of the egg. During de\elop- 
ment the embryo becomes richer in mineral matters, fat, and albumen, and the 
dry substance of the whole contents of the egg diminishes considerably. 

The yelk of many different kinds of ova has been analyzed, and the com- 
ponent substances distinguished as Ichthin (fishes), Emydin (tortoise), and the 
hke. More important were the discoveries of c/ioles/eri?t, vitelliii, imclciii, 
lecithin, and, in association with the latter, neuriij. As we can not here enter 
into the physiological import of such substances, it is enough to say that the 
nutritive material in ova usually consists of a mixture of comple.x, unstable, and 
highly nutritive substances. 

VIII. Maturation of the Ovum. — When the egg-cell has 
attained its mature size, a more or less enigmatical occurrence takes 
place. The nucleus, hitherto generally central, moves to the pole, 
alters considerably in its structure, and divides. A minute cell, with 
half of the nucleus, and a small amount of protoplasm, is given off 
Not long after, the nucleus remaining within the ovum repeats the 



THE EGG- CELL OR OVUM. 



99 



process, and another tiny cell is expelled. This process, which the 
majority of- investigators regard as one of normal cell-division or cell- 
budding, is known as the extrusion of the polar globules. Of gen- 
eral, and probably of universal occurrence, it has been but rarely 
observed in fishes and amphibians, and not as yet demonstrated in 
reptiles or birds. It was for long thought to be absent in arthropods, 
but the researches of Weismann, Blochmann, and others, have shown 
that this is not the case. An interesting peculiarity, which we shall 
afterwards notice, has been demonstrated by Weismann in regard to 
parthenogenetic ova. There is considerable diversity as to the exact 
time at which the extrusion occurs; generally, however, it precedes 
the entrance of the fertilizing sperm. The minute extruded cells never 
have any history, though they occasionally linger for a considerable 
time on the outskirts of the ovum. As an exception, they have been 
seen themselves to divide; and, with equal rarity, a misguided sper- 
matozoan has been observed to penetrate them. Usually, however, 
they simply dwindle away. The remaining female nucleus of the 
ovum is now ready to unite w 'th the male nucleus of the spermatozoan. 
By the twofold div'ision just described it has been considerably reduced 
in size, though not a whit in complexity, or in the number of its 
chromatin elements. At this point, awaiting the essential moment of 
fertilization, we shall for the present leave it. 

Within the last two years Weismann, assisted by C. Ischikawa, 
has demonstrated an exceedingly interesting fact in regard to polar 
globule extrusion in parthenogenetic ova. Instead of the two polar 
globules which are usually extruded, parthenogenetic ova were shown 
to form only one. This was demonstrated in a variety of cases, — in 
water-fleas (daphnids and ostracodes) and rotifers, — and is believed 
by this eminent authority to be a general fact. Blochmann, who has 
been successful in demonstrating polar globules in several orders of 
insects, has also observed that in the parthenogenetic ova of the plant- 
louse or aphis, only one polar globule was formed, while in the eggs, 
which only developed after fertilization, two occurred as usual. To 
these facts we must afterwards recur in connection with par- 
thenogenesis. 

IX. Theories of the Polar Globules.— The polar globules appear to have 
been first observed in 1848 by Fr. Miiller and Lov(Jn, but it is only within recent 
years that much has been made of them. Thanks to the masterly researches of 
BQtschli and Hertwig, Giard, Fol, and others, it became possible to interpret 
the extrusion as a case of cell-division or budding. More recendy, Van 
Beneden, whose monograph on the ovum of the threadworm (Ascaris) will 
remain one of the classics in this department of research, has raised a protest 
agamst regarding the extrusion as a normal cell-division. The details of the 



lOO THE EVOLUTION OF SEX. 

process, as interpreted by him, seemed to mark out the extrusion as something 
unique. The latest resuhs of Boveri, Zacharias, and others, however, confirm 
the older view, that the process is essentially one of normal cell-division. 

But while this structural fact may be regarded as certain, there is no 
unanimity as to what the process means. The chief opinions on this subject, 
only a mere outline of which can be given, are three, not including a number of 
suggestions according to which the extrusion of the globules is a kind of 
"excretion" of the ovum, or a "rejuvenescence" of the nucleus. 

{a) According to some, the egg-cell is in a sense hermaphrodite, and the 
polar-globule formation is an extrusion of the male element. Balfour expressed 
his view in somewhat teleological language: "I would suggest that in the 
formation of the polar cells, part of the constituents of the germinal vesicle, 
which are requisite for its functions as a complete and independent nucleus, is 
removed to make room for the supply of the necessary parts to it again by the 
spermatic nucleus. ... I will venture to add the further suggestion that the 
function of forming polar cells has been acquired by the ovum for the express 
purpose of preventing parthenogenesis." To this it must now be pointed out 
that so far as one polar globule is concerned, extrusion does not prevent 
parthenogenesis. This view seems, according to Brooks, to have been first 
advanced by M'Crady. It has been most carefully elaborated by Minot. 
According to Minot, "in the cells proper, both sexes are potentially present ; to 
produce sexual elements the cell divides into its parts ; in the case of the egg- 
cell, the male polar globules are cast off, leaving the female ovum." In 
parthenogenetic ova, he supposes that enough male element is retained, since 
only one polar globule appears to be formed. Van Beneden, whose opinion 
is entitled to great weight, also inclines to regard the polar globules as male 
e.xtrusions. 

Sabatier distinguishes, besides true polar globules, other extrusions, and 
believes the eliminated parts to be male elements. His views are connected 
with an elaborate theory of polarities, according to which, for instance, the 
peripheral extrusions are male, while central cores (in the development of 
sperms) are female residues. 

{b) A very different view — morphological rather than physiological — has 
been maintained by Biitschli, Whitman, and others. The formation of polar 
globules is an atavistic reminiscence of the primitive parthenogenesis. Just as 
the mother sperm-cell or spermatogonium, which corresponds in the male to the 
ovum in the female, divides up into what form spermatozoa, so the ovum retains 
a slight power of division. Yet parthenogenetic ova, so far as polar globules 
are concerned, show this least, nor can we well conceive an atavism so 
universally present without some important physiological necessity directly 
behind it. To Biitschli's view, however, such an authority as Hertwig inclines, 
and Boveri likewise interprets the polar globules as "abortive ova." 

(c) Weismann's view is different from either of the above, though nearer the 

first. He distinguishes in the nucleus of the ovum two kinds of plasma, (i) the 

ovogenetic or histogenetic substance, which enables the ovum to accumulate 
yelk, secrete membranes, and the like ; and (2) the germ-plasma, which enables 
the ovum to develop into an embryo. When the ovum is mature the 
ovogenetic substance has served its turn ; it is henceforth only an encumbrance ■ 
it is extruded as the first polar globule. This is all that is extruded in 
parthenogenetic ova. The second extrusion is a reduction of the germ-plasma 
itself by half, and the same must occur in the male germ-cell too. What is lost 



THE EGG-CELL OR OVUM. loi 



in the second polar globule is supplied by the fertilizing sperm. The beginning 
of development depends upon the presence of a definite quantity of germ- 
plasma. This the normal egg attains by first losing half and then regaining it, 
while the parthenogenetic egg attains the same result by never losing any at all. 

In this, too, there is much hypothesis. The two kinds of nuclear plasma, tlie 
diiTerence between the two polar globules, the necessity for a definite quantity 
before development begins, are all assumptions. Nor is it at all evident how the 
advantage of fertilization (as a source of progressive change and so on) could 
operate so as to induce the ovum to go through the circuitous process of losing 
half its "germ-plasma," and then gaining it again. 

(d) It appears simpler to us to suppose that the ovum, like any other cell, 
tends to divide or bud at the limit of growth, — a view in no way inconsistent 
with regarding the process as an extrusion of male elements. The precise 
homologies of the process will be clearer on reference to the diagram at 
page 108. 



I02 'fJ!i' Ei'OLrrioy of s/:x. 



SUMMARY. 

I. The ovum presents all the essential features of a cell ; its substance 
and nucleus described. The chromatin-elements of the latter are the essential 
parts. 

II. The ovum usually grows from an amceboid to an encysted phase, with 
increase of nutrition and size. 

III. The yelk is derived from the vascular fluid, or surrounding cells, or 
special glands, and is present in vao'ing quantity and dispo.sition If little, it 
is diffuse ; if much, it is polar or central ; and the difilerent modes of egg- 
division are associated with this. 

IV. In some cases the ovum is surrounded by a number of nutritive cells 
(composite ova), and often becomes what it is by preying upon its neighbors. 
This hardly affects its unicellular character. 

V. Egg-envelopes are produced from the ovum itself (for example, viteline 
membrane), or from surrounding cells (folicular sheath), or from special glands 
(the outside shell). 

VI. Bird's egg noted as a concrete illustration of facts and problems. 

VII. The egg, so far as its nutritive material is concerned, includes a 
mixture of complex, unstable, highly nutritive substances. 

VIII. The maturation of the ovum is usually associated with a double cell- 
division or budding, known as the extrusion of polar globules. In partheno- 
genetic ova only one seems to occur. 

IX. This polar globule formation has been interpreted variously : — (a) As 
an extrusion of male elements (Minot, Balfour, Van Beneden); (b) as an atavistic 
occurrence of cell-division (Biitschli, Whitman, Hertwig, &c.); ic) by Weis- 
mann's more complex hypothesis. It seems to be a case of cell-division at the 
limit of growth. 



LITERATURE. 

Balfour, F. M.— Op. cif. 

Van Beneden, E.— Recherches sur la Fecondation. Arch, de Biologie, 

IV., 1SS3. 
Carnov.— La Cellule II., 1S86, &c. 
Geddes, p. — Op. cit. 
Haddon, a. C. — Op. cit. 
Hensen, V. — Pp. cit. 
Hertwig, O. — Op. cit. 
Hatchett Jackson,— Introduction to his edition of Rolleston's Forms of 

Animal Life. 
McKendrick, J. G. — On the Modern Cell Theory', &c. Proc. Phil. Soc. 

Glasgow, XIX., 18SS. 
Minot, C. S.— American Naturalist, XIV., 1S80. 
Thojison, J. A.— Recent Researches on Oogenesis. Quarterly Joum. Micr. 

Sic, XXVI., 1S86. 

Art Embrj'ology, Chamber's Encyclopsedia. 

Weismann, a. — Die continuitat des Keimplasmas. Jena, 1885. 

Die Bedeutung der sexuellen Fortpflanzung. Jena, i885. 

And other papers recently translated, " Heredity." Oxford, 1889. 



THE MALE-CELL OR SPERMATOZOAN. 103 



CHAPTER IX. 

THE MALE-CELL OR SPERMATOZOAN. 

I. The General Contrast between Ovum and Sperma- 
tozoan. — Just as the ovum — large, well nourished, and passive — is 
a cellular expression of female characteristics, so the smaller size, less 
nutritive habit, and predominant activities of the male are summed up 
in the sperm. As the ovum is usually one of the largest, the sperm 
is one of the smallest of cells. The yelk or food-capital, and encysting 
membranes, which are often so prominent in the former, are as con- 
spicuously absent in the latter. The contrast, though less accented, 
is still quite discernible in plants. In fact, the two kinds of cells are 
just as widely opposed in their general features as they are funda- 
mentally complementary in their history. Before this opposition and 
complementai"iness can be fully understood, however, we must briefly 
sum up the characters and history of the male elements. 

II. History of Discovery. — In 1677, one of Leeuwenhoek's students, 
Hamm by name, called his master's attention to the minute elements actively 
moving in the male fluid. Leeuwenhoek, who some years previously for the 
first time obsen-ed what we now know as unicellular organisms, was at once 
impressed by the import of the marvelously active male units. Almost too 
much impressed, in fact, for he interpreted them as minute preformed germs, 
which only required to be nourished by the ovum to unfold into embrj-os. Thus 
the unfortunate aberration, already noted as the doctrine of the animalculists, 
had its origin. For long no progress whatever was made ; some naturalists, like 
Vallisneri, depreciating the import of the sperms altogether, and regarding them 
as worms which hindered the coagulation of the seminal fluid ; others going to 
the opposite extreme, and regarding them as nests of germs. Thus Haller at 
first considered them to be what Leeuwenhoek had suggested, but afterwards 
admitted them merely as nativi hospites scminis. In 1S35, even Von Baer was 
inclined to interpret them as minute parasites peculiar to the male fluid ; and if 
the curious student will turn up the article " Entozoa " in Todd's Cyclopcrdia of 
Anatomy and Physiolo(;y, of about the same date, he will find that the veteran 
Owen includes the spermatozoa under that strange heading. The ver\- name 
spermatozoan recalls the view which so long prevailed. 

In 1837, R. Wagner emphasized their constancy in all the sexually mature 
males which he examined, and their absence in infertile male hybrids ; Von 
Siebold demonstrated their presence in many of tlie lower animals ; and lastly, 
in 1841, Kolliker made one of his many important contributions to biology, in 
proving that the sperms had a cellular origin in the testis. 

III. Structure of the Sperm. — The sperm, then, is a cell. 
Though some, such as Kolliker, have inclined to regard it rather as a 
nucleus, its truly cellular character may be regarded as proven beyond 



104 



THE EVOLUTION OF SEX. 



dispute. We have, as in die ovum, to deal with cell-substance and 
nucleus, with this marked difference, that the cell-substance is generally 
reduced to a minimum. 

The sperm is almost always, moreover, a cell of a very definite 
type or phase. It is like one of the highly motile Protozoa, like a 
flagellate infusorian. Usually it consists of a minute ' ' head, ' ' consist- 
ing almost entirely of nucleus, and of a long contractile tail, which, 
working behind like a screw, propels the essential ' ' head ' ' through 
the water or along the ducts. Occasionally, as the diagram shows, 
there is a departure from the predominant phase of cell-Hfe. Thus in 
the threadworm Ascaris, the sperm has a blunt pear-shaped form, and 
exhibits slight amceboid movements. In some crustaceans and other 
arthropods, the cell is even more quiescent, and may exhibit curious 
forms such as that figured for the crayfish. The relatively dormant 




Fig. 27. — " Spermatic Animalculi" of the Rabbit ana tlie Dog. 
— From BufFon, after Leeuwenlioek. 



activity may however wake up, and the sperm exhibit active amceboid 
movements. Zacharias has made some interesting experiments, 
showing the modifiability of sperms under reagents ; thus, in a little 
crustacean {^Polyphemus pedicuhis) he first caused the cylindrical 
sperm to form amoeboid processes, and afterwards to replace these by 
what were to all intents and purposes cilia. This is entirely congruent 
with other experiments and observations on the passage of cells from 
one phase of the cell cycle to another. 

The progress of microscopic technique has demonstrated many complexities 
in the sperm as well as in the ovum. For a discussion of some of the more 
important of these, the reader is referred to the Encyclopcsdia Britannica 
article "Reproduction." A few points only need be noticed here. Thus most 
spermatozoa exhibit not only a head (almost wholly from the nucleus of the 
mother-cell), and a mobile tail (from the substance of the mother-cell), but a 
median portion connecting these. The tail is not unfrequently, as in salamander 
and man, furnished with a very delicate undulating or vibratile band. Com- 
plexities such as axial filaments, striations, and the like abound. In a few 
cases, as in the threadworm, the sperm is not left without any nutritive capital 
but furnished with this in the form of a cap, which falls off before the essential 



THE MALE-CELL OR SPERM ATOZOAX. 



105 



moment of fertilization arrives. Important perhaps is tlie observation, mainly- 
due to Flemming, tiiat the head of the sperm not only arises from the nucleus of 
the mother-cell, but almost wholly consists of the chromatin-elements of the 
same. 




Fig. 28. — Spermatozoa of crayfish {a), lobster {I), crab (c), ascarid {d), 
water-flea — moina (f), man {J), ray {g), rat (A), guinea-pig (z)» 
a beetle — immature stage (-^), sponge (/). 



IV. Physiology of the Spermatozoan. — A few facts in regard 
to the physiology of the sperm demand notice, (a) It is specialized 
as a highly active cell ; its minimal size, the usual absence of any 
encumbering nutritive material, the contractility of the tail, and the 
general shape, all fit it for characteristic mobility. More than one 
histologist has likened it to a free muscle-cell, and its resemblance to a 
flagellate monad has already been noted. (J) Furthermore, the sperm 
has very considerable power of persistent vitality. Not only does it 
often remain long unexpelled in the male animal, without losing its 
functions, but it may retain its fertilizing power after remaining for 
weeks, or even months, in the female organism. In the earthworm, 
the spermatozoa pass from one worm to another, not directly to the 
ova nor to female ducts, but to be stored up in special reservoirs or 
spermathecae. So it is with many animals. The spermatozoa received 
by the queen-bee during her single impregnation, are for a considerable 
period — even for three years — used in fertilizing successive sets of 
worker and queen ova. Quite unique, however, is the case of one of 
Sir John Lubbock's queen-ants, which laid fertile eggs thirteen years 
after the last sexual union with a male. The spermatozoa had 
apparently persisted all that time. Hensen cites the facts that a hen 
will lay fertilized eggs eighteen days after the removal of the cock ; 
and that in bats, spermatozoa may remain alive a whole winter in the 
uterus of the female, (c) Remarkable too, and again like monads, is 



io6 



THE EVOLUT/O.V OF SEX. 



the power the sperms have of successfully resistini; great deviations 
from the normal temperature. The presence of acids has usually a 
paralyzing influence, but alkaline solutions have, on the whole, the 
opposite result. 

V. Origin of the Sperms. — A primitive female cell in the ovary 
grows in bulk and nutriment, and remains intact, but a primitive male 
cell in the testis undergoes repeated division into secondary cells, 
which either themselves, or by further division, form the sperms. For 
the last twenty years the development of spermatozoa has been the 
subject of almost continuous research and controversy, and the all too- 
abundant nomenclature affords a suggestive index to the confusion out 




Fig. 2g. — Diagram of the Development of Spermatozoa (upper line), of the Maturation and Fertili- 
zation of the Ovum (lower line). 

a, an amosboid sex-cell : A, Ovum, with germinal vesicle, n; B, Ovum extruding first polar body, 
/i and leaving nucleus reduced by half; C, extrusion of second polar body, /2, nucleus «2, now 
reduced to one fourth of original, i, a mother sperm-cell, dividing (2, 3) into immature and 
mature spermatozoa (^/.). 

D, the entrance of a Spermatozoan ; £, the male and female nuclei jr/. « and «2 approach one 
another. 



of which the subject is now emerging. In a general way, the process 
is simply that of the varied segmentation of a mother-sperm-cell, and 
the occurrence of a series of preparatory stages before the sperm is 
finally matured. In detail, however, there are many variations, and 
these are described in a maze of often tautologous and ambiguous 
terms, such as spermatogonium, spermatoblast, spermatospore, sperma- 
togemma, spermatomere, spermosphere, and a dozen more. 

One of the most defensible set of terms is that used by Voigt after Semper, 
and also by Von la Valette St. George, who has worked persistently at the 
subject for over twenty years. The sperm or spermatozoan is differentiated 
from an immature cell or spermatide ; this is modified from or descended from 
a spermatocyte ; the spermatocytes result from the division of the mother-sperm 
cell or spermatogonium ; and this finally is a modified form or a descendant of 
the primitive sex-cell or male ovule. 



THE MALE-CELL OR SPERMATOZOON. 



107 



Difficulties become thick, however, when we inquire into the division of the 
mother-sperm-cell or spermatogonium, and it is here that the observations of 
recognized authorities so much disagree. Accepting the results of competent 
observers, we have elsewhere endeavored to rationalize and unify the con- 
flicting observations by comparing the different modes of spermatogenesis vdth 
the different forms of ovum-segmentation. It has been already incidentally 
noticed that the egg-cell may divide wholly and equally, or unequally, or only 
very partially, or round a central core. Just in the same way the mother-sperm- 
cells may divide into a uniform ball of cells, or only at one pole, or only at the 
periphery round a central residue. Balfour and others had hinted at this com- 





A* 















Fig. 30. — Comparison of Spermatogenesis and Ovum Segmentation. 



EXPI.ANATION. — The first line, A-E, exhibits types of ovum segmentation: — A, reg^ular morula; B, 
unequal segmentation, for example, in some MoUusks; C, centrolecithal or peripheral type, for 
example, in a shrimp Peneus: D, partial segmentation: E, the same, with the cells less marketily 
defined off from the yelk. 

In the next two lines various types of spermatogenesis are collated with the above to illustrate the 
parallelism; — A' and A", morula type, as in Sponge, Turbellarian, Spider, &c.: B' and B", where the 
division is unequal, and one large nutritive cell is seen (Plagiostome fishes, Von la Valette St. George) ; 
C and C", after Blomfield, Jensen, &c., showing central cytophoral or blastophoral nutritive portion; D' 
and D", sperm-blastoderm, with a few formative cells on large nutritive blastophore, after Gilson, &c.; E' 
flind E", the same, with the sperm-cells less definitely separated off, after Von Ebner and his followers. 



parison in the use of terms like sperm-morula; and Herrmann had also con- 
cluded, "that the division of the male ovule into a series of generations of 
daughter-cells, is a phenomenon comparable to that exhibited by the ovum in 
the formation of the blastoderm. . . . It seems, then, more important to deter- 
mine exactly the mechanism of division than to give a particular name to each 
stage of segmentation." 

Although this interpretation of spermatogenesis by collating it with ovum- 
segmentation appears to Minot " a fanciful comparison," in favor of which he 
is "unable to recognize any evidence," neither the initial homology between 
the mother-sperm-cell and ovum with which we start, nor the striking parallelism 



io8 THE EVOLUTION OF SEX. 




between tlie modes of division of these homologues seems thereby even dis- 
puted, much less shaken. The widely different conditions in which these two 
processes occur, and their very different meaning to the organism, are of course 
as obvious to us as to any; but here, as elsewhere, the morphologist's com- 
parisons are strictly independent of the approval of the physiologist. 

VI. Further Comparison of Ovum and Sperm.— It is often said that the 
sperm is the male cell which corresponds to the ovum. This is only true in a 
certain sense. In function the two elements are indeed, in a general way, of 
equal rank, and are obviously complimentary. But even in this respect, the two 
elements, which unite in equal proportions in the essential act of fertilization, are 
not e.xactly sperm and ovum, but (a) the head or nucleus of the sperm and {b) 
the female nucleus doubly reduced by the extrusion of two polar globules. The 
accurate structural resemblance or homology is not between ovum and sperm, 
but between ovum and mother-sperm-cell.* This fact, pointed out by Reichert 
in 1S47, corroborated by Von la Valette St. George, Nussbaum, and others, is 
fundamental to a clear comparison of the history of ovum and sperm, and is pos- 
tulated as an accepted fact in the rationale of spermatogenesis suggested in this 
chapter. It is possible to follow out the homology into even further detail: 
thus the antithesis seen in polor-globule formation may be fairly collated with 
similar separations occurring in spermatogenesis. 
Van Beneden and Julin, in their researches in 
oogenesis and spermatogenesis in Ascaris, have 
noted the morphological correspondence of the 
polar globules, as we may call them, of both 
ovum and sperm. Again we have a recent micro- 
chemical demonstration of the similar staining Fig. 31.— Diagrammatic comparison- 
reactions of polar globules in ova, and the corre- I- f™ale /.' and male ,ii cell formed 

,. ._ r ^^ 1. 11 • from the division of a single cell in 

spondmg remnant of the parent cell m sperma- ^^^ development of the hermaphro- 
genesis. In the differentiation of the reproduc- dite reproductive organs of the worm 
five cells in plants, both higher and lower, simi- SagiUn ; IT. ovum /a and polar 

, . . .L i_ u J r^r *u body a^\ TTl. stump of mother- 

lar extrusions are to be observed. Of this ' „ ... , ., „ . 

_ sperm-cell M and the spermatozooD 

Strasburger has given numerous illustrations, ^s. 
crowned by his own demonstration that the 

nucleus of the pollen grain, in its germination upon the stigma, separates into a 
vegetative, relatively unimportant, and a generative or essential nucleus. Even 
in Protozoa Blochman and others have found analogues. A process so general 
is capable of a unified explanation, more specific than that of simply referring 
the matter to the mysterious necessities of cellular physiology. Just as in the 
development of the "worm" Sagitta a single cell divides into two, which 
become the starting-points of male and female organs respectively, so the cell 
divisions above alluded to express antitheses between more katabolic and more 
anabolic protoplasmic constituents. 

VII. Chemistry of the Sperm. — Comparatively little has been done in 
regard to the chemistry of the male elements in different animals. The most 



* Since the above was written, Platner has in a remarkable manner demon- 
strated the unity between the division of the ovum in extruding polar globules 
and the division of the spermatocytes. In both cases occurs the unique phe- 
nomenon of a second nuclear division following on the heels of the first without 
the intervention of the usual resting phase. 



THE MALE-CELL OR SPERMATOZOON. 



109 



important observations are those of Miescher, on the milt of salmon. His anal- 
ysis demonstrated the presence of lecithin, fat, and cholesterin — also component 
parts of the ovum. Besides these, after the heads of the spermatozoa have been 
formed, Meischer detected the abundant presence of a substance which he 
called protaniin, which occurs in association with the nuclein already noted as 
present in the yelk. Albuminoid material, and products of decomposition, such 
as sarkin and guanin, were demonstrated, according to Hensen, by Picard. 

Miescher emphasized the interesting fact that while the sperm is being formed 
in the Rhine salmon, the animal is fasting. As no food whatever is taken and as 
the muscularity of the fish is well known to decrease greatly, Miescher directly 
connected the degeneration of the lateral muscles with the development of the 
spermatozoa. 

Zacharias has more recently made a micro-chemical comparison of the male 
and female elements in Characece, mosses, ferns, phanerogams, and amphibians. 
He finds that the male cells are distinguished by their small or absent nucleoli, 
and by their rich content of nuclein; while the female elements exhibited a pov- 
erty of nuclein, an abundance of albumen, and one or more nucleoh, more or 
less large in proportion. The male cells have, in relation to their protoplasm, a 
larger nuclear mass than the female elements. 

It is interesting to notice that two investigators have recently pointed out that 
an analysis of two different kinds of pollen showed a great analogy of composi- 
tion between these male reproductive cells and those of the salmon and ox. 



IIQ THE EVOLUTION OF SEX. 



SUMMARY. 

I. The contrast between the elements is that between the sexes. The 
large, passive, highly-nourished anabolic ovum ; the small, active, katabolic 
sperm. 

II. Hamman's discovery, 1677 ; Leeuwenhoek's interpretation ; the school 
of animalculists ; KoUiker's demonstration of the cellular origin of the sperm, 
1841. 

III. Structure of the sperm, — nuclear "head" of chromatin, protoplasmic 
"tail," middle portion. The sperm in reality comparable to a monad or 
flagellate infusorian, only with less cell substance. Its occasional degreda- 
tion into the amceboid phase. 

IV. Physiology of the sperm ; its locomotor energy at a ma.ximum, but 
yet great power of endurance, like a monad or bacillus. 

V. Origin of sperms from the division of a mother-sperm-cell homologous 
with the ovum. The different modes of "spermatogenesis" may be collated 
with the different modes of ovum-segmentation. 

VI. The occurrence in sperm-development of phenomena comparable both 
structurally and functionally with polar globula formation. 

VII. Chemistry of the sperm ; resemblance between pollen and sperma- 
tozoa. 



LITERATURE. 

Geddes, p., and Thomson, J. A. — History and Theory of Spermatogenesis. 
Proc. Roy. Soc. Edin., i885, pp. 823-824, i pi. See also Zoological 
Record from 1886, 



THEORY OF SEX— ITS NATURE AND ORIGIN. 



CHAPTER X. 

THEORY OF SEX — ITS NATURE AND ORIGIN. 

TJTAVING got SO far in our analysis, and before passing to the study 
of the processes of reproduction, we must add up the results in a 
general theory of the nature and origin of sex. After this has been 
done, we shall be in a better position to deal, in Book III., with fer- 
tihzation, parthenogenesis, and the like. The number of speculations 
as to the nature of sex has been well-nigh doubled since Drelincourt, 
in the last century, brought together two hundred and sixty-two 
' ' groundless hypotheses, ' ' and since Blumenbach quaintly remarked 
that nothing was more certain than that Drelincourt' s own theory 
formed the two hundred and sixty-third. Subsequent investigators 
have, of course, long ago added Blumenbach' s " Bildungstrieb " to 
the list; nor is it claimed that the generalization we have in our 
turn offered has yet received "final form," if that phrase indeed be 
ever permissible in an evolving science, except when applied to what 
is altogether extinct. This much, however, is distinctly maintained, 
that future developments of the theory of sex can only diifer in degree, 
not in kind, from that here suggested, inasmuch as the present theory 
is, for the first time, an expression of the facts in terms which are 
agreed to be fundamental in biology, those of the anabolism and kata- 
bolism of protoplasm. 

I. Suggested Theories. — According .to Rolph, — a fresh and 
ingenious thinker, removed before attaining his mature strength, — 
' ' the less nutritive, and therefore smaller, hungrier, and more mobile 
organism [cells, he is speaking of] we call the male; the more nutri- 
tive and usually more quiescent organism is the female. ' ' He goes 
on vividlv to suggest why " the small starving male-cell seeks out the 
large well-nourished female-cell for the purposes of conjugation, to 
which the latter, the larger and better nourished it is, has on its own 
motive less inclination." 

Minot, in his "theory of genoblasts," or sexual elements, ventures 
little further than regarding male and female as derivatives of primi- 
tive hermaphroditism in two opposite directions. ' ' As evolution con- 
tinued, hermaphroditism was replaced by a new differentiation, in 
consequence of which the individuals of a species were — some 
capable of producing ova only, others of producing spermatozoa only. 
Individuals of the former kind we call females, of the latter males, anp 
they are said to have sex. " "At present all we can say is, we do 



112 THE EVOLUTION OF SEJC. 



not know why or how sexual individuals are produced." In regard 
to the sex-elements, we have already noticed his opinion that they are 
at first "hermaphroditic or asexual," and that both dififerentiate by 
the extrusion or separation of the contradictory elements, the ovum 
getting rid of male polar globules, the sperms leaving behind a female 
mother-cell remnant. 

Brooks has emphasized rather a different aspect of the question. 
' ' A division of physiological labor has arisen during the evolution of 
life, the functions of the reproductive elements have become specialized 
in different directions. " " The male cell became adapted for storing 
up gemmules, and, at the same time, gradually lost its unnecessary 
and useless power to transmit hereditary characteristics. " " The 
males are, as a rule, more variable than the females; the male leads, 
and the female follows, in the evolution of new races. ' ' Brooks does 
not exactly attack the problem of the nature and origin of sex, but 
his emphasis on the greater variability of males is of much importance. 

These three positions must be taken as representative; others 
which appeal to superiorities, polarities, and like mysteries, can hardly 
claim scientific standing, and have been already sufficiently referred to 
at p. 28. To those which interpret the sexes in terms of the advan- 
tages of sexual reproduction, and to those which deal almost exclu- 
sively with the problem of fertilization, we shall afterwards return. 
The truth, in fact, is, that it is difficult to find any answer at once 
serious and direct to the question of the fundamental difference between 
male and female. 

II. Nature of Sex as seen in the Sex-elements. — The Cell- 
cycle. — As ova and sperms are the characteristic products of female 
and male organisms, it is reasonable that an interpretation of sex should 
start at this level. Here, assuredly, the difference between male and 
female has its fundamental and most concentrated expression. For 
the bodies, after all, as Weismann has so clearly emphasized, are but 
appendages to this immortal chain of sex-cells. 

We have already pointed out that the sex-cells are more or less 
on a level with the Protozoa. If we only knew, they probably differ 
widely from them in those intricacies of nuclear structure of which we 
only see the surface; yet as single cells the sex-cells are comparable 
with the Protozoa. For the moment, let us study those simplest 
organisms. Even a student, shown an extended series of unicellular 
forms, amoebae, foraminifers, sun-animalcules, infusorians, gregarines, 
and some of the simplest algae as well, might gradually begin to 
group these in his mind under three divisions. First there are highly 
active cells, — infusorians of all sorts; at the opposite extreme there 



THEORY OF SEX— ITS NATURE AND ORIGIN. 



113 



are quiescent forms, in which the Hfe seems to sleep, and locomotion 
is almost absent, — the gregarines, and some unicellular algae; and 
between these there are forgis which in a 
via media have effected a sort of compromise 
between activity and passivity, which are 
without the celia of the one or the self-con- 
tained stagnancy of the other, but possess 
outflowings of their living substance, — the 
familiar amoeboid processes. He would thus 
reach, almost by inspection, a rough and 
ready classification of the Protozoa, into 
active, passive and amoeboid cells, — a classi- 
fication however which, under varying titles, 
is more or less distinctly recognized by all 
the authorities on the subject. 

But if he went further than casual inspec- 
tion, and studied the life-history of some of 
the very simplest forms, such as some of the 
primitive molds or Myxoniycetes, and followed 
Haeckel's account of the life-cycle in Proto- 

myxa, he would gain new light on his classification. For in these life- 
histories he would find the cells now encysted, now active lashed 
spores, and again sinking down into the compromise of equilibrium 
effected by amcebae. He is now in a position to recognize that the 




Fic. 32.-1116 divergence of male 
and female cells from primitive 
amoeboid indifference. 




Fig. 33.— The encysted Protomyxa, and its division into numerous individuals within the cyst 

— From Haeckel. 



chapters in the life-history of the simplest forms are, as it were, proph- 
ecies of each of his three groups. Before final differentiation has taken 
place, the organisms pass through a cycle of phases, on eof whchi is 
accented by each of the different groups of the Protozoa. Thus an 



114 



THE EVOLUTION OF SEX. 



infusorian has its encysted chapter, a gregarine its amceboid stage, and 
a rhizopod may begin as a mobile cihated spore; for each group, while 
decenting one phase of the cycle, retains embryonic reminiscences of 
the others. 

A conviction that the triple division really meant much would 
grow in our student's mind, if he passed from the Protozoa to the 
cells which compose higher animals. There he would find active 
ciliated cells in most of the classes — from the ciliated chambers which 
lash the water into a sponge, to the cells lining the air-passages in 
man; passive encysted cells would be illustrated in some forms of 
connective, fatty, and skeletal tissue; while the white blood-corpuscles 
would be at one recognized as amcebse. Extended observation here 
also would show him the cells passing from one phase to another.. 




Fig. 34. — The cyzX. oi Protoinyxa busting, the flagellate young stages becoming at once amoeboid, 
eventually to unite in a composite amceboid mass, or "Plasmodium." — After Haeckel. 



His rough classification of the Protozoa would be verified in the his- 
tology of higher animals, and would reappear in the study of their 
diseases. He would be thus at length in a position to say that how- 
ever these three phases were brought about, the forms characteristic 
of them were of such wide occurrence through Nature as to justify his 
restatement of the familiar cell-theory in terms of a larger conception, 
that of the cell-cycle; that is to say, from the conception of the cell as 
a unit-mass of living protoplasm, amceboid, encysted, or ciliated, as 
the case might be, he would come to regard these forms as the pre- 
dominant phases of a cycle, — primeval, certainly, in the history of the 
organic world, and largely so even in the individual cell. 

All this time, however, our student has remained a morphologist, — 
his use of terms, Hke active and passive, simply expressing change of 
place. Not on this path of structural observation alone is it possibe 



THEORY OF SEX- ITS NATURE AND ORIGIN. 



115 



to understand what the. forms and phases of cells really mean. A 
final corroboration of the cell-cycle, and at the same time a rationale 
of it, is obtainable only on physiological lines, when we begin to 
inquire into the protoplasmic processes which lie behind any change in 
the form and habit of a cell. We have already spoken of the modern 
physiologist's conception of living matter, or protoplasm, as an 
exceedingly complex and unstable substance or mixture of substances, 
undergoing continual chemical change or metabolism. On the one 
hand, it is being continually reconstructed by an income of nutritive 
material, which, at first more or less simple, is worked up by a series 
of chemical changes till it reaches the climax of complexity and 



V 



3^ 




@ 



■^7-"!»J 






Fig. 35. — Diagram of the Cell-cycle, — of encysted, ciliated, and amosboid 
phases. I., II., III., in Protozoa — IV., ovum and sperm of fern pro- 
thallus — v., encysted, ciliated, and amoeboid animal cells — VI., ciliated 
animal cell pathologically becoming amcehoid — VII., sperm and amoe- 
boid sperm — VIII., amoeboid and encysted ovum. — From Geddes. 



instability. These upbuilding, constructive, synthetic processes are 
summed up in the phrase "anabolism." But, on the other hand, the 
protoplasm is continually, as it " lives, ' ' breaking down into more and 
more stable compounds, and finally into waste products. There is 
a disruptive, descending series of chemical changes known as kata- 
bolism. Both constructive and disruptive changes occur in manifold 
series. The same summit (see page 76) may be gained or left by 
many different paths, but at the same time there is, as it were, a dis- 
tinct watershed, — any change in the cell must tend to throw the pre- 



ii6 THE EVOLUTION OF SEX. 

ponderance toward one side or the other. In a certain sense, too, 
he processes of income and expenditure must balance, but only to the 
tusual extent — that expenditure must not altogether ou trunincome, 
else the cell's capital of living matter will be lost, — a fate which is 
often not successfully avoided. The disruptive, or katabolic, or energy- 
expending set of changes, may be obviously greater in one cell than 
in another, in proportion to the constructive or anabolic processes. 
Then, we may shortly say that the one cell is more katabolic than 
the other, or vice versa on the opposite supposition. Just as our 
expenditure and income should balance at the year's end, but may 
vastly outstrip each other at particular times, so it is with the cell of 
the body. Income, too, may continuously preponderate, and we 
increase in wealth, or similarly, in weight, or in anabolism. Conversely, 
expenditure may predominate, but business may be prosecuted at a 
loss; and similarly, we may live on for a while with loss of weight, 
or in katabolism. This losing game of life is what we call a katabolic 
habit, tendency, or diathesis; the converse gaining one being, of course, 
the anaboHc habit, temperament, tendency, or diathesis. The words 
' ' anabolic ' ' and ' ' katabolic ' ' are, of course, new, unfamiliar, and 
undeniably ugly. Habit and temperament have very vague associa- 
tions, and tendency sounds metaphysical; diatheses, again, seems no 
better than the medical equivalent of this. These things the reader 
must naturally feel; yet the medical man is nowadays quite scientific 
and definite in speaking of gouty or neurotic diathesis, of bilious 
habit, strumous tendency, or the like. The metaphysical vagueness 
is no longer chargeable to him; still less, we trust, to us. 

We are now in a position profitably to return to the Protozoa, to 
the phase of cell-life, and to the sex-elements. After what we have 
just said, it is evident that there are but three main physiological pos- 
sibilities, — preponderant anabolism, or predominant katabolism, or 
an approximate (that is, oscillating) equilibrium between these ten- 
dencies. A growing surplus of income, a lavish expenditure of energy, 
or a compromise in which the cell lives neither far .below nor quite up 
to Its income. Great passivity, great activity, or a safe average 
between these; conservative accumulation, spendthrift liberalism, and 
a compromise between these. In many different ways, more or less 
metaphorical, may we express the plain and indubitable facts oi 
anabolism and katabolism within the living matter. The student may 
think of the processes, with some degree of accuracy, under the meta- 
phor of an eddy in a stream, or of a ceaseless fountain, which, while 
remaining approximately constant, is the expression of continual 
ascent and descent of drops. The protoplasm itself must often be in 
as ceaseless change as the apex of the jet. 



THEORY OF SEX —ITS NATURE AND ORIGIN. 



117 



In active, motile, ciliated, or flagellate cells, whether they be 
constant forms or only temporary phases, there is predominant 
katabolism, — predominant when compared with the life-expenditure 
of a passive, quiescent, enclosed, or encysted cell. In amceboid 
organisms these extremes are avoided; there is certainly great ampli- 
tude of variation still, but neither anabolism nor katabolism gains the 
ascendant in any marked degree. 

Suppose, then, in such an amceboid cell, a continued surplus of 
anabolism over katabolism, the result is necessarily a growth in size, 
a reduction of kinetic energy and movement, an increase in potential 




FlQ, 36. Diagram sKowing the divergence of ovum and spermatozoon 

from a undifferentiated amoeboid type of cell. 



energy and reserve food-material. Irregularities will tend to disappear, 
surface-tension, too, may aid, and the cell acquires a spheriodal form. 
The result — surely intelligible enough — is a large and quiescent 

ovum. 

It will be remembered that young ova are very frequentiy amoeboid; 
that with a copious nutrition this disappears in varying degrees of 
encystment; that ensheathing envelops arising from the ovum, sweated 
off like cysts round Protozoa, are exceedingly common; and that ova 
are the largest of all animal cells. 

Starting once more from an amoeboid cell, if katabolism comes to 
be more and more predominant, the increasing liberation of kinetic 
energy thus implied must find its outward expression in increased 
activity of movement and in diminished size; the more active ceD 



THE EVOLUTION OF SIlX. 



becomes modiiied in form, in adaptation to passage through its fluid 
environment, and tlie natural result is a flagellate sperm. 

In short, then, the respective morphological characters of the sex- 
cells, female and male, find the same physiological rationale as do the 
large passive encysted and smaller active ciliated phases of the cell- 
cycle in general, and are alike the outcome and expression of predom- 
inant anabolism and katabolism respectively. Here again we reach 
the same formula as before; or more cumbroush' in words — the 
functions are either self-maintaining or species-maintaining, individual 
or reproductive; the former are divided into anabolic and katabolic, 
the latter into male and female. But the second set of products and 
processes, so far from being unrelated to the other as is commonly 
supposed, are in complete parallelism. Femaleness is anabolic pre- 
ponderance in reproduction, hence the ovum has necessarily the 
general character which this "diathesis " produces in non-reproductive 
cells; and, similarly, katabolic preponderance stamps its character of 
active energy upon spermatozoon as naturally as upon the cilitated 
cell of the monad. 

Rolph's characterization of the male cells as hungiy and starving (katabolic) 
has been experimentally confirmed by their powerful attraction to highly nutri- 
tive fluids, and is every day illustrated in their persistent attraction to the ova. 
Platner has suggested, in the intimately hermaphrodite gland of the snail that 
the external cells which form the ova are better nourished than the central cells 
which divide into sperms. Just as an infusorian in dearth of food is l^nown in 
some cases to divide into many small individuals, so the mother-sperm-cell is 
perhaps the seat of similar katabolic necessities. The long persistence of vitality 
seems at first sight a difficulty, if the sperms are highly katabolic cells. It must 
be noticed, however, («) that there is often only retention, not continuance of 
activity, for example, when the sperms lie closely packed in the special storing 
reservoirs; [b) that the secretions of the female ducts probably afford some nutri- 
ment to the sperms, which expose an exceptionally large surface in proportion 
to their mass; and (c) that to a certain extent we may think of them as proto- 
plasmic explosives, which may remain long inert, but on the presence of the 
required stimulus are able to start again into extraordinary activity'. 

III. The Problem of the Origin of Sex. — We must now 
return once more to the standpoint of the empirical naturalist, and set 
out toward the interpretation of sex from a different side — that of its 
origin. 

It has often been raised as a reproach against the now fortunately 
dominant school of evolutionist naturalists, that they could o-ive no 
account of the origin of sex. Some people, like children, wish every- 
thing at once. Yet it must be admitted that there has been a lack of 
any sure and certain voice on this question. Apart from the simple 
fact that evolutionist biology is still young, there are three reasons for 
the comparative silence in regard to the origin of sex. 



s 



THEORY OF SEX— ITS NATURE AND ORIGIN. 119 

1. The first of these is the still curiously prevalent opinion, that 
when you have explained the uHlity or advantage of a fact, you have 
accounted for the fact, — an opinion which the theory of natural 
selection has done more to foster than to rebuff. Darwin was, indeed, 
himself characteristically silent in regard to the origin of sex, as well as 
ol many other ' ' big lifts ' ' in the organic series. Many, however, 
have from time to time pointed out that the existence of male and 
female was a good thing. Thus Weismann finds in sexual reproduction 
the chief, if not the sole source of progressive change. Be that as it 
may at present, it is evident that a certain preoccupation with the 
ulterior benefits of the existence of male and female, may somewhat 
obscure the question of how male and female have in reality come 
to be. 

2. A second reason for the comparative silence may be found in 
the fact that the problem remains insoluble until it is analyzed into its 
component problems. The question of the origin of sex to a mind 
unprepared for the consideration of such a problem suggests quite a 
number of difficulties : What is the import and origin of sexual repro- 
duction (_the setting apart of special cells) ? What is the meaning and 
beginning of fertilization (the interdependence and union of sex-cells)? 
What is the reason of the individual, male or female, sex in any one 
case (the determination of sex) ? And lastly, what is the nature and 
origin of the difference between male and female? — the question at 
present under discussion. For purposes of analysis, those questions 
must be kept distinct, though in the final synthesis they are all 
answerable in a sentence. 

3. A third reason why the problem of the origin of male and 
female has been so much shirked, why naturalists have beaten so much 
about the bush in seeking to soh-e it, is that in ordinary life, for 
various reasons, mainly false, it is customary to mark oif the repro- 
ductive and sexual functions as facts altogether per se. Modesty 
defeats itself in pruriency, and good taste runs to the extreme of 
putting a premium upon ignorance. Now this reflects itself in biology. 
Reproduction and sex have been fenced off as facts by themselves ; 
they have been disassociated from the general physiology of the 
individual and the species. Hence the origin of sex has been involved 
in special mystery and difficulty, because it has not been recognized 
that the variation which first gave rise to the difference been male and 
female, must have been a variation only accenting in degree what 
might be traced universally. 

IV. Nature of Sex as seen in its Origin among Plants. 
— In tracing the origin of sex, we would wish to guard against any 
impression of having consciously or unconsciously arranged our facta 



1 20 THE E VOL UTION OF SEX. 

in the light of the theory we hold. Hence we prefer to follow some 
accessible account, taken essentially from the morphological point of 
view. We shall follow Prof Vines in his article ' ' Reproduction — 
Vegetable," in the Eiicyc/opcsdia B ritannica, aX Qach stage, however, 
endeavoring to interpret the facts, physiologically, in the light of 
protoplasmic processes. 

1. The simple alga Protococciis — which, in the widest sense of that term, 
evei-y one knows in some form or other, on tree-stems, in pools, wells, and the 
like — reproduces itself in a simple fashion. The cell divides into a number of 
equal units or spores ; these are set free, are mobile for a while, eventually come 
to rest, and develop to the normal size. A hint, however, of the beginning of a 
difference is seen when the cell occasionally divides into a larger number of 
smaller spores. These, however, show no difference in history. They settle 
down, and develop just like their more richly-dowered neighbors. We find here 
the occurrence of units of smaller size, that is to say, less predominantly ana- 
bohc, but still these are able to develop independently. 

2. In a higher alga, Ulothrix — one of the series known as ConfervcE — 
both large and small reproductive cells are developed. The large ones 
develop always of themselves, and so may the smaller forms. But the smaller 
forms may also unite in pairs, and then start a new plant from the double capital 
thus attained. When one of the smaller cells develops by itself, the result, in 
some cases at least, is a weakly plant. They have what Prof. Vines calls an 
"imperfect sexuality," for while they are in part dependent upon union with 
other cells, they are not wholly so. They are anabolic enough, we may say, 
sometimes to develop independently, but often they are individually too 
katabolic for anything but weak independent development. In uniting, 
however, in mutual nutrition, they are strong. The student will already see the 
relative femaleness of the large units, the maleness of their smaller neighbors. 

3. A third stage is reached in another alga {Ectocarpus) which is peculiarly 
instructive. This may separate off large cells which develop by themselves like 
parthenogenetic ova. From other parts of the plant smaller units are liberated, 
which generally, though not yet invariably, unite with one another before 
developing. But between these smaller units a most important physiological 
difference has been observed by Berthold. Some soon come to rest and settle 
down, and with these their more energetic neighbors by-and-by unite. We 
have here a very distinct beginning of the distinction between male and 
female elements. The comparatively sluggish, more nutritive, preponderat- 
ingly anabolic cells, which soon settle down — are female; the more mobile, 
finally more exhausted and emphatically katabolic cells — are male. As Vines 
says, "the one is passive, the other active ; the former is to be regarded as the 
female, and the latter as the male reproductive cell." 

4. Further, in another alga (Cnfleria') the differentiation may be traced. 
Two kinds of units result, which must unite with one another if development is 
to take place, but these units arise from perfectly distinct sources in the parent 
plant. The larger less mobile cells, which soon come to rest, are fertilized by 
the smaller more active units. The more anabolic or female cells are fertilized 
by the more katabolic or male cells, which have now gone too far for the 
possibility of independent development. 

5. To complete the series, we may simply mention such a case as that to 
which we shall presently return — those forms of Volvox, where an entire 



THEORY OF SEX- ITS NATURE AND ORIGIN. 121 

colony of cells produces either female or male elements, thus representing the 
beginning of an entirely unisexual many-celled organism. 

While the above cases also involve the problem of the origin of 
fertilization, which is left over for the present, they confirm most clearly 
our general conclusion that preponderant katabolism or anabolism are 
the ruling characteristics of male or female respectively. 

V. Nature of sex as seen in Origin among Animals. — 

Among the Protozoa, also, we can trace the beginnings of the same 
"dimorphism" between male and female. A union between similar 
cells is of course frequent, but that is not at present to the point. 
What we refer to, are the numerous cases, especially among flagellate 
and vorticella-like infusorians, where the two individuals which unite 
are quite unlike one another both in form and history. "There can 
be no doubt," Hatchett Jackson remarks, "that the process is 
essentially a sexual one ; when the individuals are invariably different, 
there is no reason why the terms male and female should not be 
applied to them." In some cases we find as before that a small active 
katabolic unit combines with a larger, more passive, and anabolic 
individual. 




Fig. 37. — Vorticella, the Eell-animalcule. — a, the normal 
individual: h, its division into two; c, the division 
accompHshed : d^ the further division of one of the 
halves into eight small (male) units; e, a minute 
individual uniting with one of normal size. 



In the bell-animalcule, which grows so commonly on the water- 
plants of our ponds, a minute free-swimming unit, formed as one of 
the results of repeated division, unites with a stalked individual of the 
normal size. In the related Epistylis, Engelmann has described how 
an individual divides first of all into two cells. One of these remains 
as such (hke an ovum), the other repeatedly divides (like a mother- 
sperm-cell) into numerous minute units. One of these subsequently 
unites with the undivided cell, and Engelmann does not hesitate to call 
the different elements male and female. In some radiolarians (for 
example, Collozoum), dimorphic spores — large and small — have been 



122 



riiE Evoi.uriox of sex. 



described, although their history has not yet been fully traced. E\-en 
in Foraminifcra, as Schlumberger, De la Harpe, and H. B. Brady 
have shown, a marked dimorphism may occur; and here again the 
disdnction seems to lie between preponderant anabolism and katabol- 
ism. 

As another illustration, it will be instructive to select the case of 
volvox. In this colonial organism, which is best regarded as a multi- 
cellular protist, the component cells are at first all alike. They are 
united by protoplasmic bridges, and simply form a vegetative colony. 
In favorable environmental conditions this state of affairs may persist, 




Fjg. 38. — Volvox globator, a colonial Alga or Infusorian, 
showing the ordinary cells (r) that make up the col- 
ony {or body), and the special reproductive cells (fr, h), 
both male and female. — After Cohn. 



or be interrupted only by parthenogenetic multiplication. When nutri- 
tion is checked, however, sexual reproduction makes its appearance, 
and that in a manner which illustrates most instructively the differen- 
tiation of the two sets of elements. Some of the cells are seen 
differentiating at the expense of others, accumulating capital from 
their neighbors; and if their area of exploitation be sufficiently large, 
emphatically anabolic cells or ova result; while if their area is reduced 
by the presence of numerous competitors struggling to become ova, 
the result is the formation of smaller, less anabolic cells, which become 
ultimately male, segment into antherozoids, meantime losing their 
vegetative greenness and becoming yellow. In some species, distinct 



THEORY OF SEX— ITS NATURE AND ORIGIN. 123 

colonies may, in the same way, become predominantly anabolic or 
katabolic, and be distinguishable as completely female or male 
colonies. Thus, again, we reach the conclusion, of a predominant 
anabolism effecting the differentiation of female elements, and of kata- 
bolism as characteristic of the male. 

VI. Corroborative Illustrations. — If the anabolic and kata- 
bolic contrast so plainly seen in the sex-elements, be the fundamental 
one, we must e.xpect to find it saturating through the entire organism. 
We have already drawn attention to the occurrence of yelk-glands in 
association with o\ aries. Or again, in the cells of a developing anther 




Fig. 39. — A Stonewort {Charafragilis), showing in two stages, adult and 
embryonic, the female organ (.5), and the male organ (a).— From 
Sachs, after Pringsheim. 

an enormous number of crystals may be often observed to occur. 
Crystals are, however, usually regarded as accumulations of waste 
products, and these anther-crystals, are, in fact, comparable to urinary 
deposits. Such accumulations do not, however, occur — at least to 
any similar extent — in the embryo-sac or in the female organs, in spite 
of the homology in male and female development. They occur as 
results of katabolism, where we would naturally expect them — in the 
tissue of male organs. 

In the stoneworts Chara or Nitella there is, as is well known, an 
alternation between nodal and internodal cells. The internodal cells 



124 '^^^ ElVLUTION OF SEX. 

are actively vegetative, and go on increasing in size; they do not 
divide, and may be justly regarded as emphatically anabolic. The 
nodal cells, on the other hand, are much smaller, and do divide. That 
is to say they are relatively more katabolic. 

A crucial test of the present theory thus suggests itself. Since the 
reproductive organs are simply, as every morphologist knows, 
shortened branch-structures, we should predict that the cell from the 
segmentation of which the antheridium is derived must correspond in 
position to a nodal and katabolic cell (that is, be based upon an inter- 
node), while the corresponding essentially female cell or ovum must be 
internodal or apical in origin (that is, based upon a node, and this 
relatively more anabolic). It is therefore not a little noteworthy that 
an examination, alike of classical figures and fresh specimens, will 
show that this imperfect homology, but perfect physiological corres- 
pondence, is invariably the fact (see figure). 

VII. Conclusion. — In conclusion, in defiance of Dr. Minot's 
recent dictum that ' ' such speculation passes far beyond the present 
possibilities of science," we believe that the consideration (a) of the 
characteristics of the sex-elements alike in history, as Minot himself 

SUM OF FUNCTIONS. 




Nutrilion. KeproductiOM. 





Anabolism. Kaiabolism. Female. Mate. 

Fig. 40. 

emphasizes, and in their finished form, (b) of the incipient sex dimor- 
phism seen among the simplest plants and animals, (c) of phenomena, 
both * normal and pathological, in the sexual tissues and organs, 
{d) of the established facts in regard to the determination of 
sex (chap. IV.) and {e) of the structural and functional, primary and 
secondary characteristics of the sexes (chap. II. and passim); — all 
lead to the general conclusion that the female is the outcome and 
expression of preponderant anabolism, and in contrast the male of 
predominant katabolism. Further corroborations will gradually 
appear in the succeeding sections, as we discuss fertilization, partheno- 



THEORY OF SEX— ITS NATURE AND ORIGIN. 



125 



genesis, or special facts like menstruation and lactation. The whole 
thesis may be once more summed up diagrammatically. 

In this way we see, with reference to the three speculations out- 
lined at the beginning of the chapter, — (i) that the penetrating insight 
of Rolph, of females as the more and males as the less nutritive, is 
fially justified; (2) that the view of Minot of the differentiation of both 
sex-cells from a primitive hermaphroditism becomes similarly devel- 
oped, and acquires greater definiteness; while (3) the view of Brooks, 
which ascribes variability primarily to the males, at least acquires con- 
siderable support from the interpretation of the males as preponder- 
atingly katabolic. For it is rather in connection with the destructive 
changes of protoplasm than with the constructive, that variations 
might be expected to .arise. 



126 THE EVOLUTION OF SEX. 



SUMMARY. 

I. Suggested theories of the nature of male and female ; their number and 
vagueness. Three recent developments — (a) Rolph's penetrating suggestion 
of more nutritive females, less nutritive males ; {b) Minot's theory of the differ- 
entiation of both kinds of se.\-cells from a primitive hermaphroditism ; (r) the 
conclusion of Brooks, that the males are more variable, and alone transmit 
new variations. 

II. Nature of sex seen in its essence in the sex-cells. The fundamental 
protoplasmic antithesis illustrated in the Protozoa^ in the cells of higher animals, 
in life-histories. The conception of a cell-cycle. The physiological import 
of this, — the protoplasmic possibilities, preponderant anabolism, predominant 
katabolism, and a relative equilibrium. The anabolic character of the ova. 
The katabolic character of the sperm. 

III. The problem of the origin of sex, so little tackled, because of (a) the 
blinding influence of teleological or utilitarian inquiries, (b) the number of 
separate problems involved, (c) the isolation of sex and reproduction from the 
general life of the organism and species. 

IV. A series from simple plants, shov\'ing the gradual appearance of 
dimorphic sex-cells, with the physiological interpretation thereof The dimor- 
phism is the result of preponderant katabolism and anabolism, and this is the 
origin of male and female. 

V. Illustrations of incipient dimorphism or sex among the Protozoa. 
Special reference to the case of volvox. 

VI. Corroborative illustrations, — anther cells and Chara. 

VII. General conclusion, (a) from the sex-cells, (b) from incipient sex, (c) 
from organs and tissues, (d) from the determination of sex, (<?) from the charac- 
ters of the sexes, — that male and female are the results and expressions of 
predominant katabolism and anabolism respectively, with consequent confir- 
mation of the speculation of Rolph and Minot, and in some measure also that 
of Brooks. 



LITERATURE. 

Brooks, W. K.— The Law of Heredity. Baltimore, 1883. 

Geddes, p. — 0pp. cit., especially "Theory of Growth, Reproduction, Sex, 
and Heredity," Proc. Roy. Soc. Edin., 1886; and Article "Sex, Encyc. 
Brit., also " Restatement of Cell Theory, " Proc. Roy. Soc. Edin. 1883-84. 

Minot, C. S. — Theorie der Genoblasten. Biolog. Centralblatt, II., p. 365. 

Rolph, W. H. — Biologische Probleme. Leipzig, 1884. 

Sachs, J. — Text-book of Botany, edit, by Vines, second edition, 1882; and 
Physiology of Plants, translated by Marshall Ward, 1887. 

Vines, S. H.— Physiology of Plants, 1886; article " Reproduction — Vege- 
table," Encyc. Brit. 

Weissmann, A. — 0pp. cit. 



BOO K III. 



PROCESSES OF REPRODUCTION. 



SEXUAL REPRODUCTION. 129 



CHAPTER XI. 
SEXUAL REPRODUCTION. 

I. DifTerent Modes of Reproduction. — It is well known that 
a starfish deprived of an arm can replace this by a fresh growth; that 
crabs can renew the great claws which they have lost in fighting; and 
that, even as high up as the lizards, the loss of a leg or a tail can be 
made good. In a great variety of cases a kind of physiological for- 
giveness is shown in the reparation of even serious injuries. Now this 
"regeneration," as it is called, is in a certain degree a process ol 
reproduction. By continuous growth the cells of a persistent stump 
are able to reproduce the entire member. We know, too, that a 
sponge, a hydra, or a sea-anemone, may be cut into pieces, with 
the result that each fragment grows into a new organism. The same 
is done with many plants; and though the division is artificial, the 
result shows how very far from unique is the process which we usually 
speak of as reproduction. In fact, as Spencer and Haeckel said long 
ago, reproduction is but more or less discontinuous growth. So 
again, we pass onward insensibly from cases of continuous budding, 
as in sponge or rosebush, to discontinuous budding in hydra, zoophyte, 
and tiger-lily, where the offspring, vegetatively produced, are sooner 
or later set free. Similarly in the Protozoa an almost mechanical 
breakage begins the series. This becomes more definite, in the pro- 
duction of several buds at once, or of only one. Budding leads on to 
deliberate and orderly division, both multiple and binary; while finally, 
in colonial forms, the liberation of special reproductive units may be 
observed. 

We shall afterwards have to discuss the relations of these and other 
processes; but just as we began the study of sex with the familiar 
contrast of male and female, so we shall begin our investigation of the 
reproductive processes with the most obtrusive mode, known as sexual 
reproduction. 

II. Facts Involved in Sexual Reproduction. — It is necessary 
at the outset, to be quite clear as to the concurrence of several distinct 
facts in any ordinary case of sexual reproduction among many-celled 
organisms, (i) There is, first of all, the fact that special reproductive 
cells are present in more or less marked contrast to the ordinary cells 
making up the body. To this antithesis we have already given due 
prominence. (2) Then there is the further fact, that these special 



THE ]-:\-OLUTION OF SEX. 



reproducti\e cells are dimorphic; that they, and the organisms which 
produce them, are distinguishable as male and female. This has beea 
the main theme of the two preceding books. (3) Lastly, we have to 
recognize that these dimorphic sex-cells are mutually dependent, — 
that if the egg-cell is to develop into an organism, it must first be 
fertilized by a male element. On the facts of fertilization, therefore, 
as observed in plants and animals, attention must now be concen- 
trated. 

III. Fertilization in Plants. — " The Newly Discovered Secret 
of Nature in the Structure and Fertilization of Flowers," so ran the 
title of a work pubhshed by Conrad Sprengel in 1793, embodying his 
pioneer investigations on a now familiar field. Though not indeed 
the first to point out the importance of insects in relation to fertiliza- 
tion — for that honorappears to belong to Kolreuter (1761) — Sprengel 
laid sure foundations, now somewhat hidden by the superstructure 
which Darwin and others have built. To Sprengel' s eyes, the many 
ways in which the nectar is protected from rain seemed full of 
"intention." He recognized in the markings of the petals illumined 
iinger-posts to lead insects to the hidden hoards; and he further 
demonstrated that in some bisexual flowers it was physically impos- 
sible for the pollen from the stamens to pass to the tips of the carpels. 
His general conclusion, freely stated, was, that "since a large number 
of flowers have the sexes separate, and probably at least as many 
hermaphrodites have the stamens and carpels ripening at different 
times, Nature appears to have designed that no flower shall be ferti- 
lized by its own pollen." A few years later (1799), Andrew Knight 
maintained that no hermaphrodite flower fertilizes itself for a perpe- 
tuity of generations. 

Sprengel' s secret of Nature had, however, to be set forth afresh 
by Darwin, who, in his "Fertilization of Orchids" (1862) and " Effects 
of Cross- and Self-Fertilization " (1876) has not only shown, with 
great wealth of illustration, the manifold devices for insuring that the 
unconscious insects carry the fertilizing pollen from one flower to 
another, but has also emphasized the beneficence of cross-fertilization 
for the health of the species. "Nature tells us," he says, "in the 
most emphatic manner that she abhors perpetual self-fertilization." 
Hildebrand, Hermann Miiller, Delpino, and others, have, with con- 
summate patience of observation, further traced out the secrets ot 
Nature in this relation; and the student may be referred to Professor 
D'Arcy Thompson's valuable edition of Miiller' s "Fertilization 01 
Flowers," Sir John Lubbock's " Flowers in Relation to Insects," and 
the classic works of Darwin. Reference must, however, also be made 



SEXUAL REPRODUCTION. 



131 



to Meehan's protest (^see pp. 71, 72), that self-fertilization is neither 
so rare nor so "abhorrent" as is now generally believed. 

In a great number of cases cross-fertilization by means of insects 
does occur; in many it must occur. In another by no means small 
set of flowering plants — usually with inconspicuous blossoms — the 
fertilizing golddust is borne by the wind, and falls, like the golden 
shower on Danae, upon adjacent flowers. In many hermaphrodite 
flowers, again, self-fertilization does certainly take place; in some this 
is necessarily so. Interesting in this connection is the indubitable 
self-fertilization which occurs in the small degenerate unopening 
(cleistogamous) flowers of some plants, such as species of balsam, 
deadnettle, pansy, c&c. These occur along with ordinary flowers, 
and, curiously enough, are sometimes more fertile than they. 





Fig. 41. — Bees visiting White Deadnettle and Broom. 



In most of the lower plants the male elements are minute and 
actively mobile. They find their way through the water, or along 
capillary spaces between the leaves, to the passive female cells. In 
some cases there is a curvature of the male organ toward an adjacent 
female organ, apparently in obedience to chemical or physical attrac- 
tion. Even here close fertilization seems exceptional, and is often 
impossible. 

So far, however, only the external aspect of the process. As 
long ago as 1694 Camerarius showed that if the male flowers of hemp, 
maize, and other plants were removed, the female flowers bore no 
seeds, or at least no fertile ones. In 1704 E. F. Geoffroy castrated 
certain plants by removing the stamens and noted that they remained 
barren. " Mirandum sane," he wrote, " quam similem servet natura 
cunctis in viventibus generandis harmoniam." Reasonable as this 
now appears to us, the fundamental fact was not only slowly recog- 
nized, but on into the present century there was found naturalists 



ir-,2 



THE EVOLUTION OF SEX. 



who strongly opposed it, and denied tlie sexuality of plants altogether. 
In 1S30, however, Aniici made a great step. He traced the pollen 
grain from its lighting on the carpel tip down into the recesses of the 
ovule. Schleiden, whose name is so closely associated with the 
founding of the "cell theory," soon confirmed Amici's obser\'ation, 
but in doing so went unfortunately much too far. Not only did the 
pollen-grain send its tube into the ovule, but there, according to 
Schleiden, it gave origin to the future embryo. This opinion, which, 
as Heyer observes, made the male element really female, was obviously 
parallel to that of the zoologists who found in the "sperm-animalcule" 
the miniature embryo. The view of Camerarius and Amici of course 
prevailed; and we now know not only the fact that the pollen-grain 




Ji 

Fig. 42. — A, Enlarged section of ripe Anther {b), liberating pollen {a). B, Diagrammatic 
section of a Flower, showing fLmale parts (r), — receiving stigma, conducting style, 
ovary with seed {d); the male parts, stamens {b) with pollen. C, The Pollen-tube (a) 
growing down to the ovule {d) and female cell {e). The pollen grain is here represented 
as distinctly two-celled, c£ pp. 131 and 212. 



is a male element which unites in fertilization with a female cell, but, 
thanks especially to Strasburger, much about the intimate nature of 
the process. In the last century Millington emphasized the difference 
between male and female flowers, and we can trace the influence of 
this discovery in Erasmus Darwin's "Loves of the Plants." 

In the last few decennia it has been shown, for many of the lower 
plants, that fertilization essentially involves the union of the nuclei of 
male and female cells. By analogy the same was belived to be true 
of higher plants, but direct demonstration has only recently been 
forthcoming. Strasburger has followed the whole history of the pollen- 
grain, from the anther of the stamen to the embryo-sac of the carpel; 
and, though some details still remain obscure, his researches have 
undoubtedly succeeded in elucidating the essential facts in the process. 
He shows how the pollen-grain divides into a vegetative and gen- 
erative cell, of which only the latter is directly important in fertiliza- 



S£.\'iL4L REPRODUCTIO: 



133 



tion. The generative cell, which consists like the sperm mostly of 
nucleus with very little directly associated cell-substance, itself divides 
to form two (^or even more) generative nuclei. One of these passes 
from the pollen-tube to enter into close union with the nucleus of the 
female cell, with which it fuses to form the double nucleus ruling the 
forthcoming development. Exceptionally the other generative nucleus 
may also unite with the nucleus of the egg-cell, but this is almost as 
rare as ' ' polyspermy ' ' among animals. According to Strasburger 
the cell-substance of the pollen-grain or pollen-tube which surrounds 
the nucleus has no direct influence in the essential act. Fertilization 
is a union of two nuclei, " the cell-substance of the pollen-tube is only 
the vehicle." He confirms the observations of Pfeffer as to the reality 
of an osmotic attraction between at least the surroundings of the two 
essential elements, in accordance with which the pollen-tube bearing 




Fig. 43. — Illustrating the contrast between male and female flowers in the 
pink campion {Lyc/uiis diur?ta). 



the o-enerative nucleus is marvelously guided to its destination. The 
differentiation of the generative nucleus, in contrast to the more vege- 
tative, and the true nuclear union which forms the climax of fertiliza- 
tion, are two very important facts, showing the unity of the process 
not only in higher and lower plants but in all organisms. 

IV. Fertilization in Animals. — That the sperms were essen- 
tial to fertilization was a conclusion by no means recognized when 
those elements were first seen. Gradually, however, the fact was 
demonstrated, both by experiment and observation. Jacobi (1764) 
artificially fertilized the ova of salmon and trout with the milt of these 
forms, and somewhat later the Abb6 Spallanzani extended these experi- 
ments to frogs and even higher animals. Even he, however, believed 
that the seminal fluid was the essential factor, not the contained sper- 



134 '■''"^ EVOLUTION OF SEX. 

matozoa. Through the experiments of Prevost and Dumas (1824), 
Leuckart (1S49J, and others, attention was directed to the real import 
of the sperms, which Kolliker referred to their cellukir origin in the 
testis. The presence of the sperm within the ovum was observed in 
the rabbit ovum by Martin Barry in 1S43; by Warneck, in 1850, for 
the water-snail, a fact confirmed about ten years afterwards by Bischoff 
and Meissner; in the frog ovum by Newport (1854J; ^^d in successive 
years it was gradually recognized in a great variety of animals. 

The external devices which secure that the sperm shall reach the 
ova are very varied. Sometimes it seems more a matter of chance 
than of device, for the sperms from adjacent males may simply be 
washed into the female, as in sponges and bivalves, with the nutritive 
water-currents. In other cases, especially well seen in most fishes, 
the female deposits her unfertilized ova in the water; the male follows 
and covers them with spermatozoa. Many may have watched from 
a bridge the female salmon ploughing along the gravelly river bed 
depositing her ova, careful to secure a suitable ground, yet not dis- 
turbing the already laid eggs of her neighbors. Meanwhile she is 
attended by her (frequently much smaller) mate, who deposits milt 
upon the ova. In the frog, again, the eggs are fertilized externally 
by the male just as they leave the body of his embraced mate. Or 
it may be that the sperms are lodged in special packets, which are 
taken up by the female in most of the newts, surrounded with one of 
the male arms in many cuttlefishes, or passed from one of the spider's 
palps to the female aperture. In the majority of animals — for example, 
insects and higher vertebrates — copulation occurs, and the sperms 
pass from the male directly to the female. Even then the history is 
very varied. They may pass into special receptacles, as in insects, to 
be used as occasion demands; or, in higher animals, they may with 
persistent locomotor energy work their way up the female ducts. 
There they may soon meet and fertalize ova which have been liberated 
from the ovary; or may persist, as we noticed, for a prolonged period; 
or may eventually perish. 

When the sperms have come, in any of these varied ways, into close 
proximity to the ovum, there is every reason to believe that a strong 
osmotic attraction is set up between the two kinds of elements. We 
have often suspected that the approach of the conjugating cells of two 
Spirogyra filaments (fig. c, d) might be directed along the line of an 
osmotic current ; and although we must confess that perhaps some- 
what rough evaporations, performed a few summers ago, gave no 
positive confirmation to the idea that glucose or the hke might be 
present in appreciable quantity in the water, a recent observer, we are 
glad to see, claims to have been more fortunate. The spermatozoa, 



SEXUAL REPRODUCTIOIi. 



135 



which seem so well to deserve Rolph's epithet of "starved," appear 
to be powerfully drawn to the well-nourished ovum, and the latter 
frequently rises to meet the sperm in a small ' ' attractive cone. ' ' Often, 
however, there is an obstacle in the way of entrance in the form of the 
egg-shell, which may be penetrable only at one spot, well called the 
micropyle. Dewitz has made the interesting observation that round 
the egg-shells of cockroach ova the sperms move in regular circles of 
ever varying orbit ; and points out that thus, sooner or later, a sperm 
must hit upon the entrance. He showed that this was a characteristic 
motion of these elements on smooth spheres, for round empty egg- 
shells or on similar vesicles they moved in an equally orderly and 
systematic fashion. It was till recently believed that more than one 
sperm might at least enter the ovum, but researches such as those of 




Fig. 44. — Different Forms of Conjugation in Plants, ^, zoospores; 
/', mold : c, d, conjugate alga; : (?,y^ desmid. 



Hertwig and Fol have shown that when one sperm has found admit- 
tance, the way is usually barred against others. The micropyle may 
be blocked, or the surrounding membrane may be altered, or in other 
ways the ovum may exhibit what Whitman calls "self-regulating 
receptivity," so as to be no longer penetrable. We are safe in 
concluding that the ovum is usually receptive only to one sperm ; that 
in most cases the entrance of more than one sperm is impossible ; and 
that where ' ' polyspermy ' ' does occur, pathological development is at 
least often the result. In the lamprey's egg quite a number of sperms 
find their way into a watchglass-shaped space at the upper pole of the 
ovum, but only one gets further, the rest remain imprisoned without 
further history of any importance. 

What takes place before fertilization is, as we have just seen, very varied 
indeed among animals ; what takes place after fertilization is of course cell- 
division, but that, though referable to certain great types, must necessarily vary 



THE EVOLVTIOiV OF SEX. 



with each species ; w liat tal<es place in the act of fertilization, howevei', is 
always essentially the same. The head of the spermatozoon becomes the male 
nucleus (or pro-nucleus) of the fertilized ovum, enteriu!,^ into close association 
with the female nucleus. The latter, as we lia\'e already noted, has had its own 
history ; it is no longer the original germinal vesicle, nor usually like it in 
appearance, it is the germinal vesicle minus the quantity of nuclear substance 
given off in forming two polar globules. This female nucleus (or pro-nucleus, as 
it is generally called) comes into close association with the sperm or male 
nucleus ; nor does it remain quite passive in the process, though the greater 
activity in bringing about the close association is certainly still exhibited by the 
male. Whitman has recently emphasized the reality of an attractive influence 
between the pro-nuclei. Fusion of the pro-nuclei was observed so long ago as 
iSsoby Warneck in the pond-snail (Lymneriis). This result, however, appears to 
have been overlooked, till the same fact was reobserved in threadworm ova by 
Biitschli in 1874. Since that date the fact has been continuously studied. Some 
observers still doubt whether what can be accurately called fusion of nuclei ever 
occurs ; and if fusion means inextricable confounding and mixing up of the male 
and female nuclear elements, it is almost certain that such does not in any case 
happen. There is no doubt, however, that the two nuclei become very closely 
associated, and according to most observers a double unity is formed, in which 
the component nuclear elements from the two origins so diverse are united in 
perfectly orderly fashion. So exact, in fact, is this duality, that when the first 
division of the egg takes place, each of the two daughter-cells has in its nucleus 
half of the male and half of the female elements, and so on perhaps in after- 
stages. 

The object upon which the intimate phenomena of fertilization have been 
most studied is the ovum of the threadworm (Ascaris niegalocephala) which 
infests the horse. Since 18S3 about a dozen important memoirs have dealt with 
this subject, and with the same material. The results of competent obserA'ers 
have varied enormously in detail, but on the essential points there has been 
(with some few exceptions) an increasing congruence of opinion. The most 
important work on the subject has been that of Prof van Beneden, whom most 
of those who have followed him unite in regarding as a master. The 
discrepancies and contradictions have been accompanied at times by not a little 
warmth of asseveration, but with ever-increasing perfection of method many of 
these are disappearing. To one alone shall we here allude. According to Van 
Beneden, the normal ovum of this threadworm contained in its nucleus one 
chromatin element, and was fertilized by a sperm also with one chromatin 
element. Carnoy, however, described the normal ovum as containing two 
chromatin elements, and as fertilized by a sperm also with two. In view of the 
perfection with which both these investigaters had unravelled the structure and 
behavior of the nuclei, the discrepancy seemed serious enough. Now, however, 
Boveri has shown that both are right ; Van Beneden's type occurs ; Camoy's 
type also occurs. Nay more, an ovem with one chromatin element seems to be 
always fertilized by a sperm with only one, while an ovum with two chromatin 
elements is fertilized by a sperm likewise with two. 

A few of the details may be summarized from the recent masterly researches 
of Boveri. The extrusion of the two polar cells from the ovum is in reality a 
double process of cell-division. The quantity of the nuclear substance in the 
germinal vesicle is thereby reduced by three fourths, but the number of nuclear 
elements remains the same. Only one sperm penetrates the ovum, unless the 
latter be unhealthy ; and with the entrance of the sperm the ovum undergoes a 



SEXUAL REPRODUCTION'. 



simultaneous change, which excludes other male elements. Onh' the head or 
nuclear portion of the spemi is of real importance in the essential act of 
lertilization ; the nutritive tail or cap simply dissolves awa}-. After the sperm- 
nucleus has penetrated to the center of the ovum, and after the extrusion of the 
polar bodies is quite completed, we have to deal with two nuclei, not only 
closely approximate in structure, but alike in further histor\'. 

In Camoy's tj-pe, both male and female nuclei contain two chromatin 
elements, in the form of bent rods ; and before union takes place, these 
undergo a marked modification, the same in both cases. Round the chromatin 
rods vacuoles are formed, limiting them from the surrounding protoplasm ; into 
these the rods send out anastomozing processes, after the fashion of little 
rhizopods ; graduall}- the rods thus resolve themselves into a network, in the 
meshes of which minute "nucleoli" are also demonstrable. 




Fig. 45. — The Process of Fertilization. — After Boveri. — a, female pronucleus; /;, polar bodies: c. male 
nucleus ; d, sperm-cap : ac, chromatin elements of uniting united female and male nucleus (a and c); 
e, protoplasmic centers; /, archoplasmic threads. 



The two nuclei thus modified then unite, but that again so precisely, as Van 
Beneden especially has shown, that each forms half of that spindle figure which 
almost all nuclei take when about to divide. This double spindle figure is the 
" segmentation nucleus," which will presently divide into the two first daughter- 
nuclei of the ovum (see figs. VI-X). 

It is not possible here to discuss certain intricate changes which take place 
meanwhile, not in the nuclei, but in the cell-substance of the ovum. Both Van 
Beneden and Boveri have recently agreed on the existence of two "central 



I3S 



THE EVOLUTION OF SEX. 



corpuscles" (centrosomata) in the protoplasm, which serve as "points of 
insertion" for protoplasmic threads, which exert a "muscular action" upon 
the nuclear elements in the forthcoming division. Boveri has traced with great 
care the history of a special kind of protoplasm (what he calls the archoplasm), 
which has its center in either " central corpuscle " (f), and sends out contractile 



vr 




.46. 



fibrils (/), which moor themselves to the nuclear elements. The movements of 
the latter during the forthcoming first division of the ovum are directly referable 
to the antagonistic action of these fibrils, and thus we have hints of an 
intracellular muscularity, the thought of which makes one dumb. 



SEXUAL REPRODUCTION. 135 

In the spindle the nuclear elements, still distinguishable in their orderly 
beliavior as male and female, eventually form what is known as the "equatorial 
plate ' ' (,\'I), lying across the center of the spindle. This is a well-marked stage, 
and one characterized by apparent equilibrium. "It is the resting-stage par 
excellence in the hie of the cell. Movement is at an end, a state of stability has 
set in, and this would continue ad infinitum, did not a factor, which hitherto has 
played no part, assert itself and bring about fresh movement. This new 
movement is the longitudinal division of the chromatin elements, an independent 
expression of life — indeed, a reproductive act — on the part of the nuclear 
elements." 

The abo\-e short sketch will show how intricate, and yet at the 
same time how orderly, are the intimate processes of fertilization. 
Variations do indeed occur, both in pathological and in apparently 
normal cases ; but a general constancy is now both clear and certain, 
not only for many different animals, but also to a certain extent, as 
Strasburger has shown, for plants. 

One marvelous fact, showing the closeness of union in fertilization, 
may be briefly re-emphasized. In the double nucleus formed from the 
union of male and female nuclei. Van Beneden, Carnoy, and others, 
have shown that both constituents have an equal share. ' ' The one 
half is purely male, the other purely female, and this is true not only 
for Ascaris (by Van Beneden) and other threadworms (by Carnoy), 
but for representatives of other worm-types, ccelenterates, echinoderms, 
moUusks, and tunicates." In division to form daughter-cells (IX X), 
half of each set of constituents goes to either cell, and the dualism is 
kept up. Furthermore, though hardly yet quite certain, it is most 
probable, that "of the four chromatin loops observed in the division- 
figure of a daughter-cell, two are derived from the male parent and two 
from the female." The importance of this fact, in relation tc the 
influence of both parents upon the offspring, is very obvious. 

V. Fertilization in Protozoa. — In the nascent sexual union observed in 
many Protozoa, — not, however, as yet m foraminifers or radiolarians, — consider- 
able diversity obtains. The individuals which unite may be to all appearance 
similar (to which cases the term conjugation is generally applied), or they may 
be materially dimorphic, as in Vorticella. The union may be permanent, when 
the two units fuse into one ; or it may only be temporary', during which an 
interchange of elements takes place. In both cases the nuclear elements play 
an important part, disrupting and reconstructing during the process, while a 
genuine fusion of the two nuclei has also been observed in permanent 
conjugation. 

In regard to the interchange of elements, there is considerable divergence of 
observation. Joseph has noted what appears to be an interchange of 
protoplasm; Schneider has observed the exchange of nuclear elements ; while 
Gruber and Maupas, and Joseph as well, have, in their studies on the union of 
ciliated infusorians, laid emphasis on an accessory nuclear body, generally 
known as the "paranucleus." This body lies by the side of the larger nucleus, 



I^O '^HE EVOLUTION OE SEX. 



and u liile the latter simply disrupts and dissolves away, or is extruded without 
playing any important part, the smaller paranucleus divides in a regular way, 
and with the results there is interchange between the two individuals. 

According to JMaupas, who has investigated the subject in most detail, the 
para- or micro-nucleus is a " hermaphrodite " sexual element, of sole 
importance in conjugation. The stages in the process of fertilization are as 
follows ; — 

1. The para -nucleus increases in size. 

2. 3. It then divides twice, and eliminates certain corpuscles. 

4. 'I'his eflected, it divides again, difierentiating a male and female pro- 

nucleus. 

5. In the ne.xt stage, the male elements of the two individuals are 

exchanged, and the new male nucleus fuses with the original female 
portion. 

6. 7. In two following stages, the nuclear dualism characteristic of the 

ciliated infusorians is re-established. The old large nucleus (macro- 
nucleus ) has broken up and been eliminated meanwhile. 

8. Finally, the individuals, separating from one another, reassume all their 
original organization before beginning again to divide in the usual 
fashion. 

The union of the male and female nuclear elements in ciliate infusorians was 
admirably figured by Balbiani so long ago as 185S; and though he does not 
seem rightly to have interpreted what he observed in this particular case, he 
was right in his contention that sexual union and fertilization really occurred in 
the Protozoa. Balbiani's view has been for long scouted, and yet, with 
renewed observation, naturalists have now come back to his conclusion. 
Maupas willingly allows that Balbiani figured beautifully what he himself has 
since reobserved and interpreted. 

The phenomena described by Maupas, as summacized above, have been 
observed in to\»fard a dozen ciliated infusorians, so that there is every reason to 
believe in their general occurrence. In three species of the slipper animalcule 
(Pai-aincrcium), and in species of Sfylonichia, Lciicop/irys, Eiiplotes, 
Onychodromus, Spirostoiniiiii, Szc. the facts are as above stated. 

It is of interest to cite the facts in regard to the common bell-animalcule 
( Vorticella), because here the conjugating individuals are like ovum and sperm 
in more ways than one. In some species — for e.xample, V. monilata — the 
adult divides equally, to form two small individuals, which conjugate with those 
of normal size. In V. micros/oiiia there is again division into two, but the 
products are of unequal size ; one is more male, than the other. In the nearly 
allied Carcheshim polypinmn, the divisions are equal, but they are repeated 
twice or thrice. The result in all cases is the production of minute individuals, 
which eventually attach themselves to adults of the normal size, first to the 
stalk, and then to the body (fig. p. 121"). The accessory nuclear bodies divide 
as usual ; the large individual ceases to feed, and hermetically closes its mouth, 
like an ovum when fertilized. The small individual is gradually absorbed by 
the larger, as sperm by ovum; and in an intricate but orderly fashion a mi.xed 
nucleus results from the ftision of the para-nuclear elements of the two. The 
adult then begins to feed, to di\'ide, and so on, as usual. Here then there is (a) 
incipient dimorphism, (h') absorption of smaller by larger, and [c) intimate 
nuclear union, — facts which we have already emphasized in the fertilization of 
multicellular animals. 



5£AXVIi REPRODUCTION. 



141 



VI. Origin of Fertilization. — To understand the origin of the 
union of sex-cells, attention must still be concentrated on the Protozoa. 
That fertihzation really occurs at that low level in a highly complex 
fashion, we have just seen. It is necessary, however, to note the 
steps which lead up to what Maupas and others have so patiently 
elucidated. 

{a). In the primitive life-cycle exhibited by Protomyxa (see fig. at 
p. 114), the units which burst forth from the cyst sink down into tiny 
amoebae, and unite together in numbers to form a composite spreading 
mass of protoplasm, technically known as a Plasmodium. This is un- 
doubtedly a very primitive union of 
cells, yet it occurs at very diverse levels 
in the organic series. It is more or less 
familiar in the ' ' flowers of tan ' ' one of 
the lowly Myxomycetes, where a nucle- 
ated mass of protoplasm, of composite 
origin, spreads over the bark in the 
tanyard. The plasmodial union also 
occurs as a definite stage in the life- 
history of the primitive neighbors of 
Protomyxa, the Monera of Haeckel. 
Pour the liquid contents or body-cavity 
fluid of a freshly dredged and still ac- 
tively living seaurchin into a bowl; the 
cells which float in it, like blood-cor- 
puscles in the blood, draw together in 
clotted masses. Watch the process un- 
der a microscope, and the formation of 
a Plasmodium is seen. The dying cells 
fiise into composite masses, just like the 
units oi Protomyxa; and it is interest- 
ing to observe that, though they are dying, the union provokes a brief 
but intense renewal of amceboid activity. To forestall our point, they 
as it were fertilize each other articulo mortis. In spite of the objection 
of Michel and others, that such union, being pathological, is not com- 
parable to the multiple conjugation normal to the myxomycete, we 
maintain the distinct analogy between the plasmodium formation in 
Myxomycetes and that exhibited by the cells in the body-cavity fluid 
of many animals, and regard this as so much additional evidence of the 
profound unity of the normal and the pathological processes. Now 
it is from this primitive union of cells, as illustrated in the lowest organ- 
isms, that we start in explaining the origin of fertilization. Just as the 
very' beginning of reproduction may be detected in the almost me- 







Fig. 47. — Diagrammatic representation of the 
stages in the origin of fertilization, — I., 
Plasmodium ; II., multiple conjugation ; 
III., ordinary conjugation ; IV., conju- 
gation of dimorphic cells; V., fertiliza- 
tion of ovum by spermatozoon. 



l^^j THE EVOLUTION OF SEX. 



chanical breakage of a form like Sckizogencs, so the very beginning- ot 
fertilization is found in the almost mechanical flowing together ot 
exhausted cells. 

(b) Between this and the process usually described as conjugation, 
there are some interesting links. Sometimes as many as three or 
four spores of lowly Alga club together, as if to gather sufficient 
momentum to make a combined start in life. The young lorms of 
the sun-animalcule {Actinosphariimi) usually unites in twos, but 
Gabriel has observed in some cases a multiple union. So in gre- 
garines (common parasites in invertebrates), while the usual union is 
certainly dual, Gruber has again observed what may be termed multi- 
ple conjugation. Union of three has also been observed as an 
exception in several infusorians. The union of more than two may 
thus be interpreted as intermediate between the formation of plasmodia 
ana the normal dual conjugation. 

{c) Conjugation of two similar unicellular organisms occurs, as we 
have seen, very generally in the Protozoa, and is also a common fact 

in the life-history of simple Alga. It 
is open to every one possessed of a 
microscope to observe what conjuga- 
tion means in such a common fresh- 
water alga as Spirogyra. Opposite 
cells of adjacent filaments are attracted 
to one another by what a recent ob- 
server calls a ' ' purely physical pro- 
cess," and the contents of the one 
cell pass bodily over into the other. 
In the great majority of cases where 



Fig. 48.— Diagrammatic representation of the Conjugation OCCUrS, the Uniting Cells 

'^:^^Z.:2X:i^^- ^re to aH appearance similar, but it 

must be remembered that it does not 
follow from this that they are physiologically alike (see fig. p. 135). 

{d) Both among plants and animals, all naturalists are agreed that 
it is impossible to draw any line between the conjugation of similar 
and the union of more or less dimorphic elements. " This differen- 
tiation presents," Sachs says, "especially \\\ Algce, a most complete 
series of gradations between the conjugation of similar cells and the 
fertilization of oospheres by antherozoids, any boundary line between 
these two processes being unnatural and artificial." The gradual 
appearance of dimorphism has been already noted in discussing the 
origin of sex, and need not be re-emphasized. 

(c) Lastly, in fertilization among higher plants and animals, 
the two elements which unite are highly differentiated, alike in con- 



SEXUAL REPRODfCTIUN. 



H3 



trast to one another and in opposition to the general cells of the 
body. A consideration of the phenomena in loose protist colonies 
like \ 'olvox or Ampullina, which suggest the bridge between uni- 
cellular and multicellular organisms, shows how gradually this latter 
contrast also may have been brought about. 

To sum up, the steps in the development of the process of fertili- 
zation may be arranged in the foUovi'ing series: — 
(a) The formation of plasmodia. 
(^) Multiple conjugation. 
{/) Conjugation of two similar cells, 
(fi?) Union of incipiently dimorphic cells. 
(«') Fertilization by differentiated sex-elements. 
One difficulty must in fairness be allowed in connection with the 
hypothesis of deriving conjugation from plasmodial union. Some 
years ago, Sachs was inclined to regard the plasmodium formation of 
Myxomycctes as a process of multiple conjugation, but has since with- 
drawn this view mainly on the ground that the nuclei have not been 
shown to coalesce. Now there seems no result of studies on fertiliza- 
tion more certain than that the union of nuclei is an essential fact, 
but in plasmodium-formation, such intimate association of nuclei can 
not be asserted. The difficulty of making this a starting-point is thus 
at first sight considerable. 

Yet it must be observed (i) that our knowledge of the nuclei in 
those lowly forms is still very inadequate; (2) that, according to Gruber, 
the behavior of the nucleus is sometimes masked by the fact, that 
instead of existing as a discrete body in the cell, it lies diffusely in the 
protoplasm: but especially (3) that it is quite consistent with the general 
evolutionary conception to suppose that the primitive union was of 
very much less definite character than that subsequently evolved. A 
reinvestigation of the whole question of plasmodium formation, from 
this point of view, is however very desirable, especially since the 
recent progress of microscopic technique has rendered the study of 
the nucleus in the lowest forms much more practicable than it was a 
few years ago. 

VII Hybridization in Animals.— Many of tlie compound names of 
animals such as leopard, point back to a once prevalent belief that animals 
of veiT 'different kinds might unite sexually and have fertile offspring;. Only 
to a very limited e.Ktent is such a notion justified. E\en' one is a\\are that by 
direct human control animals like horse and ass, dog and wolf lion and tiger, 
hare and rabbit, canary and finch, pheasant and hen, goose and swan, have 
been successfully crossed. In nature, however, «'e kno\v very little of the 
occurrence of any such hybridization. It seems to occur m some fishes ; 
different species of toad are often seen in sexual union, but the result 13 
unknown • in higher animals it seems confined to varieties of a species. The 



144 



THE EVOLCTION OF SEX. 



demarcation of a species is tlie vague line uhicli marks tire physiological range 
of natural and successful intercrossing. Domesticated breeds are usually 
fertilizable mutually, and their progeny is fertile ; we regard them as mere 
varieties. Nearly related species, however, only rarely admit of being crossed 
even when under man's control ; and species-hybrids tend to be themselves 
sterile. In structural characters two varieties of dog may seem more widely 
separate than two nearly allied species, yet the varieties of dog may be inter- 
crossed, while this verj' rarely occurs with the two species. The difference in 
the reproductive elements must often be greater than the structural differences 
of the adults would suggest. Hertwig has experimented of late with the 
artificial hybridization of echinoderms, and Born with that of amphibians. Both 
emphasize the difficulties of the process, and the vaiying degrees of success 
that may result, hr three cases Born brought about reciprocal hybridization 
but this is by no means always the case. Sometimes real fertilization took 
place, but nothing followed ; in other cases the ova segmented ; in a few the 
larval stages were reached ; and in two cases metamorphosis was survived. 
The hybridization is the more readily effected the nearer the elements are to 
perfect maturity. Sometimes the success seemed greatly to depend on the 
concentration of the sperm-fluid, — the more dilute this was, the fewer sperms 
were there to overcome the difficulties of eiTecting entrance to the ovum. 

There is no doubt that at least many species-hybrids tend to sterility but 
this is exhibited in varying degrees. The male mules are always sterile but 
the females may be successfully impregnated by horse or ass. In manv cases 
the hybrids are not fertile with one another, but remain so with the parent form 
In a few cases the reproductive functions seem for a time at least to be 
exag.gerated rather than diminished as the result of crossing. It seems also 
certain that while variety-hybrids are usually fertile, their constitution is more or 
less unstable. They are often very variable, and apt to die out, as has been 
repeatedly observed in the human species. The illnatured sayino-, "God 
made the white man, God made the black man, the devil made the mulatto " 
refers to the frequently inconvenient variability of those variety-hybrids. 

Brooks has laid considerable emphasis on the variability of hybrids in 
connection with his theory of heredity. "Hybrids and mongrels," he says 
"are highly variable, as we should expect from the fact that many of the cells 
of their bodies must be placed under unnatural conditions, and must therefore 
have a tendency to throw off" gemmules." "Hybrids, from forms which have 
been long cultivated or domesticated, are more variable than those from wild 
species or varieties, and the children of hybrids are more variable than the 
hybrids themselves." "But domesticated animals and plants live under 
unnatural conditions, and they are therefore more proHfic of gemmules than 
wild species ; and as the body of a male hybrid is a new thing, the cells will be 
much more likely than those of the pure parent to throw off" gemmules. The 
fact that variation is due to the male influence, and that the action upon the 
male parent of unnatural or changed conditions results in the variability of the 
child, is well shown by crossing the hybrid with the pure species ; for when the 
male hybrid is crossed with a pure female, the children are much m(jre 
variable than those born from a hybrid mother by a pure father." 

When we regard the male as katabolic, his variability becomes intelligible ; 
while in hybridization, which means the sexual union of organisms with a ''more 
than usually divergent life-experience, the reproductive elements which are 
intermingled have probably a corresponding divergence in chemical 
constitution. 



SEXUAL REPRODUCTIOX 145 

Tlie early researches of Kolreuter (1761) gave a firm basis to the study of 
Iiybridization among plants. The comparative easiness of experiment has 
advanced the botanical side of the subject to far greater certainty than the 
zoological conclusions can pretend to. Among plants, as we should expect 
from their greater vegetativeness, the fertility of hybrids seems frequently 
established. Knight, Gartner, Herbert, Wichura, and others, have brought 
together a great number of reliable observations, and the whole subject has 
been admirably discussed by Nageli. For a copious resume of the general 
results, for the most part after Nageli, the student may be referred to chap. vi. 
of Sachs' Textbook of Botany, while Wallace's "Darwinism" [No. 115 and No. 
116 of The Humboldt Library] should be consulted for its rediscussion of 
hybridization in animals. ' 



146 THE EVOLUTION OF SEX. 



SUMMARY. 

I. Reproduction is but more or less discontinuous growtli. 

II. Sexual reproduction normally implies {a) special reproductive cells, 
distinct from the bcdy ; (4) the dimorphism of these cells ; {c) their physio- 
logical dependence, — the ovum being unproductive without the spermatozoon, 
and vice versa. 

III. The discoveries of Camerarius, Amici, Kolreuter, Sprengel, and others, 
laid the foundations of our knowledge of sexual reproduction in plants. 

IV. The history of research on fertilization in animals well illustrates the 
gradually increasing precision of scientific inquiry. 

V. The conjugation processes seen in Proto"oa are of much importance in 
suggesting the origin of differentiated fertilization. 

VI. The origin of fertilization maybe traced through the following grades: — 
(a) plasmodial union, (5) multiple conjugation, (c) ordinary conjugation, {d) 
union of dimorphic cells, ((?) fertilization of ovum by spermatozoon. 

VII. Both in plants and animals hybridization is often successful, but the 
oftspring frequently tend to be sterile. This, however, must not be exag- 
gerated. 



LITERATURE. 

See the already noted works of Balfour, Van Beneden, Carnoy, Geddes, 
Haddon, Hensen, Hertwig, McKendrick, Sachs, and Vines. 

For recent papers see Boveri, Th., Zellen Studien); Jenaische Zeitschrift fiir 
Naturwissenchaften, 1S87-S8 ; Zoological Record, from 1S86 ; and Journal of 
Royal Microscopical Society. 



THEORY OF FERTILIZATION. 147 



CHAPTER XII. 
THEORY OF FERTILIZATION. 

TN his forty- ninth Exercitation on the "efficient cause of the 
chicken," Harvey thus quaintly expresses what has always been, and 
still is, a baffling problem: " Although it be a known thing, subscribed 
by all, that the foetus assumes its original and birth from the male and 
female, and consequently that the egge is produced by the cock and 
henne, and the chicken out of the egge, yet neither the schools of 
physicians nor Aristotle's discerning brain have disclosed the manner 
how the cock and its seed doth mint and coine the chicken out of the 
egge. ' ' The old theories on the subject are more curious than profit- 
able, a fact not to be wondered at since it is really only within the last 
fifty years that the fundamental fact of the union of the sex-cells has 
been observed. 

I. Old Theories of Fertilization. — (a) From Pythagoras and 
Aristode on to the "Ovists," of whom we have already spoken 
(p. 81), numerous naturalists have held the opinion that the ovum was 
the all-important element, which only required to be awakened to 
development by contact with the male fluid or male elements. It 
must be allowed that while ova may exceptionally develop without 
sperms, the latter never come to anything apart from ova. This will 
be less insisted on, however, when it is recognized that in reality the 
ovum is not so fairly comparable with the spermatazoon as with the 
mother-sperm-cell. It must be allowed, too, that there is much to 
warrant us in thinking of the sperm as an element which stimulates 
the ovum to division; yet this will be recognized as only approximate 
language, when the facts of the intimate nuclear union are fully 
appreciated. 

(J)) In contrast to the above opinion, we find ingenious thinkers, so 
widely separate in time as Democritus and Paracelsus, regarding the 
male fluid as very important— forestalling Buffbn and Darwin— in fact, 
in considering it in a sense an extract or concentrated essence of the 
whole body. But it was only after the spermatozoa were themselves 
detected that their importance became unduly exaggerated, m^ the 
minds of those who seem almost to have been nicknamed " animal- 
culists." It seems probable enough that Leeuwenhoek himself (1677) 
saw the spermatozoon entering the ovum,— he at least said that he 
did,— but that did not prevent him from ascribing to the male ele- 
ments all the credit of development. This became, as we have seen. 



148 THE EVOLUTION OF SEX. 

a favorite hypothesis, and imagination supplied more than modern 
magnifiers to those observers who detected in the spermatozoon the 
members and lineaments of the future organism. After this the dis- 
covery that the sperm supplies half the nucleus of the fertilized ovum, 
and half the nuclei of the two first daughter-cells, seems almost a little 
thing. The polemic of modern science has this advantage at least, 
that when two competent authorities on the same subject assert the 
same thing, we may generally believe them. 

(r) The third opinion, that both elements are of essential and 
inseparable import, is obviously alone consistent with the facts. This 
view also has had its gradual development, only one phase of which 
need be noticed. Even after the nature of the spermatozoa as male- 
cells was recognized, that is to say even within the last fifty years, an 
old conception of the male influence lingered persistently. This, 
namely, that contact was not essential, but that a " sort of conta- 
gion," a " breath or miasma," "a plastical vertue," " without touch- 
ing at all, unless through the sides of many mediums," was sufficient 
to effect what we call fertilization. The above expressions are used 
by Harvey, who further says, " this is agreed upon by universal con- 
sent, that all animals whatever, which arise from male and female, are 
generated by the coition of both sexes, and so begotten as it were per 
contagium aliqiwd. ' ' De Graaf attempted in vain to give more pre- 
cision to this "contagion" in his theory of an " mira seminalis," 
or seminal breath which passed from the male fluid to the ovum. But 
the conception of an "aura" was only a verbal cloak for that absence 
of definite knowledge which the slow progress of observation still 
necessitated. The theory was pardy strengthened by a number of 
erroneous observations, which seemed to show that successful fertiliza- 
tion could occur when the genital passages of the female were appar- 
ently blocked by malformadon or disease. Spallanzani gave a death- 
blow to the theory of an "aura," by showing experimentally that con- 
tact of the male fluid with the ovum was absolutely necessary. Even 
he, however, went away from the true conclusion, by maintaining that 
the fertile male fluid of toads was desdtute of spermatozoa. That 
the above vague conceptions have been replaced by the certain con- 
clusion that intimate cellular union is the sine qua non of fertiliza- 
tion, we have already emphasized. 

II. Modern Theories of Fertilization.— Morphological.— 
Recent investigators of the facts of fertilization have generalized their 
results in different ways according to their dominant bias. Some 
mainly restrict themselves to stating the morphological facts, and to 
emphasizing the relative importance of cell-substance and of nuclei in 



THEORY OF FERTILIZATION. 



149 



the union; others attack the deeper problem of the physiological 
import of the process, — a problem the fuU solution of which is still 
remote; while others have confined themselves rather to discussing the 
uses of fertilization in relation to the species. Some representative 
positions on each of these planes must be sketched; and, first of all, 
the more morphological theories, and the very important question 
whether the union of nuclei is everthing, or whether the union of cell- 
substance has also its import. 

(a) Hertwig's View. — Professor O. Hertwig, who was one of the first 
carefully to follow out the details of fertilization in animals, thus sums up his 
" Theorie der Befruchtiing" : — "In fertilization, distinctly demonstrable 
morphological processes occur. Of these the important and essential one is the 
union of two sexually differentiated cell-nuclei, the female nucleus of the ovum 
and the male nucleus of the sperm. These contain the fertilizing nuclear 
substance, which is an organized substance, and acts as such in the process. 
The female nuclear substance transmits the characters of the mother, the male 
nucleus those of the father, to the offspring." The nucleus is thus the essential 
element both in fertilization and in inheritance. 

(b) Strasbiirger' s View. — What Hertwig maintains for animals, Strasburger 
does for plants. "The process of fertilization depends upon the union of the 
sperm nucleus with the nucleus of the egg-cell ; the cell-substance (cytoplasm) 
does not share in the process." "The cell-substance of the pollen-grain is only 
the vehicle to conduct the generative-nucleus to its destination." It may 
become nutritive, he allows however, to the germ-rudiment. "Generally the 
uniting nuclei are almost perfectly alike," though there may be slight differences 
in the size of the nucleoU. "The two cell-nuclei do not differ in their nature, 
they are not sexually differentiated in the ways that the individuals are from 
which they originate. All sex-differentiations onlj' serve to bring together the 
two nuclei essential to the sexual process." 

The opinions of these two authorities are certainly representative, and they 
both agree in emphasizing that the nuclei are all-important, and that it does not 
matter much about the union of cell-substance. Some objections to this view 
must be noticed, (a) It is permissible to doubt whether the recent concentration 
of attention upon the nucleus has not led to some under-appreciation of the 
general protoplasm. In the permanent conjugation of two cells, the entire 
contents of the two cells are obviously fused; and even when the union is 
temporary, Joseph has observed what looks like an interchange of protoplasmic 
as well as of nuclear substance, {b) There are a few observers still, such as 
Nussbaum, who maintain that in fertilization in animals the substance of the 
sperm is important as well as its nucleus, (c) Strasburger notes the minimal 
quantity of cell-substance so often present round the male nucleus, and urges 
that if it were important there would surely be more of it. But it is quite 
conceivable that a minimal quantity of highly active protoplasm might have, like 
a ferment, a momentous influence on a large quantity of a different character. 
(rf)The researches of Boveri show that though the union of nuclei is so essential, 
the protoplasmic activity and share in the process are also considerable. It 
appears to us a fact well worthy of consideration, that according to this 
authority the sperm brings with it into the ovum a protoplasmic center — a 
"centrosoma" — which appears to be of much importance in the preparation 



I50 



THE EVOLUTION OF SEX. 



for division. In this preparation, according to Boveri, the "muscular fibrils " of 
a special kind of protoplasm (or archoplasma) literally move the nuclear 
elements. ' ' The movement of the elements is wholly the result of the contrac- 
tion of the attached fibrils, and the final arrangement of these nuclear elements 
in the ' equatorial plate ' is the result of the action of the archoplasmic sphere 
exerted through the fibrils." Now this specially active protoplasm, which the 
skillful observer seems to have succeeded in fixing, has its center. There are 
two central corpuscles, each " ruling a sphere of archoplasms." Where, then, 
do these centers come from? "It is probable," Boveri says, "that the 
spermatozoon brings a centrosoma into the ovum, and that this by division 
forms two centers. Since these two corpuscles condition the division, the 
dependence of this upon the presence of the spermatozoon is for the Ascaris 
ovum explained." We have given these details, technical as they are, because 
they seem to us to show clearly that it is rash to deny that even the minimal 
cell-substance of the spermatozoon may, as well as its nucleus, have a 
momentous influence in fertilization. 

III. Physiological Theories of Fertilization. — The morpho- 
logical facts, established and verifiable by observation, form the basis 
from which to attack the deeper problem of the physiology of fertili- 
zation. Here experiment is almost insuperably difficult; only a few 
incidental results are as yet available; the suggestions thrown out by 
various naturalists must therefore be appreciated according to their 
consistence with the general principles of physiology, and with the 
general theory of sex and reproduction. To some they may still appear 
a page of probabiUties. 

Sachs compares the action of the male element upon the egg-cell 
to that of a ferment. De Bary also suggests that profound chemical 
differences exist between the two elements. Very suggestive is the 
view of Rolph, who regarded the process as essentially one of mutual 
digestion. His words well deserve quotation: — 

" Conjugation occurs when nutrition is diminished, whether this be due to 
want of light, or to the lowered temperature of autumn and winter, or to a 
reduction of the organisms to minimal size. It is a necessity for satisfaction, a 
gnawing hunger, which drives the animal to engulf its neighbor, to 'isophagy.' 
The process of conjugation is only a special form of nutrition, which occurs on a 
reduction of the nutritive income, or an increase of the nutritive needs, in 
consequence of the above-mentioned conditions. It is an 'isophagy,' which 
occurs in place of ' heterophagy. ' The less nutritive, and therefore smaller, 
hungrier, and more mobile organism we call the male, — the more nutritive and 
usually relatively more quiescent organism, the female. Therefore, too, is it 
that the small starving male seeks out the large well-nourished female for 
purposes of conjugation, to which the latter, the larger and better nourished it 
is, is on its own motive less inclined." Cienkowski has also inclined to a 
similar view, regarding conjugation as equivalent to rapid assimilation. 

Simon also seeks to establish the following among other vague conclusions : 
Sexuality has, he says, arisen twice (we should say much oftener), once among 
plants, again among Protozoa. Two similar cells unite " in order to reach the 
limits of their individuality." _ In both kingdoms the union is at first protective. 



THEORY OF FERTILIZATION. 



151 



though in a different fashion in the two cases. In the progressive differentiation, 
these two sex-cells are usually so constructed that the loss of substance in the 
union is reduced to a minimum, hence the small mobile male and the large 
quiescent female cells. The union brings about a chemico-physical process, 
which makes the female cell capable of independent nutrition and growth, and 
evokes potential properties into actual life. 

In marked contrast to Rolph's suggestion, and the view of all 
those who believe that the sex-cells are profoundly different, is the 
opinion maintained by Weismann. He denies that there is a dynam- 
ical action in fertilization. The momentous effect is merely the 
sudden doubling of the mass of the nucleus. ' ' The physiological 
values of sperm and egg-cell are equal; they are as i-i. We can 
hardly ascribe to the body of the ovum a higher import than that of 
being the common nutritive basis for the two conjugating nuclei." 
The external differences which are so obvious are only important as 
means toward the conjugation of similar nuclei. "The germ-plasma 
in the male and female reproductive cells is identical. ' ' Previous to 
the essential moment of fertilization, half of the germ-plasma is given 
off from the germinal vesicle of the ovum in forming the second polar 
body. Development will not take place unless the loss be made 
good, and the original mass restored. This is what the sperm does 
in fertilization. In short, to Weismann the process is quantitative 
rather than qualitative. 

This supposition appears to us to be open to criticism, (i) That the nuclei 
are alone important in fertilization, and that the cell substance is a mere 
adjunct, can not be said to be proved, and we have already noted some of the 
facts which tell the other way. (2) The structure of a cell is recognized by all to 
be an expression of its dominant protoplasmic processes. The sex-cells are 
usually highly dimorphic, and even Strasburger allows that there may be minor 
differences in their nuclei, as well as the marked divergence in their cell- 
substance. The nucleus can not be regarded as an isolated element, but as one 
which shares in the general life of the cell. We have already interpreted the 
differentiated male and female cells as respectively katabolic and anabolic, and 
see no reason for doubting, in spite of structural resemblance in the rough 
features of nuclei (all that we know), that this difference saturates through the 
elements. (3) If the only important matter be the quantitative restoration of the 
original amount of germ-plasma in the female nucleus, it seems difficult to 
understand the phenomena of conjugation, whether permanent or transitory, 
from which we believe fertilization to have originated. (4) That the normal 
ovum should lose half its quantity of germ-plasma, only to regain a similar 
quantity in fertilization, certainly appears a curiously circuitous process. (5) 
The occasional possibility of inducing division by replacing the sperms with 
other stimuli, seems to point to a dynamical or chemical action, which Weismann 
denies. 

We are bound, of course, to admit the importance of the established facts of 
nuclear union, and agree with Boveri that the complexity of the morphological 
facts show the present impossibility of supposing that they can be fully expressed 



1^2 THE EVOLUTION OF SEX. 

in chemical terms. But a due impression of the marvelous "individuality " of 
the nuclear elements may be combined with a general physiological interpreta- 
tion of the entire process. 

It has been already noted, in regard to the origin of fertilization 
that the almost mechanical flowing together of exhausted cells is 
connected by the stages of multiple conjugation with the ordinary 
form of the latter, while the respective differentiation of the two 
elements efl'ects the transition to fertilization proper. Historically, 
then, fertilization is comparable to mutual digestion, and, though 
bound up with reproduction, has arisen from a nutritive want. With 
the differentiation of the elements on anabolic and katabolic lines, the 
nature of the fertilizing act becomes more definite. The essentially 
katabolic male cell, getting rid of all accessory nutritive material 
contained in the sperm-cap and the like, brings to the ovum a supply 
of characteristic waste products or katastates, which stimulate the 
latter to division. The profound chemical difl^erences surmised by 
some are intelligible as the outcome of the predominant anabolism and 
katabolism in the two elements. The union of the two sets of 
products restores the normal balance and rhythm of cellular life. 
Rolph's suggestion is thus included and defined. 

IV. Use of Fertilization to the Species. — Not a few natural- 
ists have passed from the individual aspect of fertilization to its general 
import in relation to the life of the species. Why should fertilization 
occur at all, if parthenogenesis in some cases works so well ? Part of 
this question is almost illegitimate, if the existence of male and female 
be, as we think, simply the expression of a more developed swing of 
"the organic seesaw" between anabolism and katabolism. The 
answers have, however, much interest, and are valuable, so long as 
they are not magnified so as to hide the deeper physiological problems 
lying below. The origin and physiological import of fertilization can 
never be explained by any elucidation of its subsequent advantageous- 
ness. 

The two naturalists who have recentiy reached the most valuable 
results in regard to the uses of fertilization are Maupas and Weismann. 
This they have done by very different paths, — Maupas, in working 
out the details of conjugation in infusorians; Weismann, in his wider 
studies on the problems of heredity and evolution. To Maupas, fertili- 
zation is necessary to prevent the death of the species; to Weismann, 
fertilization is the ever recurrent beginning of new vital changes, and 
the continual preservation at the same time of the relative constancy 
of the species. Several naturalists of the highest reputation have 
regarded fertilization as a process which supplied a fresh life-impulse 
to the species. Thus Gallon has insisted, with much clearness and 



THEORY OF FERTILIZATION. 153 

force, on the liability of asexiial, or what he calls unisexual multiplica- 
tion to end in degeneration or extinction, and on the necessity of 
double parentage for the preservation and progress of the species. 
Similarly, Van Beneden, Biitschh, and Hensen have all spoken of 
the process as a rejuvenescence (rejeunissement, Verjiingung). The 
asexual process of cell-muitiplication is Umited; conjugation in lower, 
fertilization in higher organisms supply the recurrent impulse which 
keeps the life of the species young. According to Van Beneden, 
' ' The faculty which cells possess of multiplying by division is limited. 
There comes a time when they can divide no further, unless they 
undergo rejuvenescence by fertilization. In animals and plants, the 
only cells capable of being rejuvenesced are the eggs ; the only cells 
capable of rejuvenescing these are the sperms. All the other parts of 
the individual are devoted to death. Fertilization is the condition of 
the continuity of life. Par elle le genSrateur 6chappe a la mort." 
Hensen, in his admirable "Physiology of Reproduction," expresses 
the same when he says : "By normal fertilization, death is warded off 
(ferngehalten) from the germ and its products." Biitschh has inter- 
preted conjugation in similar terms. 

Weismann quotes the three opinions just mentioned, and vigorously 
criticises them. He demands evidence for the limitation of asexual 
reproduction assumed above, and speaks of the ' ' impossibility of 
proof" The whole "conception of rejuvenescence has something 
indefinite and misty about it." (Some may be obliged to plead 
guilty to a similar impression in regard to Weismann' s keimplasma). 
"How can one think that an infusorian, which by continued division 
has at length exhausted its reproductive capacity, will regain the same 
by uniting and fusing with another which has also lost its power of 
further division ? Twice nothing can not give one ; or if one assumes 
that in each animal there persists only half the reproductive capacity, 
so that the two together would form one, this one can hardly call 
'rejuvenescence.' It would be simply an addition, as is under other 
circumstances attained by simple growth, — that is, if we leave out of 
account what in my eyes is the most important moment in conjugation, 
namely, the mingling of two heredity-tendencies ( Vererbung- 
stendenzen)." Does Professor Weismann not feel that there is some- 
thing "indefinite and misty" even about this? He sarcastically 
compares the two exhausted individuals to two exhausted rockets, 
which are supposed to rejuvenesce in mutually affording the 
constitutents of nitroglycerine. More forcibly he urges the difficulty 
suggested by continued parthenogenesis, — a difficulty which we shall 
afterwards have to discuss. "To the conception of rejuvenescence," 
he says, in conclusion, "I could only agree, if it were proved that 



154 ■^'^'^ EVOLUTION OF SEX. 

multiplication by division can never, — not merely in certain condi- 
tions, — but never continue unlimitedly. This can not, however, be 
proved, just as little as the reverse. ' ' But Weismann must surely admit 
that the demonstration of even some cases where species, normally 
reproducing asexually, come to an absolute standstill if conjugation 
be prevented, would give considerable strength to the interpretation 
of fertilization as rejuvenescence. Such cases have, happily, come to 
hand, as we shall now see. 

We have already referred to Maupas's proof of true sexual union in 
ciliated infusorians. By an elaborate process of nuclear division, 
disruption, elimination, interchange, union, and reconstruction, two 
' ' slipper animalcules ' ' fertilize one another. What is the meaning of 
all this? 

Each infusorian, after conjugation, proceeds to divide, but the 
results are to all appearance the same as it previously produced. 
There is no special sexually produced generation. 

It has been often alleged that the subsequent dividing is accelerated 
by conjugation ; but Maupas finds that this is not so. The reverse in 
fact is true, — it is a loss of time. While a pair of infusorians 
(^Onychodromus grandis) were indulging in a single conjugation, 
another had become, by ordinary asexual division, the ancestor of 
from forty thousand to fifty thousand individuals. 

Moreover, the intense internal change preparatory to fertilization, 
and the general inertia during subsequent reconstruction, not only 
involve loss of time, but expose the infusorians to great risk. It 
seems then like a condition of danger and death rather than of multi- 
pHcation and birth. 

The riddle was, in part at least, solved by a long series of care- 
ful observations. In November, 1885, M. Maupas isolated an infuso- 
rian {Stylonichia pustulatd), and observed its generations till March, 
1886. By that time there had been two hundred and fifteen genera- 
tions produced by ordinary division, and since these lowly organisms 
do not conjugate with near relatives, there had, of course, been no 
sexual union. 

What was the result? At the date referred to, the family was 
observed to have exhausted itself They were not old exactly, but 
they were being born old. The asexual division came to a stand- 
still, and the powers of nutrition were also lost. 

Meanwhile, however, several of the individuals, before the genera- 
tions had exhausted themselves, had been removed to another basin, 
where they conjugated with unrelated forms of the same species. One 
of these was again isolated, and watched for five months. The usual 
richness of successive generations occurred; members removed at dif- 



THEORY OF FERTILIZATION. 



155 



ferent stages were again observed to conjugate successfully with unre- 
lated forms, and this was done on to the one hundred and thirtieth 
generation. After that, however, the family being again near its end, 
the removal was no longer of any use. About the one hundred and 
eightieth generation, the strange sight was seen of individuals of 
the same family attempting to unite with one another. The results 
were, however, nil, and the conjugates did not even recover from 
the effects of their forlorn hope. 

Without the normal sexual union, then, the family becomes senile. 
Powers of nutrition, division, and conjugation with unrelated forms, 
come to a standstill. This senile degeneration is very interesting. 
The first symptom is decrease in size, which may go on till the 
individuals only measure a quarter of their normal proportions. 
Various internal structures then follow suit, ' ' until at last we get 
formless abortions, incapable of living and reproducing themselves." 
The nuclear changes are no less momentous. The important para- 
or micro-nucleus may partially or completely atrophy, and conjuga- 
tion is thus fatally sterile. The larger nucleus may also become 
affected, ' ' the chromatin gradually disappearing altogether. ' ' Physio- 
logically too, the organisms become manifestly weaker, though there 
is what the author calls a ' ' surexcitation sexuelle. ' ' Such senile 
decay of the individuals and of the isolated family inevitably ends 
in death. 

The general result is evident. Sexual union in those infusorians, 
dangerous perhaps for the individual life, — a loss of time so far as 
immediate multiplication is concerned, — is in a new sense necessary 
for the species. The life runs in cycles of asexual division, which are 
strictly Hmited. Conjugation with unrelated forms must occur, else 
the whole life ebbs. Without it, the Protozoa, which some have 
called "immortal," die a natural death. Conjugation is the neces- 
sary condition of their eternal youth and immortality. Even at this 
low level, only through the fire of love can the phoenix of the species 
renew its youth. 

At the beginning of this century, the too-much-forgotten biologist 
Treviranus directed attention to fertilization as a source of variation, 
and his suggestion has been several times independently revised. 

Thus Brooks, to whose works we have repeatedly referred, has 
emphasized not only the importance of fertilization as a source of 
progressive change, but further, that the male element is much the 
more important in this connection. 

Similarly, though on somewhat different lines, Weismann finds 
in the mingling of male and female keimplasmas the source of those 
variations on which natural selection operates. Rejecting as he 



156 THE EVOLUTION OF SEX. 

does the alleged inheritance of acquired characters, he finds the 
fountain of change in sexual reproduction. ' ' Sexual reproduction 
is well known to consist in the fusion of two contrasted reproductive 
cells, or perhaps even in the fusion of their nuclei alone. These 
reproductive cells contain the germinal material of keimplasma, and 
this again, in its specific molecular structure, is the bearer of the 
hereditary tendencies of the organisms from which the reproductive 
cells originate. Thus in sexual reproduction, two hereditary tend- 
encies are in a sense intermingled. In this mingling, I see the cause 
of the hereditary individual characteristics; and in the production of 
these characters the task of sexual reproduction. It has to supply 
the material for the individual differences from which selection 
produces new species." 

But this very reasonable contention hardly appears to consist 
with Weismann's quantitative interpretation of the process of fertili- 
zation. Nor is it evident how the diversities of the male and female 
plasmas became such as their results indicate them to be, if Weismann 
be correct in maintaining that no modifications of the body influence 
the reproductive elements. 

Brooks and Weismann have at any rate maintained a thesis which 
we will be inclined to oppose, that se.xual union is productive of 
variation. To discuss the relations of this view to other theories of 
variation is not here revelant, nor can we do more than mention the 
reasonable suggestion of Hatschek, that sexual reproduction is a 
remedy against the operation of injurious variations. For we can 
readily imagine that the excess of some particular line of anabolic or 
katabolic differentiation may be neutralized through fertilization. In 
this way one is led to speculate as to whether the constant pairing 
of diseased individuals may not sometimes be more mercifully con- 
doned by nature than we have been accustomed to think. 



THEORY OF FERTILIZATION. 



157 



SUMMARY. 

I. Old theories of "ovists," " animalculists, " and of the "aura seminalis." 

II. Modem morphological theories incline to lay the whole emphasis upon 
the nuclei. The conclusions of Hertwig and Strasburger are strongly in favor ot 
this view. The claims of the cell-substance and general protoplasm must not, 
however, be overlooked. Many facts, such as those demonstrated by Boveri, 
show that the protopasm is also important. 

III. Modern physiological theories of fertilization are necessarily very 
tentative. Sachs compares it to fermentation ; Rolph to mutual digestion. To 
Weismann, the process appears quantitative rather than qualitative, so far as 
the subsequent division is concerned. Half of the keimplasma which the ovum 
has surrendered with the second polar globule is restored by the sperm-nucleus. 
The two nuclei are alike, and thus there is virtually no sex. Protest against 
this. The male cell brings to the ovum a supply of characteristic katastates. 

IV. Uses of fertilization to the species. Many regard fertilization as a 
necessary rejuvenescence of the Hfe of the species. Weismann criticises this 
view, but his criticism must be read in the light of the researches of Maupas, 
who has shown that without conjugation the members of an isolated family of 
infiisorians eventually cease to feed and divide, passing through stages ot 
degeneration and senility to extinction. In this case, conjugation is essential to 
the continued vitality of the species. According to Brooks, fertilization is an 
important source of variation ; according to Weismann, it is really the sole 
source. 



LITERATURE. 

Works already cited. 

Hertwig, O.— Das Problem der Befruchtung, &c.; Jenaische Zeitschrift fiir 

Naturwissenschaften, XVIII., 1885. 
Maupas, E.— Comptes Rendus, 1886, 1887; and Archives de Zoologie 

experimentale, 18S8. 
Strasburger, E.— Neue Untersuchungen iiber den Befruchtungsvorgang 

bei den Phanerogamen, als Grundlage fiir eine Theorie der Zeugung. 

Jena, 1884. 
Weismann, K.— 0pp. cit., especially Die Bedeutung der sexuellen Fortpflan- 

zung fiir die Selektions-Theone. Jena, 1886. 



I5« 



THE E VOL UTIGN OF SEX. 



CHAPTER XIII. 
DEGENERATE SEXUAL REPRODUCTION OR PARTHENOGENESIS. 

I. History of Discovery. — From very early times there 
appears to have been an impression, that in exceptional circumstances 
reproduction might occur without fertilization. Even Aristotle gave 
reasons for believing that, without sexual union, the unfertilized eggs 
of the honey-bee might give rise to perfect adults. We now know 
that he was right, in his conclusion at least, so far as the develop- 
ment of drones is concerned. In the early belief in Lucina sine 
concubitu, much that was erroneous was intermixed with a prevision 
of the truth; nor could we expect at an early date that asexual 
multiplication (that is, apart from ova altogether) would be kept 
distinct from what we now mean by parthenogenesis, or the develop- 
ment of ova without union with sperms. In 1701, Albrecht observed 
that a female silkmoth, which had been isolated in a glass case, laid 
fertile eggs; and though this was for long discredited, the occasional 
parthenogenesis of this insect has been repeatedly confirmed by 
competent observers. 

In 1745 the ingenious Bonnet drew attention to what is now a 
very familiar fact — the successive generations of virgin plant-lice or 
aphides. Throughout the summer, he observed the production of 
numerous generations of these little insects, all females, necessarily 
therefore all virgins, and yet fertile. So strange did the fact appear, 
that it was for long utterly discredited. Reaumur eluded the diffi- 
culty, by affirming that the aphides were hermaphrodite; but Dufour 
soon proved that this was erroneous, though he could only confess 
his ignorance in referring the phenomena to ' ' spontaneous or equi- 
vocal generation, ' ' in which ' ' the act of impregnation was in no 
degree concerned." The facts, however, were repeatedly re-observed. 
Kirby and Spence admitted them as incontestable, but could regard 
them only as ' ' one of the mysteries of the Creator, that human 
intellect can not fully penetrate." 

Meanwhile Schaffer had observed the occurrence of partheno- 
genesis in minute aquatic crustaceans, the study of which has since 
shed some vivid light on the whole subject. Pastor Dzierzon had 
also clipped the wings of the queen-bees, and in thus preventing their 
nuptial flight and impregnation, observed that the eggs they laid 
developed only into drones. The facts soon began to be recognized, 



DEGENERATE SEXUAL REPRODUCTION. 159. 

extended, and thought over by naturahsts of the standing of Owen 
(1843), Von Siebold (1856), and Leuckart (1858), whose conclusions 
have afforded a firm basis for the abundant subsequent observation 
and speculation of this interesting subject. 

II. Degrees of Parthenogenesis. — If we start, then, with 
Von Siebold' s definition of parthenogenesis, as the power possessed 
by certain female animals of producing offspring without sexual 
union with a male, it will clear the ground to notice, in the first 
place, the numerous different degrees in which this development 
without fertilization may occur. 

{a) Artificial Parthenogenesis. — There are a few curious obser- 
vations which go to show that in exceptional circumstances ova may 
develop when the male stimulus is replaced by some artificial reagent. 
These observations must still be taken cuvi grano salts, but they may 
be at least suggestive of further experiment. Dewitz observed unfer- 
tilized frog-ova to undergo segmentation \sic\ in corrosive sublimate 
solution. In some cases one division occurred, in others several; in 
some cases irregularly, in others normally. It happened both when 
the ova were left in the reagent, and when they were merely dipped 
and returned to water. The eggs experimented on were those of the 
two common frogs Ra7ia fusca and R. esculenta, and of the tree-frog 
{By la arborea'). But it must be noted that Leuckart long ago 
noted the occurrence of spontaneous division in frog ova. Similarly, 
Tichomiroff, experimenting with the unfertilized ova of the silkmoth, 
which are occasionally parthenogenetic, was surprised to observe that 
ova, which would not of themselves develop parthenogenetically, 
might be induced to do so by certain stimuli. These consisted in 
rubbing the unfertilized ova with a brush, or in dipping them for 
two minutes in sulphuric acid and then washing them. In both cases, 
he says, a percentage of the ova thus artificially stimulated developed. 
It must be remembered that occasional parthenogenesis occurs in this 
insect, and all that Tichomiroff" did was to incite this. There is 
no doubt that reagents may considerably •inodify ova; thus the 
brothers -Hertwig showed how it was in this way possible to over- 
come the non-receptivity of the ovum to more than one sperm. Nor 
can one forget how sexual reproduction in parasitic fungi tends to 
disappear, being apparently replaced by the stimulus afforded Irom the 
waste products of the host. In a similar way, the multiplication of 
cells, so frequently associated in disease with the presence of bacteria, 
has been referred by more than one pathologist to the "sper- 
matic influence" of these micro-organisms, or of the katastates which 
they form. 



i6o THE EVOLUTION OF SEX. 

(b) Pathological Parthenogenesis. — It has very occasionally been 
noticed in higher animals, where true parthenogenesis is wholly 
unknown, that an unfertilized (t^g starts off on its own resources 
without any male stimulus whatever. This is noted by Leuckart for 
frog-ova, by Oellacher for hens' eggs, and by Bischoff and Hensen 
even in mammals. Such cases must be regarded as rare abnormalities, 
comparable perhaps to parthological formations which not unfre- 
quently take place in the ovary, and it is hardly necessary to say 
that in no case did the development proceed far. Balfour has also 
cited a remarkable observation of Greff, who saw unfertilized ova 
of the common starfish developing in ordinary sea-water, in a perfectly 
normal fashion, only more slowly than usual. 

(r) Occasional Parthenogenesis. — In some of the lower animals, 
which are not themselves normally parthenogenetic, but have relatives 
so addicted, occasional parthenogenesis has been frequently observed. 
These differ from the above cases, since the results are more suc- 
cessful, often in fact reaching maturity, and also in this, that since 
related forms are parthenogenetic, the ' ' abnormality ' ' is evidently 
of a much milder type. The common silkmoth is a good example of 
this occasional parthenogenesis, which certainly occurs, though rare 
both in the genus and family. " A whole series in insects," Weis- 
mann says, "reproduce exceptionally by parthenogenesis, for instance 
many butterflies, but that never to the extent that all the eggs which 
an unfertilized female lays develop, but only a traction, and usually 
a very small fraction of the total number, the rest perishing. 
Examples of successful occasional parthenogenesis (to the extent 
at least of producing males) are furnished by those worker bees, 
wasps, and ants which exceptionally become fertile." 

{d) Partial Parthenogenesis. — The queen-bee, as has been already 
mentioned, is impregnated by a drone in her nuptial flight. The 
sperms thus received are stored up, and used to fertilize the eggs as 
she lays them in the cells. Not all the eggs, however, but only those 
which will produce futwre queens or else workers. Other eggs, to lall 
appearance similar, are unfertilized, and these, as Dzierzon first 
clearly showed, develop solely into drones. We can not, however, 
say that the absence or presence of fertilization is the sole difference, 
though if fertilization be prevented by the imperfect development of 
the wings, or by clipping them, the queen only lays drone-eggs. The 
same happens when she is old and her store of male elements 
exhausted, or when the sperm receptacle has been removed. Von 
Siebold carefully examined the eggs from drone-cells, and found 
that they never contained spermatozoa. Hensen notes an interesting 
side feet, obviously corroboratory, that "German queen-bees, ferti- 



DEGENERATE SEXUAL REPRODUCTION. i6l 



lized by Italian or Cyprian drones, produced hybrid females but pure 
drones, a proof that on the latter the sperm does not operate." Again, 
it sometimes happens that what are called " fertile workers " crop up, 
which in consequence of some accident or misdirected intention in 
the nutrition, become less abortive than the host of semi-females 
which make up the body of workers. They are fertile enough to 
lay eggs, but their female organs do not seem to admit of their being 
impregnated. Certain it is that they only produce drones. What 
has just been said in regard to bees, is also true of some wasps and 
ants. 

{e) Seasonal Parthenogenesis. — In some of the minute aquatic 
crustaceans {Cladocera), popularly included under the general tide 
of water-fleas, parthenogenesis only occurs for a season, and is peri- 
odically interrupted by the birth of males, and the occurrence of the 
ordinary sexual reproduction. Males generally reappear in the disad- 
vantageous conditions of autumn, but Weismann denies that there 
is a direct connection between these facts. The common aphides are 
parthenogenetic for a succession of generations, sometimes as many 
as fourteen, throughout the summer, but the cold and hard times of 
autumn bring back the males and the sexual process. The fertilized 
egg lives on through the winter, and develops with the warmth of 
the next spring. By keeping up the temperature and nutritive 
optimum for three or four years in the artificial summer of a glass 
case, Reaumur and Kybur succeeded in rearing as many as fifty con- 
tinuous parthenogenetic generations. In the gall- wasps {Cynipidiz) 
there is usually only one parthenogenetic generation between the 
normal sexual reproductions, but in many insects besides aphides 
there are several. It ought to be noted that the parthenogenetic 
aphides are hardly at the same structural level as the females which 
are fertilized; but as the differences mainly lie in the absence 
of certain accessory genital organs, there is no reason for regarding 
the parthenogenetic forms, as some have done, as larval. 

(y") Juvenile Parthenogenesis. — Cases do occur, however, where 
larval forms become precociously reproductive (as sometimes happen* 
among higher organisms), and produce offspring parthenogenetically. 
Such precocious production of parthenogenetic ova must be distin- 
guished from the entirely asexual reproduction exhibited by many 
larvae. No very firm line indeed can be drawn, but in the last cases 
no cells which can be called ova are present. In 1865 Professor N. 
Wagner observed what has been much studied since, that in the larvae 
of some two-winged or dipterous midges (for example, Miastor), the 
cells of the reproductive rudiment develop into larvae within the 
mother-larva's body. The mother falls victim to her precocity, for the 



1 62 THE El'OLUTlON OF SEX. 

brood of seven to ten larvae literally feed upon her to the death. They 
finally leave the corpse and begin life for themselves, only, however, 
to fall themselves victims to a similar fate. The process may thus go 
on for several generations, during which the ova, or pseudova as some 
would insist upon calling them, become smaller and smaller. Event- 
ually the larvae become too constitutionally poor to be precociously 
parthenogenetic, and develop into adult midges — male and female, the 
latter producing, however, only a few eggs. 

In another dipterous insect known as Chiro7iomus, the ova begin to 
be produced at a very early stage, are laid just at the time when the 
larval life ends, and develop parthenogenetically. According to 
Jaworowski, by the rupture of the ovarian membrane the ova fall into 
the body-cavity, where the abundant nutritive stimulus takes the place 
of fertilization. Juvenile parthenogenesis is also said by Von Siebold 
to occur among the Strepsiptera, little insects which infest bees. 

i^g) Total Pa7'thenogenesis. — Lastly, in some of the minute aquatic 
crustaceans and in many rotifers no males have ever been found. 
There is every probability that the parthenogenesis is thus total; and 
as the numbers are abundant, it has apparently been established with- 
out detriment to, at least, the continuance of the species. 

III. Occurrence of Parthenogenesis. — In these distinct sets 
of animals — rotifers, crustaceans, and insects — parthenogenesis has 
become a confirmed physiological habit. 

(a) Take first the curious little rotifers, or wheel-animalcules, which abound 
both in fresh and salt water. They are usually placed in the chaotic alliance of 
worm-types, and have long been famous for their alleged power of surviving 
prolonged desiccation. With one or two exceptions the males are markedly 
different from the females, and are usually small and degenerate. In one group 
{Philodinada:) the females have two ovaries, while males have never been 
found. They have dwindled out of existence. In the rest the females have one 
ovary, part of which has degenerated into a yelk-gland, and small males occur. 
These are quite superfluous as mates, however, for parthenogenesis prevails. 
Even whep impregnation, which is a peculiarly random process, occurs, the 
sperms appear to miss their mark, and to perish in the body-cavity. The 
numbers keep up, notwithstanding, so that we have here an entire class where 
parthenogenesis has firmly established itself 

(b) Among crustaceans, parthenogenesis is restricted to the lower orders, 
namely, branchiopods and ostracods. In the former, it is exhibited by the 
brine-shrimp Artemia and the common fresh-water Apus in one division ; by 
daphnids (for example, Daphnia and Moina, common "water-fleas") in the 
other. In ostracods, some species of the common Cypris are parthenogenetic. 
If a female water-flea, say Daphnia, be isolated from birth, she becomes the 
mother of an abundant progeny of females. Males and sexual reproduction do 
however eventually return, and the same is probably true of the majority. 
Among three thousand specimens of the brine-shrimp only one male occurred ; 
while Von Siebold repeatedly investigated every member of a colony of Apus, 



DEGEN URATE SEXUAL REPRODUCTION. 



163 



once over five tliousand in number, without finding a single male. At other 
times he found one per cent., while in certain unknown conditions (probably 
when food is scarce and life generally unfavorable) the males may be developed 
in crowds. 

In the daphnids, which have been so successfully studied by Weismann, the 
facts are more complex. There are two kinds of eggs — winter ai.d summer 
ova. The former are large, thick-shelled, capable of resisting drought and the 
like, and of remaining lc:ig latent. They only develop if fertilized, and always 
produce females. In every way they are highly ana- 
bolic ova. The summer eggs, on the other hand, are 
smaller, and thinner in the shell. They can develop 
without fertilization, and that is indeed in some cases 
physically impossible. Males are produced from sum 
mer eggs alone. They usually appear in autumn, when 
life is becoming harder, or tlie conditions more kata- 
ixjlic. 

In the little cyprids the reproductive relations are 
very varied. Thus in Cypris ovum and Notodro?nus 
monachus the males are abundant all the year round, 
and parthenogenesis is unknown. In other species — 
for example, Candona Candida — the males are still 
frequent, but parthenogenesis nevertheless occurs. 
Lastly, parthenogenesis prevails in some cases, like 
Cypris fusca and C. pubera, and the males are rare, 
appearing usually in spring. 

(c) In insects, as we have seen, the degrees of 
parthenogenesis are very varied ; so, too, is the 
systematic position of the forms in which normal 
parthenogenesis occurs. Two butterflies [Psyche helix: 
and Solcnobia, 2 sp.) and a beetle (Gastrophysa); 
some coccus-insects and aphides ; certain saw-flies 
{Tenthredinidcs) and gall-wasps [Cynipida;), are nor- 
mally parthenogenetic. In the butterflies just noticed, 
the males seem to disappear for a stretch of years, 
and the species gets on without them. The male of 
Psyche helix is verj' rare, and was for long unknown. 
When the males are developed in Solenobia trinque- 
trella, it is interesting to notice that they may predom- 
inate in numbers over the females. A whole brood 
may be male ; they are brought back with a rush. 
About a score of moths, including the silkmoth (Boiii- 
byx niori) and death's-head {Sphinx atropos) have 
been known to exhibit casual parthenogenesis ; but 
the beede above noticed stands alone. Bassett, Adler, 

and others, have demonstrated an interesting alternation of parthenogenesis 
and ordinary sexual reproduction in numerous gall-wasps. Forms which had 
been regarded as quite distinct, and had recei\'ed different generic titles, have 
been shown in about a score of cases to be merely the parthenogenetic and 
normal forms of the same insects. From a winter-gall the parthenogenetic form 
emero-es which produces a summer-gall. In this a sexual form is produced, 
which eventually gives rise to the winter-gall. 




Fig. 49. — Owen's figure of tlie 
Generations of Aphides. 
At the base an individual 
arises from a fertilized egg- 
ceU ; this gives origin par- 
theno-genetically to a 
brood, and so on through 
a siiccession of genera- 
tions. At the top the male 
and female forms reappear, 
and sexual reproduction 
returns. At the side an 
earlier appearance of sex- 
ual forms is suggested. 



1 64 THE EVOLUTION OF SEX. 

IV. Parthenogenesis in Plants. — The passive bias is so strong in plants, 
that it is easy to understand the rarity of parthenogenesis. The egg-cell which 
develops of itself must retain the stimulus which the male element in other cases 
supplies. It is natural, then, that what predominates in the active rotifers 
should be uncommon in the sleeping plants. In some of the flowering plants, 
what look like parthenogenesis has repeatedly been described, especially in 
regard to a native of New Holland, known as Ccelebogyne. When cultivated in 
Europe, the male flowers degenerate, and according to Braun and Hanstein 
disappear. Yet fertile seeds are produced. Karsten found, however, that 
stamens often persisted ; while Strasburger has shown that what developed 
were not true egg-cells, but adventitious growths from cells outside the embryo- 
sac. The same is true of some other cases. Dr. A. Ernst has recently 
described what he calls true parthenogenesis in a Menisperm found by him in 
Caracas, and named Disciphania Ej-nstii. "Female plants, which bore no 
male flowers, and which were grown perfectly isolated where there was no 
possibility of the access of pollen from another plant, produced in three succes- 
sive years an increasing number of fertile fruits." 

In the lower plants, however, there is no doubt on the subject. Partheno- 
genesis frequently occurs as one of the stages in the degeneration of se.xual 
reproduction. It has been casually observed of a species of the stonewort 
{Ckara), that when grown in certain waters the male organs disappear, yet the 
plants continue multiplying. More interesting are the fungi. To illustrate 
sexual degeneration, De Bary gives a series from fungi like those which kill the 
salmon and potato {Sapro/egnics and Peronosporecs). What happens first, is 
the degeneration of the male organs. The katabolic sex from beginning to end 
is the more unstable. The male function goes first, but the form remains after 
the reality has ceased. After a while, that is in related species, the form goes 
too. Sometimes the function is changed, and the male organs become a sort of 
protective sheaths. His series may be briefly summed up. 

1. In Pythium, the male organ discharges most of its protoplasm into the 

the female, — the usual story. 

2. In Phytophthora, only a very small portion is thus given, and we may 

almost say asked, for there are curious demand and supply arrant^e- 
ments and compulsions between the male and female organs in these 
fungi. 

3. In Peronospora. there is no perceptible passage of protoplasm from male 

to female, though, without going back to the " aura seminalis," we may 
allow the possibility of subtle osmosis. 

4. In some Saprolegnia, there are indeed the usual antheridia or male 

organs, which are directed toward the female organs, but do not open. 
The "explosive" character is diminishing. 

5. In others, the male organs never get near the female. 

6. In others, there are no male organs at all, but the female cells develop as 

usual. 

Parthenogenesis is thus reached, as the result plainly of a degenerative 
process. We can follow the story further, however, forestalling for the moment 
the subject of the next chapter. The male organ has degenerated, we have 
seen, while the female organ holds on its course. But this is not always so ; in 
many cases it follows suit, and asexual reproduction remains. 

Now why should these fungi among plants exhibit numerous instances of 
parthenogenesis ? The more intimate the parasitism, the more degenerate the 
sexual reproduction, and all traces of it is often lost. The fungus fe'rtilizes itself 



DEGENERATE SEXUAL REPRODUCTION. 



165 



fruni its host. In the fungus on the coffee-plant, for example, the stimulus of 
ertilization is replaced as it were by an " essence of coffee." 

Male parthenogenesis, paradoxical as it sounds, is really exhibited among 
lowly alga;. That is to say, a small spore (or male-cell) which normally unites 
with a larger and more quiescent one (or female-cell), may occasionally start 
developing on its own resources. The result, however, is poor enough. As 
those spores are on the border-line between asexuality and differentiated sex- 
elements, the retention of a vegetative power of division even by the incipient 
male-cell is not surprising. Nor must it be forgotten that the mother-sperm-cell 
itself has a power of parthenogenetic development. It divides, like its 
homologue the ovum, into a ball of cells, but having none of the conservative 
coherence of the latter breaks up into spermatozoa. It is exactly comparable 
to the interesting protozoon (Magosphczra) which Haeckel saw, which did its 
best to get beyond the Protozoa, but failed as soon as it had succeeded. A 
single infusorian-like cell divided into a ball of cells, but the ball had no 
coherence and broke up into infusorians once more. 

V. The Offspring of Parthenogenesis. — The fate of parthenogenetic ova is 
very diverse. They may all perish, or all succeed ; they may turn out wholly 
males or wholly females. Hensen notes the following suggestive series, with 
decreasing reproductive, as opposed to constitutional, energy at each level : — 

1. Hermaphrodites, then only females. 

2. Series of females, then mixed brood. 

3. Several females, mixed brood, then only males. 

4. Series of mixed broods, then males, or death of ova. 

5. Mixed brood, with much mortality. 

6. Males only. 

7. Development only for a few stages. 

Rolph has a different arrangement, but the same idea :— 

1 . Exceptional parthenogenesis with uncertain result (for example, Silkmoth). 

2. Normal, producing males only (female solely from fertilized ova) (for 

example. Bees). 

3. Mostly males, with occasional females (for example, Nematus). 

4. Mostly females, with exceptional or periodic males (for example, Apus, 

Artemia). 

5. Only female, males unknown (for example, many Rotifers.) 

That parthenogenetic ova should develop with such diverse results is not at 
all surprising. The absence of fertilization removes one of the factors 
determining sex ; but food, temperature, age of ovum, &c., remain, and 
produce bias now to one side, now to the other. To this we shall presently 
return ; meanwhile the facts of offspring may be more cleariy expressed thus :— 



> 

O 
o 

w 
"z, 

w 
o 
o 
a; 
w 



Result. 

Nil 

Partial and pathological development 
Great mortality in a mixed brood . 
$ 's alone 



$ 's mostly, a few 9 's 

$'s and S>'s (one generation"! 

^'s, and more than a few ? 's 

9 9 9 (a succession), then a predommance of $ 

999, then equal numbers of <y 's and 9 's 

9 9 9, then a minority of S's among 9's 

9 9 9 9, very rare ^ 's . 

9 9 9 9, non-functional $ 's among 9 's 

9 9 9 9, ad infinitum, no ,J 's 



Example. 

Most organisms. 

Rarities mentioned. 

Many insects. 

Hive-bee and some 
other forms. 

Nematus (allied to bee). 

Most gall-wasps. 

Some sawflies. 

Some water-fleas. 

Solenobia sometimes. 

Aphides ; some water- 
fleas. 

Many water-fleas. 

Most rotifers. 

Many rotifers. 



1 66 THE Kl'OLCTlOy OF SEX. 

VI. Effects of Parthenogenesis. — Since partlienogenesis is 
doniinaiU m rotifers, and well-established among water-fieas and plant- 
lice, it is very plain that whatever else it affects, it is anything but 
prejudicial to numbers. An aphis will continue for days producing a 
viviparous brood, at the rate of one per hour; the offspring soon begin 
themselves to multiply; and Huxley calculates that if this continued for 
a year without mortality, a single aphis would be the ancestor of a 
progeny which would weigh down five hundred millions of stout men! 
Not gardeners only have cause for gratitude that climate and enemies 
prevent such untoward increase. But there are other desiderata besides 
numbers. Can it be said that parthenogenesis favors the general life 
and progress of the species? It will be at once recognized that rotifers, 
brine-shrimps, water-fleas, aphides, coccus-insects, and so on, are 
relatively low forms. Only two or three butterflies and one beetle are 
parthenogenetic. Higher up in the scale virgin birth never occurs 
except in a very partial and pathological degree. But we can go 
further. More than one of the old naturalists, and in recent years 
Brooks, Galton, Weismann, and others, have laid emphasis on the 
value of fertilization as a fountain of change. To Weismann the inter- 
mingling of the male and female ' ' germ-plasmas ' ' in fertilization is 
really the source of variation. That it is a source, all will admit. If 
it be removed, therefore, as in rotifers, the species will be so much the 
less likely to progress. Weismann holds that it will not progress at 
all; and though we should not go quite so far, we are bound to allow 
that the establishment of parthenogenesis is a handicapping of evo- 
lution. 

We can not, however, follow Weismann in his next step. If all 
change springs from the sexual intermingling, the rotifer species can 
not change at all. They can not go forward, nor yet backward. 
Having attained to a physiological state when males became super- 
fluous, they remain in stati quo. So he emphasizes that superfluous 
organs, such as the sperm-receptacle, do not become rudimentary in 
parthenogenetic species, — "rudimentary organs can only occur in 
species with sexual reproduction." This is a corollary of Weismann' s 
contention that no individually acquired characters, either plus or 
minus, can be transmitted, and that the sexual intermingling is the 
sole source of change affecting the species. Were the main propo- 
sitions proven, the corollary would follow, but there are still many 
dissentient voices. Without going into the general question at present, 
let us take the corollary by itself d) Cases where males are quite 
unknown are comparatively few ; in most cases they reappear at 
intervals. It is not possible, therefore, as Weismann will allow, to be 
certain that the sperm-receptacle becomes superfluous to the species. 



DEGENERATE SEXUAL REPRODUCTION. 



167 



(2) He also allows that it does degenerate in the summer aphides, 
where the periodic disappearance of males is well known. (3) In spite 
of the absence or else futihty of impregnation in rotifers, we find the 
males obviously in process of degeneration. 

In conclusion, we beheve with Weismann and others, that the 
absence of fertilization is a minus in evolution, but see no warrant for 
supposing that it absolutely precludes either progress or the reverse. 
The power of parthenogenetic birth has two different results, (i) 
The female cell has a certain maleness about it ; it retains the stimulus 
which the male element usually affords ; the species will therelbre be 
frequently of active male-like habit, for example, rotifers and water- 
fleas. (2) On the other hand, the long-continued production of 
females means an anabolic preponderance, a weighting of the species; 
and this is seen in the sluggish plant-lice, coccus-insects, and the like. 

VII. Peculiarity of the Parthenogenetic Ova. — Before a 
theory of parthenogenesis is sought, the natural question arises. Are 
these eggs that develop of themselves in any way peculiar ? [a) For 
a while it was supposed (for example, by Balfour) that parthenogenetic 
ova did not ibrm polar globules, and the theory based upon that 
regarded the retention of these bodies as taking the place of fertili- 
zation. The demonstrated occurrence of one polar globule in several 
parthenogenetic eggs partially demolished this theory, and it is only 
within the last two or three years that it has been restated in accurate 
form, {b) Simon shrewdly points out that in some of the most 
marked cases of parthenogenesis the sex-cells are insulated from the 
body at a very early stage. This is notably so in those midges which 
reproduce parthenogetically even before maturity. It is certainly 
striking that these forms should unite an extreme earliness in the 
embryonic separation of the germ-cells with a most precocious 
reproduction. These germ-cells are ova which have a much less 
circuitous history than in most cases; they have far fewer cell-divisions 
behind them, they have thus a reserve power of division which other 
ova have not; they are able, in fact, to develop of themselves. This, 
unfortunately, is not known to be true of some of the most signal 
cases of parthenogenesis (for example, rotifers); but it is true of some, 
and that to a greater extent than was kno\\-n when Simon wrote. On 
the other hand, some forms where parthenogenesis is unknown (for 
example, leeches and Sagifta), also exhibit the same early differentia- 
tion of germ-cells, so that we can only look upon the fact as one of 
the auxiliaries of parthenogenesis. 

{c) The peculiarity of parthenogenetic ova, which has of late 
attracted much attention, is that they extrude only one polar cell,^ 
not two, like other eggs. This discovery is due to Weismann, who, 



1 68 THE El'OLUTlON OF SEX. 



with the assistance of Herr Ischikawa, has verified it in about a dozen 
species, Lcptodora hya/iiia, Sida crystallina, Cypris reptans, and other 
water fleas. Blochmann has also corroborated Weismann's discovery, 
in his observations on aphides. What theoretical importance Weis- 
mann attaches to the fact will be immediately noticed.* 

VIII. Theory of Parthenogenesis. — We may begin with 
Balfour's view of the case, though that of Minot has the priority. 
' ' The function of forming polar cells has been acquired by the ovum 
for the express purpose of preventing parthenogenesis. ' ' If they were 
not formed, parthenogenesis would normally occur. This is expressed 
in curiously teleological language, but the main idea is clear enough, — 
the retained polar cells replace the sperm nucleus. It is only necessary 
to change cells into cell to make it reasonable today. One must not 
forget, however, that in higher animals, where parthenogenesis is 
unknown, polar cells have not been found often as yet, nor ever seen 
in birds and reptiles. And one would fain get further back still, and 
know why only one polar globule is formed in parthenogenetic ova. 
" In accordance with Minot's hypothesis of sexuality, it might be 
assumed that in parthenogenetic ova the male element was retained, 
and that the cell remained a true asexual cell, and did not become a 
sexual element." "Blochmann and Weismann have shown that this 
is the case, by their discovery that in parthenogenetic ova only one 
polar globule is formed, while there are always two in ova which are 
impregnated ; hence it is probable that one polar globule (by 
hypothesis, male) is retained." 

Minot's words are not beyond criticism either, though they are not 
teleological. An ovum which retains a male element is not happily 
described as remaining asexual; it would be better to call it a case of 
intra- cellular hermaphroditism. Nor can it yet be said that there are 
always two polar globules in ova which are impregnated. The 
discovery referred to is historically Weismann's, while a corroboration 
is due to Blochmann. It is more important, however, to notice how 
Minot cleverly adapts himself, and rightly too, to increased knowledge 
of the facts. The parthenogenetic ovum only retains one polar globule, 
— one male element is enough; two would be "polyspermy," which 
is abhorred. 

There was no fear that Rolph would indulge in teleology, rigid 
necessitarian as he was. Parthenogenesis of ova was to him the 
more natural process, the sperm a subsequent importation. "There 
is for the ovum a certain minimal mass, which must be surpassed it 

* Blochmann, however, claims to have demonstrated the formation of two 
polar bodies in those unfertilized eggs which are to give birth to drones. 



DEGENERATE SEXUAL REPRODUCTION. 169 

it is to develop at all; and a second minimum, which the ovum must 
attain, if a female is to be produced. ' ' Abundant nutrition of the 
ovum tends to parthenogenesis, producing male offspring, as the 
lower stage; but if the second limit be attained, resulting in females. 
In the opposite direction, if the ovum have fewer resources, it requires 
to be fertilized. Females or males will again result according to 
the state of the elements. If no fertiUzation occur, the dependent 
ovum must of course die. Rolph is always suggestive, but he erred 
in regarding the sex-elements too quantitatively, in missing the quali- 
tative antithesis of sex, and the opposition observed in cell-division. 
id) Strasburger also lays emphasis, in a subtler and more technical 
way, on nutritive conditions. " In the rare cases of parthenogenesis, 
specially favorable nutritive conditions may counteract the lack 01 
nuclear plasma." He notes three different ways in which this may 
happen, and also inclines to believe that retention of polar globules 
would favor parthenogenetic development. It is important to notice 
how two naturalists, so very different in their manner of attacking a 
subject as Rolph and Strasburger are, come to this conclusion at 
least in common, that favorable nutritive conditions favor partheno- 
genesis. All the cells in the body tend to multiply, the ova retaining 
this power develop embryos. 

(e) Weismann has a peculiar right to be heard on the nature of partheno- 
genesis. For not only has he been for many year's an investigator of the tiny 
daphnids or water-fleas, but he has recently made the important discovery, 
already noticed, that parthenogenetic ova extrude only one polar globule. 
There has not been time yet to prove that this is an absolute fact, but the 
probabilities are strong that it is. Before stating his theory, it is necessary 
to remember that the "germ-plasma " of Weismann is a specific and essential 
portion of the nucleus of ovum or sperm, part of which keeps up the continuity 
of heredity', by passing intact into the reproductive cells of the next generation. 
Besides this all-important "germ-plasma," the nucleus of the ovum contains, 
according to Weismann, an " ovogenetic nuclear plasma," which is of no direct 
importance in development, but is useful to the ovum simply as an ovum. It 
is the substance which is supposed to have to do with the general upbuilding 
of the egg-cell, with the accumulation of yelk, secretion of membranes, and 

the like. 

"The first polar body implies the removal of the ovogenetic nuclear plasma, 
which has become superfluous when the egg has attained maturity. The 
second polar body, on the other hand, implies the removal of a portion of 
the germ-plasma itself. This is so effected that the number of ancestral elements 
{Ahnen-idioplasmen) which composes it is reduced to a half A similar reduc- 
tion must also take place in the number of the male germ-elements. 

"Parthenogenesis occurs when the entire sum of the ancestral elements 
persist in the nucleus of the ovum. Development by fertilization demands, 
however that half these ancestral elements must first be extruded from the 
ovum, whereupon the remaining half, in uniting with the sperm nucleus, regams 
the original number. 



jyo THE EVOLUTION OF SEX. 



' ' In both cases the beginning of development depends upon the presence 
of a definite, and indeed similar mass of germ-plasma. In the ovum which 
requires fertilization, this is afforded by the importation of the sperm-nucleus, 
and development follows on the heels of fertilization. The parthenogenetic 
ovum already contains the necessary mass of germ-plasma, and this becomes 
active as soon as the single polar body has freed the ovum from the ovogenetic 
nuclear-plasma." 

Now if it be true that a constant difference between an egg which can 
develop of itself and one which can not, is that the former extrudes one tiny 
cell, and the latter, so far as yet observed, two, Weismann must be right in 
emphasizing that part at least of the secret of parthenogenesis lies here. Partly 
hidden still, however, if one dare ask what there is about the parthenogenetic 
ovum which limits its primitive budding to once instead of twice. Not 
altogether so subversive of Minot's theorj' either, as Weismann would make 
out. Minot, as we saw, accepts the facts, but ingeniously supposes that the 
polar element retained in parthenogenetic ova is a male element. It is neces- 
sar>', however, to examine Weismann's theory more closely, not only in its 
direct relation to the problem of parthenogenesis, but because of its postulates, 
which run so directly counter to our reading of the phenomena of sex. 

1. Weismann's theory obviously differs very emphatically from those pre- 
viously suggested. The first polar body is no skimming of antagonistic male 
material ; the very reverse, it is an extrusion of ovogenetic nuclear material 
which had to do with the upbuilding of the ovum, an emphatically female 
function. Nor is the second polar extrusion in any way an expulsion of male 
elements ; it is a giving away of some of the precious germ-plasma, the bearer 
of hereditary characteristics. Furthermore, even the sperm nucleus is in no 
peculiar sense male material ; it might as well be another ovum-nucleus. It 
has only a quantitative value, to restore to the nucleus of the ovum an amount 
of germ-plasma equivalent to that which has been so I'ecklessly squandered. 

2. But Weismann's theory, based on the observation of facts, is in itself full 
of hypotheses. This distinction between ovogenetic and germ-plasma within 
the germinal vesicle is an unverifiable myth. That the first polar body is an 
extrusion of one kind of nuclear substance, and the second something quite 
different, is another unproved hypothesis. Were the extrusions markedly 
different, one might believe it, but they are the same. When a large cell 
divides very unequally, as in polar-body formation, there is some warrant for 
supposing that the little bud is different /rom the large cells ; but that two 
successive divisions, entirely similar in character, are conspicuously different, 
requires faith. It is allowed by all that each polar division lessens the mass 
(not the number) of the chromatin elements in the nucleus by a half, but so far 
as nucleus is concerned there is nothing whatever to show that the first division 
is qualitatively different from the second. The first may have more cell- 
substance extruded along with it, and the second may be rather a nuclear than 
a cell-division, but as regards "plasma" the two are, so far as the facts go, 
absolutely alike. The second division also follows on the heels of the first 
without the intervention of the usual resting-stage. Nor of course is there any 
proof that a parthenogenetic ovum does not part with half its "germ-plasma" 
in the first division. The distinction between the two kinds of nuclear plasma 
is, in plain words, a myth. 

3. Weismann's preoccupation with questions of inheritance has given a bias 
to his theorj', making it morphological rather than physiological. A given 
quantum of germ-plasma, he says, fits the ovum to develop. The partheno- 



DEGE2,'ERATE SEXUAL REPRODUCTION. 171 

genetic ovum has this and keeps it. Tlie ordinary ovum has it too, but extrudes 
it, to get it back again from another source. If this is all the sperm does, one 
can not help wondering that such a circuitous process could ever arise. The 
entrance of the sperm must be looked at in two ways, — (a) it bears with it 
certain hereditary characteristics, doubtless in the nucleus for the most part ; (6) 
it brings with it a stimulus to division of a qualitative character, doubtless in some 
part in its small cell-substance. The last function — the dynamic function — 
Weismann wholly denies. The sperm has to him only a quantitati\e function. 
Yet in spite of this virtual denial of sex, — that is, of any deep difference 
between male and female whether elements or organisms, — he does admit a 
qualitative action after all, for it is out of the mingling of the male and female 
germ-plasma that all variations arise. 

4. Boveri makes an interesting note in regard to Weismann's disco\ery and 
theory. There is a tendency, illustrated in ascarids, for the second polar 
division to limit itself to the chromatin elements, to be a nuclear division rather 
than a genuine cell-budding. Such a second division may possibly occur in the 
parthenogenetic ova, while there may be in reality one extrusion. A second 
nucleus may be formed, and retained, and act the part of a spermatozoon, very 
much as IMinot's theory supposes. 

(^cr) Our theory of parthenogenesis is not so subtle as Weismann's 
nor so simple as Minot's. Just as the spores which illustrate the 
beo'innings of sex inay sometimes dispense with conjugation and ger- 



f disease (D). 
Female < se.\ (s). 

(parthenogenesis (P). 



( parthenogenesis (P). 
Male] sex (s). 

( disease (D). 

Fig. 50.— Diagram illustrating the theorj' of parthenogenesis. 

minate independently, so may ova develop parthenogenetically. 
These are to be regarded as incompletely differentiated female cells, 
which retain a measure of katabolic (relati\-ely male) products, and 
thus do not need fertilization. Such a successful balance between 
anabolism and katabolism is indeed the ideal of all organic life. That 
the extrusion of one polar globule still occurs, only shows that some 
katabolic products are still expelled. In parasitic fungi, sexual 
reproduction disappears, and surrounding waste products presumably 
help the purpose otherwise effected by sexual organs, so peculiarities 
in the condition of parthenogenetic o\'a may explain the retention of 




172 



THE EVOLUTION OF SEX. 



the normal balance which makes division possible without the usual 
stimulus of fertilization. Abundant and at the same time stimulating 
nutrition (Rolph), early differentiation of the sex-cells (Simon), the 
general preponderance of reproductive over vegetative constitution 
(Hensen), their liberation before the anabolic bias has carried them 
too far, are among these favoring conditions. The incipient segmen- 
tation observed in a few ova is an independent effort to save them- 
selves from being too big to live, since they are not passive enough 
to remain dormant. Waste has set in, self-digestion begins, the cell 
is forced into the expedient of division. In higher animals this is 
all in vain; in lower animals "such imperfectly differentiated female 
cells are commoner; they form the parthenogenetic ov^a. 

IX. Origin of Parthenogenesis. — From the occurrence of 
parthenogenesis in the animal series, it is certain that it has originated 
as a degeneration from the ordinary sexual process. It is no direct 
persistence of a primitive ideal state, though to some degree a recapit- 
ulation of it. One hypothetical mode of origin, which may well apply 
to the rotifers, is easily sketched. In conditions favoring katabolism 
the males wore themselves out, the females became katabolic enough 
to do without them. We find the males, where they persist, much 
smaller than the female rotifers, often extremely degenerate, in one 
section wholly unknown. Again, from the fact that the interruption 
of a parthenogenetic series of females by the appearance of males 
usually occurs in hard times, we may infer the prosperous vital con- 
ditions induced parthenogenesis. Why, then, are not internal para- 
sites parthenogenetic? They are very generally hermaphrodites, and 
have moreover gone beyond parthenogenesis to prolific asexual 
multiplication. 

It is misleading to interpret the occurrence of parthenogenesis as 
due to "motives" and "important advantages." These are after- 
thoughts of our importations. It is not easy indeed to keep from 
metaphorical language which suggests that polar globule-formation is 
a "contrivance," and parthenogenesis a "device." Such casual 
words are of little account; but to say, as Weismann does, "that 
sexual reproduction has here been given up, not by any chance nor 
from internal conditions, but from quite definite external grounds of 
utility fzweckmassigkeitsgrunden)," is to say the least misleading. A 
species of crustacean is being decimated by enemies, increased multi- 
plication would lessen the danger of extinction, parthenogenesis is 
established, and for every one before producing eggs there are now 
two — voila tout. Against this short and easy method with Nature 
we emphatically protest, and maintain that the origin of partheno- 



DEGENERATE SEXUAL REPRODUCTION. ly-. 



genesis was not for any subsequent advantage, but purely from 
necessary internal conditions. 

X. The Case of Bees. — We have already spoken of the " voluntary 
parthenogenesis" of bees. All the eggs are supposed to have the power of 
parthenogenesis, but all are not allowed so to develop. The fertilized eggs 
develop into queens and workers, the unfertilized give rise to drones. Weis- 
mann emphasizes the fact that the ova are all alike. ' ' There is no difference 
between those which are, and are not to be fertilized. The difference first 
appears after the maturation of the egg, and the removal of the ovogenetic 
plasma." The state of the polar bodies is not known, so the question need not 
be complicated by suppositions about them.* Writing before his discovery in 
regard to parthenogenesis, he says the sine qua non of development is that the 
nucleus acquire a certain quantity of germ-plasma ; the fertilized egg gets its 
quantum in the usual way by aid of the sperm, the unfertilized gets it by simple 
growth ; the difference of sex in the result need not be further taken into 
account. Again we remark that this matter of a quantum of "germ-plasma," 
and the two ways of getting it, is a pure supposition, both in general and in this 
particular case. Again we must note, that if parthenogenesis be decided on 
utilitarian principles, and if the difference of se.x need not be taken into account, 
and if the eggs are all the same to start with, we see some difficulty in under- 
standing the persistence of drones and sexual reproduction at all. It is a 
laborious and expensive way of attaining no obvious gain. But we should, 
indeed, like to be sure that the ova are all tlie same to start with. Von Siebold 
said that the queen was moved by the sight of the different size of the cells to 
fertilize or refrain from fertilizing. This may be so. Impressive as a queen's 
cell is, the difference between a worker's and drone's is much less striking. 
We suspect the impulse lies somewhere else. But barring tiiis, the eggs laid 
first, when the queen is at its prime, develop into females ; the eggs which give 
rise to drones come later, when the mother is more exhausted. They have had 
less chance of differentiation — they are parthenogenetic ova. So with old 
queens, when the stock of sperms is also of course exhausted. Weismann 
quotes the experiment which Bessels made, after Dzierzon. The nuptial flight 
was prevented, and ova which, in the course of nature, would have been 
fertilized and given rise to queens and workers, were of course unfertilized, and 
developed parthenogenetically into males. This proves, he says, that the ova 
are all the same to start with. But one would like to know whether the 
prevention of the nuptial flight had not also its effect upon the ova, and whether 
the parthenogenetic ova are not always less differentiated. 

*See, however, p. i68, note. 



ly^ THE EVOLUTION OF SEX. 



SUMMARY. 

I. Parthenogenesis was formerly believed to be of wider occurrence tlian 
it really is, but it is definitely Icnown to be not uncommon in lower animals. 

II. Artificial, pathological, occasional, partial, seasonal, juvenile, and total 
parthenogenesis must for clearness be distinguished. 

in. The occurrence of parthenogenesis is especially well seen in rotifers, 
crustaceans, and insects. 

W . It is rare among plants, but certainly occurs in some of the lower 
forms. 

V. The offspring of parthenogenetic ova is very diverse. 

VJ. The effects of parthenogenesis on the species deserve consideration, 
especially by those who find in sexual intermingling the sole fountain of specific 
variation. 

VII. Parthenogenetic ova, so far as observed, form only one polar body. 

VIII. Parthenogenetic ova are here regarded as imperfectly differentiated 
female cells, retaining certain male or katabolic characteristics. 

IX. In origin parthenogenesis is regarded as a degeneration from the 
ordinary sexual process. 

X. The voluntary parthenogenesis of bees is taken as a concrete illustration. 



LITERATURE. 

See especially the already cited works of Balfour, Brooks, Hensen, Minot, 
Rol|5h, Sachs, Weismann ; also - - 

Ow'EN. — Parthenogenesis ; or, The successive production of Procreating Indi- 
viduals from a single ovum. London, 1S49. 
Von SiEBOLD. — Beitriige zur Parthenogenesis. Leipzig, 1S71. 
Leuckart. — Art "Zeugung" in Wagners's Handworterbuch d Physiol, Bd. 

IV., 1S53. 
GersT/ECKER. — Bronn's Klassen und Ordnungen des Thierreich, Vol. V. 

Arthropoda. 
Brooks, W. K. — Law of Heredity. Baltimore, 18S3. 
Simon, F. — Die Sexualitat, &c., Inaug. Dissertation. Breslau, 1S83. 
Blochmann. — Uber die Richtungskorper l^ei Insekteneiern, Biolog. Cen- 

tralblatt, VII., and Morpholog. Jahrbuch, XII. 
Weismann, A. — Beiter, zur Naturgeschichte der Daphnoiden. Leipzig, 1876- 

79. Uber die Zahl der Richtungskorper und fiber ihre Bedeutung fur die 

Vererbung. Jena, 1887. 
Weismann, A., and Ischikawa, C— Brichten der nautrforsch. Gesellschft., 

Freiburg, III., 1.S87. 
Hudson and Gosse.— The Rotifera. London, 1S86. 
Plate. — Beitriige zur Naturgeschichte der Rotatorien, Jenaische Zeitschft. f. 

Naturwiss, XIX., 1886. 
Karsten, H. — Parthenogenesis und Generations-Weschel im Thier und 

Pflanzenreiche. Berlin, 1S88. 



AS EX UAL REPR OD UC TION. 



175 



CHAPTER XIV. 
ASEXUAL REPRODUCTION. 

I. Artificial Division. — Weeping willows are by no means 
scarce trees in Britain, yet as they never flower they must all have 
grown from slips; or, in other words, artificial asexual multiplication. 
So, too, only more naturally, the Canadian pondweed has spread 
prodigiously in our lochs, canals, and rivers, never flowering, but 
owing its increase wholly to the asexual process. Every one knows 
how the gardener increases his stock by slips and cuttings, thus taking 
advantage of the power a part has to reproduce the whole. Quite in 
the same way, cultivators of bath-sponges bed out little fragments to 




Fig. 51. — A group of Sea-Anenioiies. — From Andres. 

keep up a convenient supply. In the last century, the Abbe Trembley 
delighted himself and others by the often repeated observation that to 
get many hydra polypes out of one, the simplest and quickest way 
was to cut it in pieces. Though the fragment be very small, it will 
reproduce the whole, provided always that it have to start with fair 
samples of the difterent kinds of cells in the body. The same may be 
done any day with the much larger sea-anemones. So the earth- 
worm, curtailed by the spade, does not necessarily suffer loss, though 
it suffer pain. The head-portion grows a new tail, and even a 
decapitated portion may reproduce a head and brain, — not that this is 
saying much for these. 



176 THE EVOLUTION OF SEX. 



II. Regeneration. — Spades and knives are not exactly instru- 
ments of Nature, but tliey have their counterparts. Fighting with a 
rival a crab may lose its claw, or the same may happen in the 
frequently fatal molting, which seems almost like a mistake in Nature. 
Slowly, however, forgiving Nature makes good the loss; the cells of 
the stump multiply, and arrange themselves in obedience to the same 
necessities as before, and a limb is regenerated. Many an appendage 
among the lower animals is from time to time nipped off, only to be 
grown again. A snail has been known patiently to regenerate an 
amputated eye-bearing horn twenty times running. Sometimes one is 
tempted to think that the animals almost understand that it is better 
for one member to perish than for the whole life to be lost, so readily 
does a starfish surrender an arm, or a lizard its tail. Yet it must be 
recognized that animals, like men, are often wiser than they wot of. 
In the panic of capture, strong convulsions may occur, which surprise 
and perhaps shock the molester of a sea-cucumber by the ejection oi 




Fig. 52. — The formation of a Sponge Colony {Olyntkus) 
by budding. — After Haeckel. 

its viscera; or a tetanic contraction of the muscles makes the slowworm 
brittle in the hands of its captor. The power of regeneration is most 
marked in echinoderms, but persists as high up as reptiles. The 
regrowth of part of a hzard's leg is the cJief-d' auvre in this line. 
Beyond that, regeneration is restricted to httle things. We constantly 
regenerate the skin of our lips, but we can not naturally replace an 
amputated hmb. It is more marvelous that we can not, than that the 
lizard can. That the cells of an irritated stump should divide and 
multiply, and that the result should be the same as it was at the first, 
is really no marvel, or rather as much as but no more than the original 
development. The dividing cells of the growing stump are simply 
repeating their original development. 

III. Degrees of Asexual Reproduction. — The keynote of the 
subject was truly struck by Spencer and Haeckel, when they defined 
asexual reproduction as discontinuous growth. All growth is a 



ASEXUAL HE PROD UC TION. 



177 



reproduction of the protoplasm and its nuclear elements, or in short of 
the cells; all reproduction (excluding the important fact of fertilization) 
is growth. The ovum, asexually produced from the parent ovum or 
its lineal descendant cells, grows and reproduces itself in turn, building 
up the embryo. The embryo grows into an adult organism, and the 
surplus of continued growing energy results in the asexual production 
of buds, or the sexual discharge of differentiated reproductive elements. 
We start from the ordinary processes of cell-multiplication and 
regeneration exhibited in the normal organism. Then come the 
processes by which lost members are regenerated, involving more or 
less serious extra growth. To these we must add the rarer and yet 
not rare cases where the artificial halves or fractions of an organism 
can grow into wholes. Normal and frequent, however, are the very 
abundant cases of budding, where a sponge or hydra, zoophyte or 
coral, has surplus enough to grow off new individuals, which remain 
continuous with itself The parent organism, whether zoophyte or 
strawberry plant, has an asexually produced progeny round about, 
and in asexual continuity with itself But they do not always remain 
continuous; the hydra produces buds, but eventually sets them adrift. 
This is still better seen in many of the hydroids, where individuals are 
separated off as swimming-bells or medusoids. The multiplication 
has become discontinuous. Continue the process, and we find the 
liberation of special cells, clinging 
often for a time to the parent, gen- 
erally dependent for development on 
union with similar cells of comple- 
mentary constitution; we find, in 
fact, the sexual reproduction which, 
in the higher organisms, so thor- 
oughly replaces the asexual process. 

IV. Occurrence of Asexual 
Reproduction in Plants and 
Animals. — In plants, as one would 
expect from their typical vegetative 
constitution, the asexual process is 
common, particularly among the 
lower forms. The most familiar of 
all cases is afforded by the common 
livenvorts {_Marchantia and Liaiu- 
larid), which through the formation 
of asexual buds or gemmae in the cups so familiar upon their thallus, 
are enabled to overrun our flowerpots, and so rapidly become a pest 
of the greenhouse. Many ferns, too, notably among the Aspleniums, 




Fig. 53. — Asexual Propagation of Grass — {a) 
the bulbils rooting on the ground: (5) 
their appearance in the inflorescence : (c) 
a small portion enlarged. — From nature. 



lyS 



THE EVOLUTION OF SEX. 



reproduce by bulbils, arising upon the frond; and the bulbils which 
arise in the axils of the leaves of the tiger-lily are familiar missiles for 
every child accustomed to a flower-garden (see figs. pp. 209 and 264). 
The alliums, and some of our common grasses also, furnish us with 
examples of the replacement of flowers by separable buds. Asexual 
reproduction or multiplication by more or less discontinuous gro^^•th, 
without the differentiation of special and mutually dependent sex-cells, 
occurs Irom the simplest animals on to the tunicates or sea-squirts, 
from the base to just over the line which separates backboneless and 
backboned animals. It is necessary, however, to review the groups. 

Protozoa. — Fertilization began in almost mechanical fusion. Reproduction 
begins with almost mechanical rupture. The unit-mass of protoplasm, becoming 
too big for control, breaks. Thus it saves itself, and at the same time multiplies. 
Such breakage may be seen in a primitive form like Schizogenes, but it also 
occurs in a few of the relatively high infusorians. That the breakage some- 
times means dissolution is certain ; nor is reproduction ever so verj' far remo\ed 
from death. 

The rupture becomes orderly and systematic in budding. This may be 
multiple, as in the common Arcella, where a number of small buds are 
constricted off all round. But the process is oftener concentrated in one 
extrusion or overflow. In budding, the separated daughter-cell is in varying 
degree smaller than the parent, and the process resembles an overflow. When 
the bud is approximately equal to the parent, and the process is of the nature of 
a constriction, it is of course division. 

The division may also be multiple, taking place in rapid succession and in 
limited space, for example, within a cyst. Then we speak of spore-formation. 
The last three modes of multiplication are exceedingly common among 
Protozoa. 

These buddings and divisions are not, of course, rough and ready processes. 
The nucleus almost always shares in them in an orderly and deliberative 
fashion. There are variations in its behavior as in higher animals, but there is 
no doubt that cell-division, with a gradient of progress like everything else, is 
essentially one and the same in the vast majority of cases. Gruber has been 
especially successful in proving that fragments oi Protozoa, artificially separated 
without nuclear elements, can not live long, though they may grow and repair 
their losses for a little. The nucleus is essential to life, though sometimes it 
seems to disappear, and become as it were a diffuse precipitate in ihe proto- 
plasm. 

Sponges. — In sponges no one can fail to recognize the impossibility of 
drawing any rigid line between growth and asexual reproduction. Between 
simple extension of the parent mass, and the budding off of new individuals, 
no sure distinction can in many cases be made out. Sponges do not divide, 
though they may be cut up, yet they give off discontinuous buds. An outgrown 
tube may lose connection with the parent, or a great tumor-like mass may be 
slowly extruded, or tiny brood-buds may be set adrift to shift for themselves. 
In disadvantageous conditions the surface of a sponge sometimes gathers into 
minute superficial buds, by means of which it is possible that the life is saved. 

In the fresh-water sponges, in disadvantageous circumstances, — of cold in 
some countries, heat and drought in others, — some of the cells club together to 



ASEXUAL REPRODUCTION. 



179 



form gemmules, which often save the hfe of the otherwise dying sponge. They 
are complex enough, with sheaths and spicules, and sometimes even witli a 
float, but in principle they simply do by a muhiple union what is otherwise 
attained by ovum and sperm. Best known in this respect is the freshwater 
sponges iSpoiigilld); they have also been described in other common sponges, 
for example, in CHnoe, the borer in oyster-shells. 

Ccelenterates. — In such names as zoophytes, sea-firs, sea-roses, there is a 
prevision of the undoubtedly plant-like character of many of the ccelenterates. 
A sessile habit is very general, though often only a phase in the life-history, 
and an asexual reproduction runs riot. A well fed hydra is prolific in bud- 
bearing ; and numerous gradations connect this with the myriad colonies 
exhibited by many hydroids. The individuals forming a united family share 
in the common life and nutriment. As the colony becomes complex, it is often 
physically impossible for all the members to remain on terms of even approxi- 
mate equality of internal and external conditions. One l^ecomes relatively 
overfed, another starved. Slight differences of function gradually become 
emphasized and exaggerated, till division of labor is established. The struc- 
tural aspect of this is differentiation of polymorphism among the members of 
the colony, and results in the establish- 
ment of nutritive and reproductive, sen- 
sitive and protective, "persons." Thus 
in the common Hydractinia, the open- 
mouthed nutritive individuals are mark- 
edly contrasted with the dependent repro- 
ductive persons ; and again, in different 
form, the rythm repeats itself in the con- 
trast between active, offensive, and sensi- 
tive elongated members, and entirely 

:, , ^' • 1 • 1 ^ Fig. 54. — One of the acarids or lice IGhci- 

passive and abortive spines, which form ^,,^^„„^ ^„,,^^^^ ^^^^.^^ ^ life-saving cyst, 

a chevaux-de-frise under shelter of which while the individual itself dies. 

the others cower. It is usually supposed 

that the sessile hydroids are in a sense degenerate from more active ancestral 
types. The free-swimming embryo becomes exhausted, settles down, and 
exhibits predominant vegetativeness with postponed sexuality. In many cases, 
however, there is a recovery of the ancestral liberty of action, for modified 
"persons" are set adrift as active, free-swimming, sexual medusoids. 

There are, however, active forms of the true medusoid type ( Trachymediisiz) 
which never descend to the sessile nadir of existence, but yet exhibit the asexual 
tendency of the class in forming temporary clusters of pendent buds. Lang has 
lately described a remarkable compound medusoid ( Gastroblasta raffaelii), 
which has sometimes as many as nine stomachs, and may be assumed to be 
highly nutritive. The remarkable point, however, is that the compound adult 
is the result not only of continued budding, but of a process of rectangular 
incomplete division. Along with some others it leads on tov\'ard the Portuguese 
man-of-war, or siphonophore series. Here the larva develops at first into a 
simple medusa-like individual, but this buds off a manifold series of "persons," 
which, by dislocation or even migration, becomes arranged in all the beauty of 
the siphonophone colonies, which surpass even Hydractinia in their division 
of labor. It is difficult enough in some cases to distinguish between true 
"persons" — which Haeckel calls " Medusomes " — and mere organs like 
protective bracts, which are also budded off 




I So 



THE EVOLUTION OF SEX. 



In another direction, namely, among the true jelly-fishes (Acraspeda), where 
an active habit greatly preponderates, we still find the occurrence of asexual 
multiplication. Some forms (for example, Pelagia) are entirely free ; at the 
opposite extreme a few {Lucernarida) may be described as sedentary; between 
these we find the common aurelia, which settles down in its youth, and gives 
rise by division to what afterwards become the large sexual jelly-fishes (see 
fig. p. 187,) 




Fig. 55. — Siphonophore Colony, showing the float (a), 
the swimming-belU {b), and the nutritive, repro- 
ductive, and other "persons" beneath. — From 
Lang, after Hacckel. 



There remains two classes of ccjelentrates — the Ctcnophora, like Beroe, 
which represent a climax of activity, and never divide ; and the Actinozoa 
(sea-anemones and corals), which lead to a passive terminus again, and exhibit 
profuse asexual multiplication. A few sea-anemones divide normally, just as 
they may be multiplied by artificial cutting. Fragments may also be given off 
in an arbritrary sort of fashion, reminding one of the overflow buds of sponges. 
The division may be either longitudinal or crosswise in sea-anemones, and the 
budding of corals takes many forms, resulting in the quaint complexity of 
brain-corals and the like. In one sea-anemone {Gonactinia proliferd), where 
transverse division occurs, it is interesting to notice that this has only been 
observed in young forms with undeveloped sexual organs. It recalled, in fact, 
the asexual multiplication of a young jelly-fish. In another of the corals 
{Antipatharia) Brooks has recently observed how a nutritive "person" may by 
constriction form a reproductive individual on either side. 

Worms. — The lower worm-types are roughly distinguishable from most of 
the higher by the broad fact that they are all of a piece, without rings or seg- 



ASEA- UAL RE PROD VC TION. 



i8r 



ments. A physiological link, however, between worms of only one segment 
and those with many, is found in the asexual chains which some of the former 
occasionally develop. Thus the little turbellarian Microstmnuni lineare may 
bud off a temporary chain of sixteen individual links. The 
budding begins at the posterior end, c^nd what is partly sepa- 
rated off is a portion in excess of the normal size. The 
second link grows till it attains the usual adult size, and as it 
exceeds this form a third link. At the same time the original 
individual may also be doing the same, and thus a chain of 
four is formed. Two more buddings by each of the links 
bring the asexual process to a climax, and then the individ- 
uals separate from one another and become sexual in free- 
dom. It is important to notice that the asexual reproduction 
takes place in favorable nutritive conditions, and as each 
individual exceeds its normal limit of growth. In some allied 
planarians the asexual multiplication is effected not by bud- 
ding but by division. Zacharias observed that when nutri- 
tion was checked the vegetative increase ceased, and sexual 
reproduction set in. Not quite parallel with the above, but 
quite asexual, is the prolific multiplication characteristic of 
the flukes and tapeworms. The common liver-fluke has often 
several asexual generations before it finds its final hosts in 
the sheep, and is surpassed in this respect by some of its 
relatives. The bladder worm, in passive ease, with a plethora 
of nutrition, may form asexually many "heads," each of 
which, inside a future host, grows out into the long series of 
joints which compose the tapeworm. In their profuse asex- 
ual multiplication these parasites are like parasitic fungi, but 
unlike them in the retention of the sexual process to boot. 

In their asexual reproduction, the Polyzoa recall sponges, 
for not only do they all multiply by budding, and that abundantly, but they form 
peculiar winter-buds like sponge-gemmules, by which on the death of the parent 
the continuity of life is nevertheless sustained. The winter-buds or statoblasts 
may further resemble sponge-gemmules in elaborateness of ex emal equipment, 
a common characteristic of passive resting structures. 




Fig. 56. — .Diagram- 
matic representa- 
tion of the forma- 
tion of a chain of 
individuals in the 
Turbellarian worm 
(Microsioinum lin- 
eare). — From IjCU- 
nis. 




Fig. S7-— a Sea-worm (Myrianida) which has budded off a chain of individuals. 

— After Milne-Edwards. 



I 82 



THE EVOLVriON OF SEX. 



In the liiglier bristle-footed worm-type (Clurtopoda), asexual multiplication 
occurs in great variety of expression. Some, when alarmed, break up in a 
]5anic, but a few are also known to do this in apparently normal life. Each 
part — there may be more than two — reproduces the whole. Thus, at a com- 
paratively high level among animals, reproduction may be literally rupture. 







Fig, 58, 



. — SyUis rajiwsa, a ringed marine worm, in which asexual multiplication has produced 
a branched appearance. — From M'Intosh, "Challenger" Rep. on AinicUda. 



Oftener, however, budding precedes the division, and curious chains of ringed 
worms are thus produced. Nor do the budded individuals always keep in a 
straight line, but, as in the freshwater naids, may abut at angles, and form a quaint 
living branch. To what degrees this irregularity of budding may attain is well 
seen in the accompanying cut of a portion of a worm (Syllis ramosa) found 




Fig. 59.— Comet form of a Starfish, showing how one arm "regenerates" or 
reproduces other four.— From Carus Sterne, after Haeckel. 

on the " Challenger " voyage. The buds occur laterally, terminally, or on any 
broken surface, and the result is an almost bush-like compound organism 
rivalling even the hydroids in the freedom of its branching. Some of the 
branches becomes males or females, and go separate, or are sent adrift. In 



ASEXCAL REPRODUCTION. 



183 



other syllids, the separation of a series of joints as a sexual individual has been 
repeatedly observed, or this may be reduced till only one joint, laden with 
reproductive elements, is set free. In many of these chaetopods the budding 
begins when the normal size of the individual has been stopped by unfavorable 
conditions, which bring about separation, and the subsequent sexuality of the 
liberated individuals. 







Fig. 60. — Adventitious buds forming at the sides of a leaf of 
BryophylhiDZ calyciituin. — From nature. 



Starfishes and the like surrender their "arms" so readily, that some have 
supposed that they might, in this way, normally multiply. A voluntary 
surrender of parts as a mode of multiplication is, liowever, in this case difficult 
to prove. So while crustaceans, insects, spiders, and mollusks may lose and 
regrow certain parts, no asexual multiplication occurs. 

In the tunicates the asexual process has again full play. It is not confined to 
the passive sessile fonns, where one might expect it, but occurs in some of the 
free-swimmers as well. From a creeping stem buds may arise, like plants from 
a rhizome ; or a parent form may bud off all round, and finally die away, 
leaving the oifspring in a circle round a cavity. Both by budding and division 
chains may be formed, as in the salpas. In these lowly vertebrates asexual 
multiplication terminates. How the process often alternates in regular rh}'thm 
with ordinarj- sexual reproduction will be discussed in the next chapter. 



1 84 



THE E VOL UTION OF SEX. 



SUMMARY. 

I. Artificial division may be readily utilized as a means of multiplication in 
plants and in lower animals. 

II. Regeneration of lost parts is very common both in plants and animals. 

III. Asexual reproduction from continuous budding to discontinuous 
multiplication has many degrees, leading on to the sexual process. 

IV. It occurs throughout the series from Protozoa to Tunicata. 



LITERATURE. 

General Works cited ; the ordinary Zoological and Botanical Text-books ; 

and, 
Lang, A. — Der Einfluss des Festsitzen auf den Thieren, und der Ursprung der 

ungeschlechtlichen Fortpflanzung. Jena, i8S6. 
Spencer. — Principles of Biology. London, 1866. 
Haeckel. — Generelle Morphologie. Berlin, 1866. 
' Fredericq.— La Lutte pour 1 'Existence chez les Animaux Marins. Paris, 

1889. (For "Regeneration of Parts," &c.) 



AL TERN A TION OF GENERA TIONS. 



185 



CHAPTER XV. 
ALTERNATION OF GENERATIONS. 



I. History of Discovery. — Early in the century the poet 
Chamisso, accompanying Kotzebue on his circumnavigation of the 
globe, observed in one of the locomotor tunicates {Salpa) that a 
solitary iorm gave birth to embryos of a different character, con- 
nected together in chains, and that each link of the chain again 
produced a solitary form. Chamisso' s observation does not seem 
to have been quite accurate, but 
there is no doubt that he first called 
attention to what is by no means an 
uncommon fact, that an organism 
produces an offspring very unlike 
itself, which by and by gives origin 
to a form like the parent. The pro- 
gress of inarine zoology and the 
study of parasitic worms gave natu- 
ralists like Sars, Dalyell, Loven, Von 
Siebold, and Leuckart, early gUmpses 
of many alternations in life-history, 
but Steenstrup was the first to gen- 
eralize the result. This he did (1842) 
some twenty years after Chamisso, 
in a work entitled "On the Alterna- 
tion of Generations; or. The Propa- 
gation and Development of Animals 
through Alternate Generations, a 
peculiar form of fostering the young 
in the lower classes of animals." 
From hydroids and flukes, he gave 
illustrations of the ' ' natural phe- 
nomena of an animal producing an offspring, which at no tmie resem- 
bles its parent, but which itself brings forth a progency that returns m 
its form and nature to the parent." The interpolated generation he 
distinguished by the name of " Amme " or "wet-nurse." In 1849, 
Owen submitted Steenstrup' s essay to stern criticism, rejecting espe- 
cially the metaphorical name "nurse" as but a verbal explanation, 
and proposing to explain what he also called ' ' alternation of genera- 
tions " along with parthenogenesis and other phenomena, by the 
supposition of a residual germ-force or spermatic power in the cells 




Fig. 61. —Diagrammatic representation of 
alternation of generations, as, asexual 
generation ; s, sexual generation. 

II. Shows alternation of asexual (as) 
and sexual {s) generations. 

In I. the sexual is becoming increas- 
ingly subordinated to the asexual <asiii 
flowering plants). 

In III. the asexual is increasingly 
subordinated to the sexual (in mosses). 



1 86 1'^Ii EVOLUTION OF SEX. 



of the apparently asexual offspring. In this he partially prophesied 
the modern conception of a residual persistent germ-plasma. Soon 
afterwards Leuckart attempted to treat all as cases of metamorphosis, 
thereby greatly extending the meaning of that term. The labors of 
some of the foremost naturalists have both extended Steenstrup's 
observations and rendered them more precise. We now know that the 
phenomenon is of wider occurrence than was at first supposed, and also 
that the title has been unduly extended to cover several entirely differ- 
ent sets of facts. It is necessary, therefore, to notice the various forms 
which the rhythm of reproduction may take. 

II. The Rhythm between Sexual and Asexual Reproduc- 
tion. — The clearest case to start with is that of many hydroids. A 
sessile plant-like zoophyte, which buds off numerous nutritive persons, 
produces in the warm months modified individuals which are set 
adrift as medusoid persons. Unlike the hydroid which bore them, 
these become sexual; and from their fertilized ova and embryo 
develops, which eventually settles down to start a new sessile colony. 
And thus through the seasons we have hydroid asexually producing 
sexual medusoids, and these again producing hydroids. The life- 
history for two complete rhythms may be written in the formula, 
in which M, F, and A stand for male, female and asexual forms 
respectively, — 

-F ^ F 




Fig. 62. — A, asexual hydroid: S, sexual medusoid; fertilized ova at base. 

Or take, in slight contrast, the life-story of the common jellyfish 
Aurclia. Large free-swimming sexual animals produce ova which are 
fertilized by sperms; the embryo develops, not however into a jelly- 
fish, but into a sessile hydroid-like organism or "hydra-tuba." By 
growth and division this asexually produces the jellyfish in turn. 
Here the sexual generation is more stable and conspicuous, the 
reverse of the former case, but the same formula apphes. 

Or take a case from another class of animals, the marine worms. 
Some of the sylids have the following life-history. A worm remains 
asexual, never attaining either the external or internal organs of the 
sexual individuals. It gives rise to these, however, by an asexual 



ALTERNATION OF GENERATIONS. 



187 



process or chain-making. Sexual individuals are budded off from 
the asexual, into which their fertilized ova in turn develop. This 
must, of course, be distinguished from cases where asexual multipli- 
cation is only a phase preceding the acquisition of sexuality. The 
above cases are again expressible in the simplest formula. 

{b) Now take a more complex case, from among the tunicates, 
the highest point at which the genuine alternation can be said to occur. 




Fig- 6:;.— I'he alternation of generations in the common jellyfish Anrclia; i, the free-swimming embryo, 
or pianola; 2. the embryo settled down; 3, 4, 5, 6, the developing ase.Kual stage, nr hydratuba: 7, 
8, the formation of a pile of individuals; 9, the liberation of these; 10, 11, the aquisition of the free- 
living se-vual medusa form. — From Haeckel- 



From a fertilized ovum in Salpa, a nurse or asexual individual 
develops. This has a root-like process or stolon, on which buds 
are formed. These are set free together, and form a chain of sexual 
salps. The chain finally breaks up. The fertilized ova of the 
sexual salps grow up into nurses again. Now the only emphatic 
complication here is the liberation of a chain of individuals at once; 
otherwise the formula holds perfecdy good. 

In the allied Dolioliim, however, the case is different. From a 
fertilized ovum a nurse, or asexual individual, develops as before. 
This produces a number of primitive buds, which cluster about the 
nurse. Many of them form nutritive individuals, and these we may 
leave alone. But others become "foster-mothers," and go free, 
carrving with them a few of the primitive buds, — as it were their 
younger sisters. The foster-mother remains asexual, is a bearer 



1 88 THE EVOLUTION OF SEX. 

merely, and need not further complicate the series. But the primitive 
buds which have been carried away give rise asexually to secondary 
buds; these become sexual, and their fertilized ova give rise to the 
original ' ' nurse ' ' forms. There are therefore several asexual gen- 
erations between the sexual, and our formula must run, — 




Fig. 64. 



III. Alternation between Sexual and Degenerate Sexual Reproduction. — 

The cases we have just noticed are both easier to state and easier to explain 
than others which are sometimes also included under the vague title of 
" alternation of generations." The above alternations were between sexual and 
asexual reproduction ; these must be distinguished, vague as the boundary 
must be, from alternation between the ordinary sexual process and a degenerate 
form of the same. 

The adventurous histoiy of some of the flukes [Trematoda) m.a.y be taken 
as a first illustration. The common liver-fluke {Distoinuni or Fasciola hepatica) 
which causes the disastrous "rot" in sheep has a life of vicissitudes. The 
fertilized ovum gives rise to an embi-yo, which passes from the sheep, which its 
sexual parent infested, to the ,vater by the field side. There it leads for a while 
an active life, knocking against many things, but finally attaching itself to a 
minute water-snail. Into this it bores, losing its covering of active cilia with 
change of habit, and becoming much altered into a passive vegetative form 
known as a sporocyst. Now this sporocyst sometimes divides ; and if this were 
all, and the results grew up into liver-flukes, we should have the old formula 
and less loss of sheep. But direct development never occurs, and we may leave 
the casual division at present out of account. Certain cells within the sporocyst 
form germs, and these serve in the place of genuine ova. They produce 
within the body of the sporocyst another brood of what are called Redics. 
There may be several generations of them, and the final result is a brood of 
minute tailed organisms (Cercariaf), \\\\\c\\ leave the water-snails, leave the 
water even, creep up grass-stems, and encyst themselves. There most wait for 
death, a few for the attainment of adult sexual life if they chance to be eaten by 
a sheep. The somewhat complex story may be written in lines : — 

The fertilized ovum gives rise to an aquatic embryo (I). 

This enters a water-snail, and becomes a " spoi-ocyst." 

(The sporocyst may divide.) 

Within the sporocyst, cells develop into " Redia;" (11). 

There may be several generations of redia (III., IV). 

The last generation ( Cercaricc) may become adult sexual liver-flukes (V). 

This can not be accurately ranked as parallel to what occurs among the 
above-mentioned tunicates, for the redia: arise from precocious reproductive 
cells. These can not be called ova, and there is no fertilization, but yet the 



ALTERNATION OF GENERATIONS. 189 

process is not one of division, or of budding. It is a degenerate process of 
parthenogenetic reproduction in early life. The facts may be again summed up 
in a formula, which does not take account of the occasional division of the 
"sporocyst." 




Fig. 65. — A, asexual larvae ; S, sexual fluke ; the upper circles 
represent the special germ-cells ; fertilized ova at the base. 

The germ-cells, which behave like ova, and yet do not rise to that level, 
appear sometimes in a central mass within the asexual individual, sometimes 
simply in the epithelium lining the body walls. There may be a long series of 
generations producing and produced in tjjis way, and these are often unlike one 
another. Fluke, embryo, sporocyst, redia, and cercaria, are all markedly 
different in structure, though embryo changes into sporocyst, and cercaria into 
fluke. 

This alternation between sexual reproduction with the usual fertilization, and 
reproduction by means of special cells which yet require no fertilization, prevails 
in many plants, for example, ferns and mosses. From a fertilized egg-cell the 
ordinary fern-plant, with which everyone is familiar, develops. But this is quite 
asexual, if we mean by that that it is neither male nor female, and that it 
produces neither male nor female elements. At the same time it produces 
special reproductive cells, — not egg-cells exactly, any more than those within 
the sporocyst were, but yet able to develop of themselves into a new organism. 
This is not another fern-plant, however, but an inconspicuous green organism, 
much less vegetative, and sexual. The so-called "spore" formed on the 
leaves of the sexless fern-plant falls to the ground, develops a "prothallus," 
which bears male or female organs, or both. An egg-cell is fertilized by a male 
element, and the conspicuous vegetative fern-plant once more arises. The 
formula is therefore as follows : — 




Fig. 66.— Where A = sexless vegetative fern-plant ; 

sp. = the parthenogenetic special reproductive cell or spore : 
S — the sexual inconspicuous "prothallus," with male and female organs. 

Now take the history of a moss. Unlike the fern, the more conspicuous 
" moss plant " is sexual. It bears male and female organs, and an egg-cell is 
fertilized by a male element. The fertilized egg-cell, however, does not lose its 
hold of the mother plant, but grows like an encumbering parasite upon it. 
Obviously, then, it does not give rise to another " moss-plant." The result of 
the fertilized eg.g-cell is a tiny sexless stalk, which bears on its apex the special 
reproductive cells or spores with which we are now familiar. In other words. 



I go THE EVOLUTION OF SEX. 



the fertilized egg-cell develops into a parasitic spore-bearing generation. 'Hie 
"spores" fall into the ground, as they did m the fern, and there grow mto a 
usually thread-like structure, from which the sexual moss-plants are budded off. 
If we do not emphasize the transitional thread-like stage, — the protonema as it 
is called, — the formula is as follows (see also fig. p. 1S5): — 




Fig. 67. — Where A ^inconspicuous sexless parasitic generation upon the "moss-plant" 
sp. =^ the special parthenogenetic reproductive cell or spore produced by A. 
S = the conspicuous se.\ual '*moss-plant," budded from the threads developed 
from the spore. 



If we do emphasize the "protonema " stage (/>), and regard the moss-plants 
as asexually budded from it, the formula runs : — 




In the fern, the vegetative se.xless generation was the more conspicuous ; in 
mosses, the sexual generation. In a way this recalls the contrast between the 
life-history of many a zoophyte, and that of the common jellyfish Aiirelia. The 
asexual hydroid colony is more conspicuous than the usually small swimming- 
bell, but the se.xual jellyfish is much more conspicuous than the minute asexual 
"hydra-tuba." The common comparison between medusoid and hydroid on 
the one hand, and prothallus and fern-plant on the other, is rather misleading, 
simply because the hydroid merely buds off the medusoid, while the fern-plant 
produces the prothallus from a special reproductive cell or spore. In some ferns 
and mosses, however, a more exact parallel is occasionally exhibited. The 
production of "spores" may be suppressed, and from the place where they 
should be formed a (sexual) fern-proth alius or a new (sexual) moss-plant is 
vegetatively developed, just as medusoid from hydroid. This exceptional 
occurrence is technically called apospory. The very opposite of this also 
occurs, the suppression not of the spore-bearing, but of the sexual generations. 
The fern-plant then arises vegetatively from the prothallus ; and this would be 
paralleled if we supposed the sporocyst of the fluke to bud offredias(as it some- 
times does), and these to continue the species without ever becoming really 
sexual, solely by means of the special cells above described. 

IV. Combination of both these Alternations. — The asexual hydroid buds 
off a medusoid, the fertilized ovum of^hich develops into a hydroid. Here 
there is simple alternation between sexual and asexual reproduction. 



ALTERNATION OF GENERATIONS. 



191 




Fig. 69. 



A sexless fern-plant forms special reproductive cells (spores), which develop 
parthenogenetically into a sexual prothallus, from the fertilized egg-cell of which 
the fern-plant arises. 




Fig. 70. 



The diiTerence between these two alternations has been as often pointed out 
as it has been ignored. The former is called true alternation of generations (or 
metagenesis) ; the latter is called by zoologists, in reference to flukes for 
instance, hetcrof^amy. Comparisons between the alternations in plants and 
animals have seldom recognized the distinction. 

Let it be recognized, however, and we can readily proceed to more 
complicated cases where the two are combined. Returning to the liver-fluke 
and others like it, we find that the sporocyst sometimes multiplies in a genuinely 
asexual fashion — without the intervention of precocious ova, special repro- 
ductive cells, germs, or spores, call them what you will — by direct division 
or budding. For such cases the formula must be modified as follows : — 




Fig. 71. 



The complication is not serious. It is simply that, before the multiplication 
by special cells sets in, there may be more than one (A', h") entirely asexual 
(and not merely sexless) generation. 

V. Alternation of Juvenile Parthenogenetic Reproduction with the Adult 
Sexual Process. — We have already noted the curious precocity of some 
midge larvae, which reproduce while still young. Cells within the body, 
apparently precocious ova, develop parthenogenetically into larva;, which prey 
upon the mother larva, eventually kill her and leave her, only themselves to 
become in turn similar victims of precocih'. This mav continue for a series 01 
generations, with continuous decrease in the size of the reproductive cells, till 
finally true se.xuality and adult life is attained. The reproductive cells here are 



ig: 



THE E VOL UTION OF SEX. 



rather more difterentiated than those in the young flukes, but the close 
parallelism is indubitable. Except that there is for a while no fertilization, the 
process can hardly be called asexual. The formula may be expressed in a 
gentle curve ; — 




Fig. 72. — Where the starting-point is as before a fertilized ovum; 
L ^ prematurely reproductive larva ; 
ps ^ precocious parthenogenetic "pseudova" : 
S = adult sexual male and female organism. 

Somewhat different is the curious case of Gyi-odactyhts, a trematode 
parasitic on fresh-water fishes, where three generations are foumd inclosed, one 
within the other, in a fashion which recalls the fancies of the preformationists. 
In this case, however, it seems likely that internal fertilization really occurs. 

VI. Alternation of Parthenogenesis and Ordinary Sexual Reproduction. — 
In our gradual ascent, we now reach the frequent alternation of parthenogenesis 
and ordinary sexual reproduction. The special cells which develop without 
fertilization are now genuine parthenogenetic ova, and the organisms which 
produce them are adults, not juveniles. The formute will differ mainly in the 
number of generations through which the parthenogenesis may be continued...^ 




Fig. 73. — Where the starting-point is a fertilized ovum. 

P ^parthenogenetic, female, producing a parthenogenetic 

ovum, from which arise other parthenogenetic fomw, 

or eventually 
S = Male and female. 



VII. Alternation of Different Sexual Generations. — The rhythm may be 
followed in yet a higher scale. In a very few cases there is an alternation 
between two different sexual generations. Thus one of the threadworms 
(Leptodera appcndiculata) found in the snail gives rise, by the ordinary sexual 
process, to a different form, which leads a free life, and subsequently gives 
origin to the parasite. In both generations the sexes are distinct. More 
remarkable still is the history of another nematode (Angiostomum nigrove- 
nosiim), found in the lung of the frog. It is physiologically hermaphrodite, 
though its organ is ovary-like ; its eggs are fertilized by its own sperms, which 
mature finst ; the progeny become sexual — males and females — in the earth, 
and their offspring return to the frog, where they become hermaphrodites. 
Another example of alternation of sexual generations is found in one of the 
threadworms which occur in man {Rhabdonema sirongyloides). 



ALTERNATION OF GENERATIONS. 



193 



VIII, Occurrence of these Alternations in Animals. — From sponges to 
tunicates such alternations occur. Beyond the latter, unless we wish to be very 
subtle, they cease. It is necessary to be clear about the fact that asexual and 
sexual reproduction may occur together in the same form. The common hydra 
gives off buds in an entirely asexual way, but it is also a sexual animal, with 
male and female organs. They may be periods of vegetative growth and 
climacterics of sexuality in the' same organism, without any alternation of 
generations. 

It is possible that the term alternation of generations may be applied to some 
of the phenomena observed in the Protozoa. Thus Brandt maintains that all 
the colonial radiolarians, known as Spharozoa, form on the one hand isospores, 
which are all equal and apparently parthenogenetic, and on the other hand 
anisosporcs, which are large and small, — in fact, sexually dimorphic. He 
believes — though the fact can not be called demonstrated — that two unequal 
anisospores unite to form a double cell, a fertilized unit, which will produce 
isospores again, and these the normal colony. The generation of these 
sphffirozoa is further complicated (a) by division of the colonies, {b) by division 
of '.the individuals of young vegetative colonies, and (c) by the formation of 
special " extra-capsular " reproductive bodies in young colonies. 

The history of the common fresh-water sponge {Spongilla), as told by 
Marshall, is one of many vicissitudes. In autumn the sponge begins to suffer 
from the cold and scarcity of food. It dies away ; but some of the units save 
themselves, and, in a sense, the parent, by forming the " gemmules " we have 
already noticed. These winter in a quiescent state within the parental corpse, 
but in spring they get out of the debris, and start male or female sponges. 
The males are short-lived, but their male elements fertilize the ova of the 
females. The fertilized ovum develops into a ciliated embryo, and this into an 
asexual sponge, which produces the gemmules. 




Fig. 74. — The starting-point a fertilized ovum, which develops into 
A = asexual sponge, which forms only 
G^ gemmules, which develop into 
S = male and female sponges 

Besides the hydroid and medusoid, the hydra-tuba and jellyfish alternations, 
which we have already noticed, there are many complications of degree among 
coelenterates. The medusoid stage degenerates by subtle gradations, ceasing 
to be free, and eventually becoming what, if its history were not known, would 
be called an organ rather than a "person " of the colony. Furthermore, it may 
itself take to budding, and continue the asexual habit of the hydroid from which 
it springs. Outside the Hydrozoa, genuine alternation of generations does not 
occur, unless that described by Semper for Fungia corals be accepted as such. 

A very interesting alternation has been recently described by W. K. Brooks 
in a remarkable medusa {Epenthesis macradyi). On the reproductive organs 
of this swim-bell there grow, like parasites, what are exactly comparable to the 
reproductive buds (blastostyles) of a hydroid, and these form medusoids by 
budding. The result is a compound colony, v\-hich approaches the Siphonophora. 
The process recalls and surpasses the apogamy of a few ferns. 



jt)^ THii i-j-oi.rrroN of si-x. 

Anions typf-wiimis, the strict alternation of generations in some of the 
marine chcetopods (sylids), the more compHcateci phenomena of so many 
trematodes, the sexual rhythms of that peculiar threadworm Angios/oiiuiiH, 
have been already discussed. It is necessary, however, to state the case for 
tapeworms, which are usualh- included among the examples of alternation of 
generations. The usual view is, that the embryo of a tapev\'orm develops into 
an asexual bladder-worm, which asexually buds off a "head," or more than 
one. Such a " head," passing to another host, buds off asexually the chain of 
reproductive joints or sexual individuals which constitute a tapeworm. Asexual 
bladder-worm, asexual "head," and sexual joints, form the series. That there 
is a genuine alternation of generation is believed by some authorities, but there 
are emphatic difficulties against this supposition, except in the occasional 
occurrence of a bladder-worm with several " heads, " each of which may 
develop into a tapeworm. The case is well stated by Hatchett Jackson in his 
monumental edition of RoIIeston's "Forms of Animal Life," and we accept his 
Terdict that there is really one individual throughout, excejjt when asexual 
multiplication of heads occurs. The tapeworm, on this view, is an adult sexual 
bladder-worm, and the joints are only highly individualized segments. 

Of the parthenogenetic cycles in crustaceans and insects, the juvenile repro- 
duction of some of the latter, and the true alternation of generations in some 
tunicates, enough has already been said. 

Von Jhering is responsible for starting the paradox that in higher animals a 
mother may bring forth her grandchildren. He refers to the case of the hyffina- 
like carnivore Praopus. where a single ovum gives rise to eight embryos, which 
are thus in a pedantic sense grandchildren ! The frequent occurrence of twins 
in all groups, the remarkable case of an earth-worm {Liimbricus trapezoides) in 
which a double embryo is constant, and the morphological resemblance of 
polar globules to abortive germs, led Von Jhering to maintain that the origin of 
multiple embryos from a single ovum is the primitive and normal condition, and 
that the development of only one is secondary and adaptive. The data are 
hardly sufficient for such a striking conclusion. 

IX. Occurrence of Alternations in Plants. — In the lower 
plants, algae and fungi, an alternation between spore-producing and 
truly sexual generations is frequent. In mosses and ferns it is almost 
constant, and yet more marked. Occasionally either spore-formation or 
sex-cell formation may be suppressed, and the life-history thus simpli- 
fied. In a few of the higher plants both are exceptionally suppressed, 
and we have thus a reversion to a purely vegetable process, just as if 
a hydra went on giving off daughter-buds without ever becoming 
sexual. In the flowering-plants, what corresponds to the sexual 
generation of a fern is much reduced; it has come to remain continuous 
with the vegetative asexual generation, on which it has reacted in 
subtle physiological influence. Just as in the higher animals, alter- 
nation of generations finds at most only a rudimentary expression. 

X. Heredity in Alternating Generations. — The problem of 
the relative constancy of inheritance is now in part solved by the 
theory of germinal continuity. The ovum which develops into an 



AL TERN A TION OF GENERA TIONS. 



I9f 



oftspring is virtually continuous, either in itself or through its nucleus, 
with the ovum which gave rise to the parent. A chain of ovum-like 
cells is only demonstrable in a few cases; but Weismann overcomes 
this difificulty by supposing that what really keeps up the protoplasmic 
tradition or continuity between the parental ovum and the next 
generation is a specific and stable portion of the nucleus — the ' ' germ- 
plasma. ' ' When a medusoid goes off from a hydroid, it carries with 
it a legacy of this germ-plasma, continuous with that which gave rise 




Fig. 75.— The hermaphrodite fern prothallus contrasted with (2 a) the male and (2 t) 
the female thallus of liverwort, and (3 a and b) male and femaJe prothal- 
lus of horsetail. Above are the corresponding reductions of the sexual 
prothallia in (4) Salvinia, (5) Isoetes, (6) Cycad and Conifer, and (7) 
Phanerogam. 



to the hydroid. This legacy forms the reproductive elements of the 
medusoid, which in turn give rise to hydroids. The medusoid itself 
is a modified asexual growth, into which some of the germ-plasma of 
the hydroid has migrated; it is literally only the bearer of the hydroid 
germ -plasma. Weismann' s classic researches on hydroids have 



igS THE EVOLUTION OF SEX. 

shown that the reproductive cells, which by hypothesis bear the germ- 
plasma, often arise far down in the hydroid body, and actually migrate 
to their final seat in the bearer. Where the alternation is not between 
sexual and asexual, but between the ordinary sexual process and 
multiplication by special parthenogenetic cells, as is the case in many 
flukes, we are in the same way bound to suppose that the cells within 
a sporocyst which give rise to rediae are, like ova, charged with this 
reproductive germ-plasma. It is very interesting to notice that, as far 
back as 1849, Owen had a distinct prevision, not only of the distinc- 
tion between body-forming cells and reproductive-cells, of which so 
much is now made, but of the essential idea of the "germ-plasma." 
Speaking of the recurrence of a parental form after numerous 
interpolated generations, he says, "the essential condition is the 
retention of certain of the progeny of the primary impregnated germ- 
cell, or, in other words, of the germ-mass unchanged in the body of 
the first individual developed from that germ-mass, with so much of 
the spermatic force inherited by the retained germ-cells from the 
parent-cell or germ-vesicle as suffices to set on foot and maintain the 
same series of formative actions as those which constituted the 
individual containing them." In this somewhat overweighted sentence, 
if we read "germ-plasma" instead of " spermatic force," we have a 
close approximation to the modern conception of Weismann. So 
again, he says, " an impregnated germ-cell imparts its spermatic power 
to its cell-offspring; but when these perish, or when the power is 
exhausted by a long descent, it must be renewed by fresh impreg- 
nation. But Nature is economical, and so long as sufficient power is 
retained by the progeny of the primary impregnated vesicle (the 
essential part of an ovum), individuals are developed from that progeny 
without the recurrence of the impregnating act. ' ' 

XI. Hints as to the Rationale of Alternation. — We shall 
have to take a fresh view of alternation of generations after the general 
theory of growth and reproduction has been discussed; meanwhile, 
however, the physiological aspect of the facts may be simply indicated. 
A fixed hydroid contrasted with a swimming-bell or medusoid, a 
sessile hydra-tuba contrasted with an actively locomotor jellyfish, 
illustrate not a peculiar antithesis, but a most general and fundamental 
rhythm of organic Hfe, — that between nutrition and reproduction. 
The hydroid has a relatively passive habit and a copious nutirtion; it 
is preponderatingly vegetative and asexual. The reverse habit, the 
physiological rebound, finds expression in the medusoid. In the 
same way, though the alternation is less strictly between asexual and 
sexual, the contrast between leafy spore-bearing fern-plant and incon- 



ALTERNATION OF GENERATIONS. 197 

spicuous sexual prothallus is again fundamentally parallel. The 
notation adopted must have already suggested our fundamental 
diagram, the different forms of which may be separated out or 
superposed: — 





Nutrition Reprodnctioa 

A A 

Aoabolism. Katabolism. Female. Muln 

Fig. 76. 

Although it has just been shown that the process of alternation 
demands a much more thorough analysis and discrimination of the 
different cases than has hitherto been customary, and this on the 
physiological as well as merely on the morphological side, the general 
aspect of the process, in which an asexual form alternates with one or 
more dimorphic sexual generations, makes it evident that we have 
here to do in two generations with what is often so obvious in one — 
the familiar antithesis between nutrition and reproduction. A consid- 
eration of the physiological distinctions between the asexual and 
sexual generations, shows that the former is the expression of favorable 
nutritive conditions resulting in vegetative growth, or at most in 
asexual multiplication, while the latter is conditioned by less propitious 
circumstances. Just as a well-nourished plant may continue propa- 
gating itself by shoots and runners, and just as an aphis in artificial 
summer may for years reproduce parthenogenetically, so a hydroid 
with abundant food and otherwise favorable environment may be 
retained for a prolonged period vegetative and asexual, while dearth 
of food and otherwise altered conditions evoke the appearance of the 
sexual generation. The contrast between the deeply-rooted well- 
expanded fern-plant and the weakly-rooted slightly-exposed 
prothallus, is obviously that between an organism in conditions 
favorable to the continuance and preponderance of anabolic processes, 
and an organism in an environment where katabolism is, at an early 
stage, likely to gain the ascendant. The former is thus naturally 
asexual, the latter sexual. A survey, in fact, of the conditions and 
characteristics of the two sets of forms, inevitably leads us to regard 
the asexual generation as the expression of predominant anabolism, 



igy 



THE EVOLUTION OF SEX. 



and the sexual as equally emphatically katabolic. Alternation of 
generations is, in fine, a rhythm between a relatively anabolic and 
katabolic preponderance. 

XII. Origin of Alternation of Generations. — Even in an individual plant or 
animal there are vegetative and reproductive periods ; alternation of genera- 
tions involves the separation of these to different individuals, by the interpola- 
tion of more or less asexual reproduction. In most hydroids, the asexual 
vegetative tendency preponderates ; in most medusoids, the sexual repro- 
ductive dominates. But the origin in each particular case is involved in the 
pedigree of the (jrganism. Thus Haeckel distinguishes a progressive from a 
retrogressive origin ; in the former, the organisms are in transition from pre- 
ponderant asexual to sexual reproduction ; in the latter, the organisms are 
returning or degenerating froni dominant sexuality to an asexual process. It 
is safe to say that the latter is more frequently the right interpretation of the 
facts. So far as reproduction is concerned, one of those medusoids ( Trachy- 
incdiisir) which have no corresponding hydroid parent, or a jellyfish like 
Pclagia which has no fixed asexual hydra-tuba stage, is nearer the ancestral 
habit than those members of both divisions which exhibit alternation of 
generations. Where we have alternating series of similar forms with different 
degrees of sexuality, for example, the rhythm between parthenogenesis and 
true sexual reproduction in aphides, Weismann once interpretated the facts as 
associated with the periodic action of external influences ("Studies in the 
Theory of Descent," chap. v.). But in contrast to such cases he distinguished, 
[a.) an origin from metamorphosis, where one stage in the life-history becomes 
precociously reproductive, for example, in the midge Cccidomyia ; {d) the case 
of the Hydroinedusa:, where sexuality is postponed in early life, and asexual 
reproduction dominates ; and (c) an origin from division of labor w-ithin a 
colony. Without entering upon a discussion of each case in relation to its 
history and environment, it is not possible to do more than reassert that in 
many different degrees the continuous alternation between growth and multi- 
plication, nutrition and reproduction, asexuality and sexuality, anabolism and 
katabolism, may express itself in the life-history of the organism. 

Postscript. — From Mr. R. J. Harvey Gibson's valuable paper on "The 
Termynology of the Reproductive Organs of Plants" (Proc. Liverpool Biol. 
Soc, Vols. III. and IV.), we take the following scheme : — 

A. Asexual stage or sporophyte produces spores in sporangia (ovosporangia 
and spennosporangia in higher Cryptogams and Phanerogams). 

B. Sexual stage or gamophyte {oophyte and spermophytc where the thallus 
is unisexual), produces ova and sperms in ovaries and spermaries ; the product 
of union of ovum and sperm being oosperm. 



ALTERNATION OF GENERATIONS. 199 



SUMMARY. 

I. The fact that successive generations may be markedly different was 
observed by the poet Chamisso, and first made precise by the zoologist 
Steenstrup. 

II. A fixed asexual hydroid buds off and Uberates locomotor sexual 
swimming-bells, whose fertilized ova give rise again to hydroids. Asexual and 
sexual generations alternate. 

III. The offspring of the liver-fluke forms from certain cells in its body a 
numerous progeny ; these repeat the same process several times ; the last 
generation grow into the sexual liver-flukes. Reproduction by special cells like 
precocious undifferentiated ova, alternates with reproduction by ordinary 
fertilized ova. So, too, the vegetative sexless " fern-plant" gives rise to special 
cells like parthenogenetic egg-cells, which develop into an inconspicuous sexual 
prothallus. From the fertiHzed egg-cell of the latter the "fern-plant" arises. 

IV. These two different kinds of alternations (II. and III.) may be combined 
in a more complicated manner. 

V. In some flies precocious. parthenogenetic reproduction alternates with the 
normal sexual reproduction of the adults. 

VI. In many insects and crustaceans, parthenogenetic reproduction alter- 
nates with the normal sexual process. There may be one or many intervening 
parthenogenetic generations. 

VII. A hermaphrodite threadworm parasitic in the frog fertilizes its own 
eggs, which develop into free-living males and females, from the fertilized ova 
of which the hermaphrodite parasites again arise. Here there is an alternation 
of sexual generations. 

VIII. In animals these alternations occur from sponges up to tunicates. 

IX. In plants they occur in algffi and fungi, are almost constant in ferns and 
mosses, but are inconspicuous in higher plants. 

X. The problem of heredity is somewhat complicated by such alternations. 

XI. Alternation of generations is but a rhythm between a relatively anabolic 
and katabolic preponderance. 

XII. The origin has varied considerably in different cases. 



LITERATURE. 

5"^^ the general works already cited; also, Steenstrup "On the Alternation of 
Generations," transl. Ray Soc, 1845; Owen's " Parthenogenesis," &c., 
1849; Haeckel's " Generelle Morphologic," 1S66 ; Weismann, A., Die 
Entstehung der Sexualzellen bei den Hydromedusen, Jena, 1S83 ; and 
Papers on Heredity, Translation, Oxford, 1889; Vines' article "Reproduc- 
tion — Vegetable," Ency. Brit.; and the ordinary Text-books of Zoology 
and Botany. 



BOOK IV 



THEORY OF REPRODUCTION. 



GROWTH AND REPRODUCTION. 203 



CHAPTER XVI. 
GROWTH AND REPRODUCTION. 

I. Facts of Growth. — In a well-known aphorism Linnaeus 
noted that living organisms were not alone in their power of growth. 
Crystals become centers for other crystals, till a large mass results; 
and the product, as every case of minerals shows, is often both orderly 
and complex. But it can hardly be said that an inorganic body has 
any control over or credit in its growth, nor does the latter follow 
as the almost necessary consequence of previous waste or liberation 
of energy. It is one of the oldest generahzations that the growth of 
organisms has a pecuhar method of its own — that of intussusception 
as distinguished from mere accretion. The new particles which are 
taken in, more than replacing previous expenditure, are not deposited 
upon the surface of already established material, as is the case with 
a crystal, but are intercalated in the interstices of previous particles. 
It is, of course, unnecessary to enter here upon the long-continued 
controversy whether such structures as the cell-wall and starch-grains 
of plants grow thicker or larger by accretion in crystal-like fashion, 
or by intercalation which is supposed to be characteristically organic. 
It is worth noticing, however, as Biitschli points out, that if the living 
matter has the form of an intricate network, the fresh material of 
replacement or growth may be added to the surfaces of the threads 
which make the web. Thus what is roughly called intercalation may 
be more literally an internal accretion. 

Hunger is a dominant characteristic of living matter. When a 
unit-mass or cell has been giving off energy in doing any kind of 
work its substance is chemically impaired, — less capable of doing 
further work until new energy has been supplied by nutrition. Some 
have even maintained that a simple organism may be physically 
attracted to, as well as psychically by, its food. The supply which the 
lifelong hunger of the protoplasm demands, is frequently afforded in 
greater abundance than the actual necessities require. There is a 
surplus for further upbuilding after mere reparation has been made. 
This surplus is the condition of growth. Popularly, but yet accurately, 
it may be said that growth or addition to the capital of the organism 
occurs when income is in excess of expenditure, when construction 
preponderates over disruption. 

But beside this familiar fact it is necessary to place another 
certainty — that of the limit of growth. We may fairly call giants a 



204 



THE EVOLUTION OF SEX. 



few of the Protozoa, such as the large amoeboid Pelontyxa, some of the 
gregarines, and even more markedly the extinct nummulites, which 
were sometimes as large as half-crowns. So an occasional alga, like 
Botrydium, may swell out into a large single cell, and the ova of 
animals, for example, birds, are often greatly expanded by the accu- 



ooo 



oo 




Fig. 77. — Cell-division at the limit of growth. 



mulation of yelk. Yet the unit-masses generally remain very small. 
They have their maximum size, approximately constant for each 
species. Up to this point they grow, but no further. The same, as 
every one knows, is true of multicellular animals. The size fluctuates 
slightly according to the conditions of individual life, but the average 
is strikingly constant. 

II. Spencer's Theory of Growth. — The first adequate discus- 
sion of growth is due to Spencer. He pointed out that in the growth 
of similarly shaped bodies the increase of volume continually tends to 
outrun that of the surface. The mass of living matter must grow 
more rapidly than the surface through which it is kept alive. In 
spherical and all other regular units the mass increases as the cube of 
the diameter, the surface only as the square. Thus the cell, as it 
grows, must get into physiological difficulties, for the nutritive neces- 
sities of the increasing mass are ever less adequately supplied by the 
less rapidly increasing absorbent surface. The early excess of repair 
over waste secures the growth of the cell. Then a nemesis of growing 
wealth begins. The increase of surface is necessarily disproportionate 
to that of contents, and so there is less opportunity for nutrition, 
respiration, and excretion. Waste thus gains upon, overtakes, 
balances, and threatens to exceed repair. Suppose a cell to have 
become as big as it can well be, a number of alternatives are possible. 
Growth may cease, and a balance be struck ; or the form of the unit 
may be altered, and surface gained by flattening out, or very frequently 
by outflowing processes. On the other hand, waste may continue on 
the increase, and bring about dissolution or death ; while closely akin 
to this, there is the most frequent alternative, that the cell divide. 



GROWTH AND REPRODUCTION. 



205 



halve its mass, gain new surface, and restore the balance. Here, in 
fact, the famous law of Malthus holds good. 

III. Cell-division. — What usually occurs, then, at the maxi- 
mum or limit of growth, is that the cell divides. This, in its simplest 
forms, is rough enough to suggest rupture or overflow ; but in the 
vast majority of cases it is an orderly and definite process, in which 
the nucleus plays an important and probably a controlling part. By 
a complicated series of changes, both in form and position, the 
essential nuclear elements group themselves so as to form the daugh- 
ter-nuclei of each product of division. The orderliness and complexity 
of these changes forbid any offhand attempt to analyze the real 
physiological movement by which the growth of all multicellular 
organisms is effected. That attractions and repulsions do exist within 







Fig. 78.— Diagram of the changes in the nucleus during cell-division :— coil stage (o), 
the formation of a double star (i, c, d), and the recession of the divided 
chromatin elements to opposite poles ic) to form the daughter-nuclei (/) 
of the two daughter-cells. — From Hatschek, after Flemming. 



the cells is certain; an analysis of their precise nature — the final 
problem of histology — is still far in the distance. We can not get 
within miles of it. The problem has always loomed before embry- 
ologists and histologists — the historians and mechanicians of the 
organism. Pander, in the first quarter of this century, was inquiring 
into the mechanics of development, and Lotze followed him with some 
luminous suggestions. The task has been continued by His and 
Rauber ; while the experimental investigations of O. Hertwig, Fol, 
Pfliio-er, Born, Roux, Schultze, Gerlach, and others, have added 



2o5 



THE EVOLUTION OF SEX. 



further stepping-stones. Observers sucli'as Van Beneden and Boveri, 
in their masterly accounts of the morphological facts, have not left the 




Fig. 79. — Illustrating the Mechanism of Cell-Division, — (a) the chromatin 
or essential elements of the nucleus forming an *' equatorial 
plate" in the one figure, drawn toward the poles to form two- 
daughter.nuclei in the other: (b) the almost "muscular" 
threads ; (r) the protoplasmic center from which these radiate. 
— From Eoveri. 



problem of the actual dynamics unessayed ; while the title of Berthold' ' 
book on " Protoplasmic Mechanics," shows how the biologist persis- 
tently seeks the aid of the student of physics in his endeavor to explain 
the architecture of the living organism. 

IV. Protoplasmic Restatement. — In the above helpful sugges- 
tion, Spencer has emphasized the reasonableness and general necessity 
of cell-division at the limit of growth, refraining from the deeper 
question of the actual mechanism involved. In truth such cautious 
reserve must still be maintained, but Spencer's analysis admits of 
being expressed in lower and more definite terms. The early growth 
of the cell, the increasing bulk of contained protoplasm, the accumu- 



GROWTH AND REPRODUCTION. 207 

lation of nutritive material, correspond to a predominance of 
protoplasmic processes, which are constructive or anabolic. The 
growing disproportion between mass and surface must, however, imply 
a relative decrease of anabolism. Yet the life, or general metabolism, 
continues, and this entails a gradually increasing preponderance of 
destructive processes, or katabolism. As long as growth continues, 
the algebraic sum of the protoplasmic processes must of course be plus 
on the side of anabolism, and growth may be now more precisely 
defined as the outcome of the preponderance of an anabolic tendency, 
rhythm, or bias. The limit of growth, when waste has overtaken and 
is beginning to exceed the income or repair, corresponds in the same 
way to the maximum of katabolic preponderance consistent with life. 
The limit of growth is the end of the race between anabolism and 
katabolism, the latter being the winner. Thus cell-division occurs 
especially at night, when nutrition is at a stand-still, and when there 
is therefore a relative katabolic preponderance ; and so explorers have 
shown us that many marine algae reproduce during the darkness of 
the Arctic winter. 

What is true for the cell, is true for cell-aggregates. Organisms in 
their entirety have very definite limits of growth. Increase beyond 
that takes place at a risk, hence giant variations are peculiarly unstable 
and short-hved. Or again, just as the single cell has found, probably 
somewhat pathologically, a surface-gaining expedient in the emission 
of mobile processes, so many organs, notably leaves, have struck a 
balance between mass and surface by becoming split up into lobes and 
more or less discontinuous expansions. 

Spencer has laid great stress on the importance of the physiological 
capital with which the organism begins ; this represents, in active 
animals at least, the start which their anabolism gets at the outset. 
Other things equal, growth varies — (a) directly as nutrition; {U) 
directly as the surplus of nutrition over expenditure ; [c] directly 
as the rate at which this surplus increases or decreases; (d) directly 
(in organisms of large expenditure) as the initial bulk ; and {e) direcriy 
as the degree of organization, — the whole series of variables being 
finally in close reladon to the doctrines of the persistence of matter 
and conservation of energy. Some apparent excepdons are readily 
explained. Thus, many plants seem to grow indefinitely, but they 
expend very litde energy, and have often enormous surface-area in 
propordon to mass. The crocodile goes on slowly growing, though 
at a gradually diminishing rate, but it again expends relatively little 
energy in proportion to its high nutrition. Birds which expend most 
energy, have their size most sharply defined. 



2oS THE EVOLUTION OF SEX. 

V. The Antithesis between Growth and Multiplication, 
between Nutrition and Reproduction. — The life of organisms is 
conspicuously rhythmic. Plants have their long period of vegetative 
growth, and then suddenly burst into flower. Animals in their young 
stages grow rapidly, and as the growth ceases reproduction normally 
begins. Or again, just as perennial plants are strictly vegetative 
throughout a great part of the year, but have their stated recurrence 
of flowers and fruit, so many animals for prolonged periods are 
virtually asexual, but exhibit periodic returns of a reproductive or 
sexual tide. In some cases, such as salmon and frog, periods of 
active and preponderant nutrition are followed by times of fasting, at 
the end of which reproduction occurs. Foliage and fruiting, periods 
of nutrition and crises of reproduction, hunger and love, must be 
interpreted as life-tides, which will be seen to be but special expressions 
of the fundamental organic rhythm between sleep and waking, rest 
and work, upbuilding and expenditure, which are expressed on the 
protoplasmic plane as anabolism and katabolism. 

The common hydra, in abundant nutritive conditions, produces 
numerous buds, and even these sometimes begin themselves to bear 
another generation. In other words, we may almost say, with plenty 
of food the polype groivs abundantly, so obviously is this asexual 
reproduction continuous with growth. A check to the nutritive condi- 
tions, however, brings on the development of the sexual organs and 
the occurrence of sexual reproduction. In planarian worms, the 
asexual multiplication of which we have already noted, Zacharias 
observed that favorable nutritive conditions were associated with the 
formation of asexual chains, while a check to the nutrition brought 
about both the separation and the sexual maturity of the links. 
Rywosch corroborates this, noting in Microstomum lineare that the 
generative organs do not become completely matured till the indi- 
viduals cease to be links in a chain, and that the sexuality is hastened 
by outside influences such as checked nutrition. The gardener 
root-prunes his apple-tree, thereby checking nutrition to improve the 
yield of fruit, in other words, to augment reproduction. Reversely, 
the removal of reproductive organs may increase the development of 
the general "body" both in plant and animal,— witness the castrated 
ox, capon, &c., or the way in which the gardener nips off" the flower- 
buds from his foliage plants. Taking a further step, we recall the 
familiar and already repeated fact that favorable nutritive and other 
conditions enable the aphides to continue parthenogenetic through the 
summer months; but both for the common plant-lice and for the vine- 
msect phylloxera, it has been shown that a check to nutrition causes 
the parthenogenesis to cease, and is associated with the return of 



GROWTH AND REPRODUCTION. 



209 



sexual reproduction. The above instances are obviously not all upon 
the same plane. They illustrate, however, at different levels, the 
same great contrast. It is necessary, however, to become more 
precise. 

VI. The Contrast between Growth and Reproduction in 
the Individual. — {a) The DistribuHcm of Organs. — The general' 
position of the flower at the end of the vegetative axis is so obvious 
a fact that its import tends to be overlooked. The end of the 
axis is furthest from the source of nutritive supply; with exag- 





FlG. 80. — The Moonwort Fern (Botry- 
chijtm hinare), showing 
the contrasted frond (li), 
and fructification (3). — 
After Sachs. 



Fig. 81. — Diagram of the Tiger Lily, 
showing bulbils {a) in 
lower axils, and flower 
above. 



geration, we might call it the starvation-point. There, with katabolic 
conditions tending relatively to predominate, the reproductive organs 
are situated. The flower occupies a katabolic position, and is often 
the plant's dying effort. 

In the tiger-lily, growth at first tends to remain continuous, and 
the base of the bulb bears simple vegetative buds. Further up, how- 
ever, where nutrition reaches its maximum, the axils of the leaves 
contain buds, which are separable though still asexual. Finally, 



2IO THE EVOLUTION OF SEX. 



further up still, where nutrition is relatively less active and katabolism 
is maximized, the formation of flowers indicates the appearance of 
sexual reproduction. 

In many ferns, the contrast between the vegetative and reproduc- 
tive regions of the organism is as marked as in the flowering plant. 
Thus the moon wort (^Botrychium') and the adder's tongue (Ophioo;Ios- 
suni) have their spore-bearing shoots standing in conspicuous antithesis 
to the leafy portion, and a similar contrast is well seen in the royal 
fern (^Osmundd) and some of its allies. 

In animals, the contrast in position between reproductive organs 
and the general body is never so marked. Yet the generally posterior 
position of the organs, their frequent close association with the excre- 
tory system, their occasional rupture as external sacs, must not be 
lost sight of 

(b) The Contrast in the Individual Life. — Growth during youth, 
sexual maturity at the limit of growth, the continued alternation of 
vegetative and reproductive periods, are common-places of observation 
which require no emphasis. If growth and vegetative increase are 
the outcome of preponderant anabolism, reproduction and sexuality as 
their antitheses must represent the katabolic reaction from these. But 
anabolism and katabolism are the two sides of protoplasmic life ; and 
the major rhythms of their respective preponderance of these, give the 
familiar antitheses we have been noting. These contrasts of metabol- 
ism represent the swings of the organic see-saw; the periodic contrasts 
correspond to alternate weightings or hghtenings of the two sides. 
Yet the contrast is less than it seems. Ih previous chapters we have 
seen how growth, becoming overgrowth, turns into reproduction ; and 
how sexual reproduction, dispensing with fertilization, may degenerate 
till we know it no longer from growth. Reproduction, moreover, is 
as primitive as nutrition, for not only do hunger and love become 
indistinguishable in that equal-sided conjugation which has been curi- 
ously called "isophagy," but nutrition in turn is nothing more than 
continual reproduction of the protoplasm. Here, indeed, we have 
been anticipated by Hatschek, who clearly states the more than verbal 
paradox, that all nutrition is reproduction. 

VII. The Contrast between Asexual and Sexual Repro- 
duction. — In plenty, the hydra buds; in poverty, it reproduces 
sexually. In the same way, the liverwort on the flower-pot bears its 
pretty cryptogamic " flowers " when its exuberant growth and bud- 
ding have come to an end. On rich soil the plant has luxuriant 
foliage; but great abundance is the reverse of conducive to the richest 
crop of flowers and fruit. Gruber, Maupas, and others, have shown 
that abundant nutrition favors the asexual multiplication, that is, the 



GROWTH AND REPRODUCTION. 211 

di\'ision of infusorians. In other words, the maximum size is rapidly 
reached when food is abundant, but the conditions at the limit of 
growth bring about reproduction. Preponderant anabolism leads up 
to the possibility of multiplication, but we need the onset of katabolism 
to bring about the reproductive crisis. Gruber also notes that in the 
very reverse of favorable conditions, rapid division with diminution of 
size and resulting conjugation sets in ; and Khawkine observes the 
occurrence of division, both at an optimum and in famine. In both 
cases a katabolic crisis is associated with reproduction, though the 
crisis may be, and often is, preceded by an anabolic preponderance. 

In regard to a common infusorian {Leiicophrys patitla), Maupas 
observes that with abundant food the ordinary fission continues, but 
with scanty nutrition a metamorphosis occurs, followed by six succes- 
sive divisions, which have for their end conjugation. That is to say, 
we have positive proof that in these lowest organisms, katabolic condi- 
tions determine the beginning of sexual reproduction, a matter of no 
small importance to the evolutionist. Generalizing, M. Maupas 
concluded, that the reproductive power of ciliated infusorians depends, 
Ti) on the quality and quantity of the food ; (2) on the temperature ; 
(3) on the alimentary adaptation of the buccal organs. He also demon- 
strates that with a vegetarian diet their rate of asexual reproduction is 
much less, and the size smaller. Taking these facts, along with his 
important demonstration that the life of ciliated infusorians runs in 
cycles of asexual reproduction, necessarily interrupted (if the life of 
the species is to continue) by conjugation or sexual reproduction, we 
again reach the general conclusion that anabolic conditions favor 
asexual reproduction rather than sexual ; and that while preponderant 
anabolism is the necessary condition of the overgrowth which makes 
the asexual reproduction possible, the onset of katabolic preponderance 
is necessary to the act itself 

Semper quotes an interesting observation by Strethill .Wright, 
unfortunately somewhat vague, that certain polyps multiply abun- 
dantly in the dark by buds, while in the light, and with insufficient 
supplies of food, they bring forth sexual individuals or medusae. More 
precise is the fact, already cited from Zacharias, that the spontaneous 
asexual multiplication of planarians went on apace when the food 
supply was copious (anabolic condition), but if the amount of food 
was reduced or altogether withdrawn (katabolic condition) the asexual 
reproduction completely ceased. Bergendal reports that in the 
transverse division of another planarian worm {Bipalmnt), the severed 
links were all sexually immature ; and the results of Rywosch demon- 
strate the same antithesis between the sexual and the asexual process. 



212 THE EVOLUTION OF SEX. 

In the same way, sexual reproduction is contrasted with its degen- 
erate expression in parthenogenesis. The conditions of the latter in 
aphides and phylloxera are demonstrably anabolic, the normal sexual 
process recurs with the periodic return of hard times, or in relatively 
katabolic conditions. In the lower crustaceans, a similar contrast of 
conditions has also been observed. 




Fig. 82. — Pollen Grain; a, the two nuclei; b, the general proto- 
plasm ; c, the outer wall. — From Carnoy. 

It is again, on the present view, readily intelligible why in the 
exceptionally favorable anabolic environment of bacteria and many 
parasitic fungi sexual reproduction should be absent. Marshall Ward 
has pointed out, that the more intimate the degree of parasitism or 
saprophytism, the more degenerate the sexual reproduction. The 
greater the anabolism, in other words, the more growth and the less 
sexuality. That such comparatively complex organisms can continue 
their asexual reproduction, dispensing altogether with the acknowl- 
edged stimulus of fertilization, may probably be at least partially 
explained on the assumption that the abundant waste products of the 
host act as extrinsic stimuli. 

On this view, moreover, alternation of generations loses much of 
its uniqueness. The contrast between the vegetative asexual hydroid 
or hydra-tuba, and the active sexual medusoid or jellyfish, is very 
marked. So too, on a higher plane, the vegetative spore-producing 
fern-plant stands opposed to the less nutritive sexual prothallus. The 
alternation is but a rhythm of large amplitude between anabolic and 
katabolic preponderance. 

What is so marked in the alternation is only a specialization of the 
reproductive or sexual parts of the organism as against the growing or 
asexual ones, — a specialization which becomes exaggerated into sepa- 
rate existences, each dominated by its own physiological bias. 

In the fern or flowering plant the vegetative or asexual existence 
has preponderated, and this is entirely consistent with the character- 



GROWTH AND REPRODUCTION. 



istic passivity of plants. This is emphatically their line of development; 
but, be it observed, that though in the flowering plants the nutritive 
generation has dwarfed, and included the sexual, which seem indeed 
to be mere organs, — the pollen-grain and embryo sac, — yet it is 
through and for these that we have all the glory of the flower (see fig. 
p. 195). In animals, with their emphatically active line of develop- 
ment, the reproductive generation is the higher; and in the higher 
forms the separate asexual existence is wholly lost. 



214 '^'^^ EVOLUTION OF SEX. 



SUMMARY. 

I. Growth is characteristic of living organisms, though analogous processes 
occur at the inorganic level. Hunger is an essential characteristic of living 
matter. As certain as the fact of growth is the definiteness of its limit alike for 
cell and for organism. 

II. Spencer has analyzed the limit of growth, in terms of the continual 
tendency that increase of mass must have to outrun increase of surface. 

III. Cell-division at the limit of growth, at the maximum or optimum of size, 
restores the balance between mass and surface. The actual mechanics of the 
process are at present beyond analysis. 

IV. Spencer's analysis may be restated in protoplasmic terms. Growth 
expresses the preponderance of anabolism ; increase of mass, with less rapid 
increase of nutritive, respiratory, and excretory surface, involves a relative 
predominance of katabolism. The limit of growth occurs when katabolism has 
made up upon anabolism, and tends to outstrip it. What is true of the unit, 
applies also to the entire multicellular organism. 

V. Throughout organic life there is a contrast or rhythm between growth 
and multiplication, between nutrition and reproduction, corresponding to the 
fundamental organic seesaw between anabolism and katabolism. 

VI. This contrast may be read in the distribution of organs, in the periods 
of life, and in the different grades of reproduction. Yet nutrition and reproduc- 
tion are fundamentally nearly akin. 

VII. The contrasts between continuous growth and discontinuous multiplica- 
tion, between asexual and sexual reproduction, between parthenogenesis and 
sexuality, between alternating generations, are all dififerent expressions of the 
fundamental antithesis. 



LITERATURE. 
Spencer, Principles of Biology ; and Haeckel, Generelle Morphologic. 



THEORY OF REPRODUCTION. 



CHAPTER XVII. 
THEORY OF REPRODUCTION. 

I. The Essential Fact in Reproduction. — In the foregoing 
chapters the facts involved in the different forms of reproduction have 
been analyzed apart, and separately discussed. Male and female 
organisms have been interpreted as relatively katabolic and anabolic ; 
the origin of sex, in the individual and in the race, has been traced 
back to the preponderance of anabolic or katabolic conditions ; the 
ultimate sex-elements were seen to exhibit the same contrast in its 
most concentrated expression; fertilization was regarded as a katabolic 
stimulus to an anabolic cell, and on the other side, of course, as an 
anaboUc renewal to a katabolic cell, as well as the union of opposed 
hereditary characteristics. Only by a separation of the problem of 
' ' sexual reproduction ' ' into its component problems can the solution 
be reached. Sexual reproduction is hke a complex musical chord in 
the organic life, combining several elements, all of which, however, 
admit of the same fundamental analysis. Two problems remain, — 
the psychical aspect of the process; and the import of that common 
feature of all reproduction, the separation of part of the parent organ- 
ism to start a fresh hfe. The latter forms the subject of the present 
chapter. 

II. Argument from the Beginnings of Reproduction. — 
Leconte and others have pointed out that reproduction really begins 
with the almost mechanical breakage of a unit-mass of living matter, 
which has grown too large for successful coordination. Reproduction, 
in fact, begins as rupture. Large cells beginning to die, save their 
lives by sacrifice. Reproduction is literally a hfe-saving against the 
approach of death. Whether it be the almost random rupture of one 
of the more primitive forms such as Schizogenes, or the overflow and 
separation of multiple buds as in Arcella, or the dissolution of a few ol 
the infusorians, an organism which is becoming exhausted saves itseh 
and multipUes in reproducing. In some cases, reproduction is effected 
by outflowing processes of the cell, which have gone a litrie too far. 
Now, such primitive forms of multiplication, gradually becoming more 
definite, express a predominant katabolism in the unit-mass. Repro- 
duction in its simplest forms is associated with a katabolic crisis. 

III. Argument from Cell-Division. — Most unicellular organ- 
isms reproduce by cell-division; and this is, of course, a precedent of 



2l6 



THE E VOL UTION OF SEX. 



reproduction in multicellular organisms, whether they multiply by 
asexual budding or by differentiated sex-elements. But in the preced- 
ing chapter, following Spencer, we have emphasized the connection 
between division and a katabolic predominance within the cell. A 




-Division of an Animal Cell, showing the nucleus {a) in process of 
forming two daughter-nuclei, showing also the protoplasmic 
network (/'). — From Carnoy. 



constructive period may precede, but a disruptive climax attends the 
division. So far then as reproduction is either wholly included in the 
process of cell-division, or has this as its necessary precedent, it is 
associated with a katabolic crisis. 

IV. Argument from the Gradations between Asexual Sev- 
erance of Parts and the Liberation of Special Sex-cells. 

Discussing asexual reproduction, we have noticed that some worm- 
types break into two or more parts, which start new individuals. That 
some nemerteans normally break up into pieces, as they do in the 
feverish anxiety of capture, is most probable; and this is certainly the 
case in certain annelids. From a syllid, which sets free a sexual 
individual, the overgrowth of an asexual parent to one which liberates 
a series of joints, or even a single joint, bearing reproductive elements 
is but a slight step. From the last case to the rupture which liberates 
sex-elements is again only a slight advance. A similar series is well 



THEORY OF REPRODUCTION. 



217 



illustrated among the Hydromedusce. The breakage or thinning away 
which sets a large portion free is a katabolic process, in a sense a local 
death. The gentleness of the gradient warrants us in concluding that 
the liberation of sex-cells, in its earlier expressions at least, is asso- 
ciated with a local or with a general katabolic crisis. 

V. Argument from the Close Connection bet'ween Repro- 
duction and Death. — Without going back to primitive disintegra- 
tions, or the asexual severance of more or less large portions, we may 
point further to the close connection between reproduction and death, 
even when the former is accomplished by specialized sex-cells. We 
shall presently discuss at greater length this nemesis of reproduction, 
but it is important here to emphasize that the organism not unfre- 
quently dies in continuing the life of the species. In some species of 
the primitive annelid Pofygordhis, the mature females die in liberating 
the ova. At a very different level, the gemmules of the common 
fresh-water sponge are formed in the decay of the asexual adult, while 
even the sexual summer forms, especially the males, are peculiarly 
unstable and mortal. The whole history of this form seems a contin- 
uous rhythm between life and growth on the one hand, and death and 
reproduction on the other. Or again, the flowering of phanerogams 
is often at once the climax of the life and the glory of death. In his 
ingenious essay on the origin of death, Goette has well shown how 
closely and necessarily bound together are the two facts of reproduc- 
tion and death, which may be both described as katabolic crises. ' 

VL Argument from Environmental Conditions which 
Favor Reproduction. — The rhythm between nutrition and repro- 
duction, or between growth and multiplication, has been as it were the 
refrain of the preceding pages. This ' ' organic seesaw ' ' is determined 
by the very constitution of the organism ; in other words, it expresses 
the fundamental characteristic of living matter. It is an incomplete 
conception, however, unless it be remembered that about this " organic 
seesaw ' ' there blows the wind of the environment, swaying it now to 
one side, now to the other. It is important, therefore, to illustrate 
how the play of external conditions accelerates or retards the repro- 
ductive function. 

The influence of heat upon the reproductive powers of infusorians 
has been carefully investigated by Maupas. The higher the tem- 
perature up to a certain limit, the faster do these organisms reproduce. 
In favorable nutritive conditions, Siylonichia pustulata divides once in 
twenty-four hours at a temperature of 7° to 10° C, twice at 10° to 
15°, thrice at 15° to 20°, four times at 20° to 24°, and five times at 
24° to 27° C. Illustrating the rapid rate of increase, Maupas notes 



2i8 THE EVOLUTION OF SEX. 



in the same paper, that at a temperature of 25° to 26° C. , a single 
Stylonichia would in four days have a progeny of a million, in six 
days of a billion, in seven and a half days of a hundred billions! In 
six days the family would weigh one kilogramme, and in seven and a 
half days one hundred kilogrammes. 

The action of heat may be twofold; up to a certain limit it quickens 
development and the general life, favoring asexual reproduction and 
parthenogenesis rather than the sexual process; beyond that limit of 
comfortable warmth, so variable for different animals, it may induce 
a feverish habit of body, and hasten reproductive maturity and sexual 
reproduction. In other words, heat may in some cases favor ana- 
bolism, in others katabolism. It is intelligible enough to find increased 
heat sometimes associated with increased asexual reproduction, some- 
times with accelerated sexuality. Instances of both may be gathered 
from Semper' s " Animal Life," the classical work on the influence 
of the environment upon the organism. 

Maupas supplies another vivid illustration of a yet more important 
enviromental influence, that of food. In another ciliated infusorian 
{Lcucophrys), so long as food is abundant, fission obtains; but when 
food grows scanty, there is a metamorphosis without encystation, 
followed by six successive divisions. These are effected, however, 
' ' without vegetative growth, and have for their final object not multi- 
plication but conjugation." In other words, abundant food is asso- 
ciated with asexual reproduction; a check to the nutrition brings 
about the sexual process. Maupas gives a vivid numerical statement 
of the stimulus to reproduction by a sudden check to the nutrition. 
Leucophrys at a temperature of 20° C. , in richly nutritive conditions, 
will give rise to sixteen thousand three hundred and eighty-four indi- 
viduals in three days; but if the food be then suppressed, this large 
number will in a few hours be multiplied by sixty-four, resulting in 
a total of one million forty-eight thousand five hundred and seventy- 
six individuals! 

From cases already cited, which may be multiplied by consulting 
Semper' s " Animal Life," supplemented by a summary of more 
recent researches by one of ourselves, the general conclusions may be 
drawn, — (a) that heat increases reproduction, either directly or as 
the result of a preliminary acceleration of growth; (b) that increased 
food will, of course, favor growth, but reproduction may follow all 
the more markedly as an exaggerated nemesis; {c) that checks to 
nutrition, especially in the form of sudden scarcity, will favor sexual 
reproduction. The clearest result of all is that a sudden katabolic 
change favors reproduction, especially in its sexual form. Anabolic 
conditions favor reproduction indirectly; the reverse conditions ha\'e 



THEORY OF REPRODUCTION. 



219 



a direct influence; in both cases, reproduction is the expression of a 
katabolic crisis. 

VII. Conclusion. — Primitively, then, reproduction was a kata- 
bolic rupture of a mass of protoplasm. This becomes more definite 
in cell-division of various kinds, tending ever to occur at the limit of 
growth when waste has made up on repair, or in katabolic conditions 
due to the environment. In multicellular animals, anabolic conditions 
favor overgrowth; a check to this brings about discontinuous asexual 
reproduction. With increasing differentiation, the asexual multiph- 
cation is replaced by the liberation of special sex-ceUs, by which the 
life-saving and life-continuing sacrifice is rendered less costly. Just 
as asexual reproduction occurs at the limit of growth, so a check to 
the asexual process is often seen to involve the appearance of the 
sexual, which is thus still further associated with katabolic preponder- 
ance. This is confirmed by the contrast observed in alternation of 
generations, where the two processes in varying degrees of distinctness 
persist in the life-history of the same organism. Corroboration is 
again afforded by the association of sexual reproduction with sundry 
environmental checks of a katabolic character. And thus the oppo- 
sition between nutrition, which, after life and death, is the most 
obvious antithesis in Nature, admits of being more precisely restated 
in the thesis, that as a continued surplus of anabolism involves growth, 
so a relative preponderance of katabolism necessitates reproduction. 
Or this may be summed up once more in our fundamental dia- 
grams : — 

SUM OF FUNCTIONS. 




Nulrition 



Reproduction, 



A A 

Anabolism. Katabolism. Female. Male 

Fig. 84. 




THE EVOLUTION OF SEX. 



SUMMARY. 

I. The essential fact in reproduction is tlie separation of part of the parent 
ciixanism to start a fresh life. 

II. Reproduction begins with rupture, — a katabolic crisis. 

III. Cell-division, which sometimes sums up, and is always associated with, 
the act of reproduction, occurs at a katabolic crisis. 

IV. The gradations between discontinuous asexual multiplication and 
ordinary sexual reproduction, show a lessening of the sacrifice ; but all demand 
a disruption, or a katabolic preponderance. 

V. From first to last reproduction is linked to death. 

VI. Environmental conditions of a katabolic character favor se.xual repro- 
duction. 

VII. General conclusion, — a relative preponderance of katabolism neces- 
sitates reproduction. 



LITERATURE. 

Geddes, p. — Theory of Growth, Reproduction, Sex, and Heredit}'. Proc. Roy. 

Soc. Edin. 1886. 
Haeckel. — Generelle Morphologie. i856. 
Spek'CER. — Principles of Biology. 
Semper. — Animal Life. Int. Sci. Series. tSSi. 
Thojison. — "Synthetic Summary of the Influence of the Environment upon 

the Organism." Proc. Roy. Phys. Soc. Edin. 1887. 



PHYISOLOGY OF SEX AND REPRODUCTION. 221 

CHAPTER XVIII. 
SPECIAL PHYSIOLOGY OF SEX AND REPRODUCTION. 

TT is no part of our purpose to discuss in detail the physiology of 
sexual and reproductive functions. The fundamental physiology of 
the essential functions has been the subject of preceding chapters ; the 
details will be found in the standard works on Physiology, Botany, 
and Zoology. For the sake of completeness, however, it is necessary 
to take a brief survey of some of the facts, which are in themselves of 
supreme importance, and which further elucidate the general biology 
of the subject. 

I. Weismann's Theory of " Continuity of the Germ- 
Plasma." — Thanks, especially to Weismann, the view that ordinary 
cells of the ' ' body ' ' become at a certain epoch changed into special 
reproductive cells, may now be put aside as exceedingly improbable. 
In a minority of cases, already quoted, the reproductive cells, or the 
rudiments of sexual organs, are demonstrably set apart at an early 
stage, before the differentiation of the embryo has proceeded far. 
They thus include some of the original capital of the fertilized parent 
ovum intact, they continue the protoplasmic tradition unaltered, and, 
when liberated in turn, they naturally enough develop as the parent- 
ovum did. Following out this important fact, various naturalists have 
reached the conception of a continuous necklace-like chain of sex-cells 
from generation to generation, — a continuous chain upon which the 
mortal individual organisms arise and drop away, like so many sepa- 
rate and successive pendents. 

But in the majority of cases, such a conception, as Weismann has 
justly insisted, gives a false simplicity to the facts. A chain of insulated 
sex-cells, connecting the parental fertilized ovum with the germ-cells 
which develop into offspring, is, so far as we yet know, only rarely 
demonstrable. In other words, the rudiments of the reproductive 
organs often appear at a relatively late stage in the development. 
Where do they come from ? Are somatic, or ordinary body-cells 
modified into reproductive elements ? Weismann's answer is a decided 
negative. Although no continuous chain of germ-like cells is demon- 
strable, there is a strict continuity oi <^Qxm-plasma. Part of the double 
nucleus of the fertilized ovum keeps its characteristics unaltered, in 
spite of manifold divisions persists intact, and is finally established in 
the rudiment of reproductive organs. Or in other words, those cells 
in which the original germ-plasma most predominates become the 



THE EVOLUTION OF SEX. 



reproductive cells. To quote Weismann's own words, "In each devel- 
opment a portion of the specific germ-plasma which the parental ovum 
contains is not used up in the formation of the offspring, but is reser\'ed 
unchanged to form the germ-cells of the following generation. ' ' In 
short, continuity is kept up by the plasma of nuclei, rather than by a 




Fig. 85.— The chromatin elements of the nuclei inc. )il (.i), double star (/)), and 
almt>st divided stages (c). — After Pfitzner- 



chain of cells. It will be obser\-ed, of course, that while early insula- 
tion of definite germ-cells is a demonstrable fact, to be seen in a few 
cases, though perhaps of wider occurrence than we know of, the 
continuity of germ-plasma is strictly an hypothesis. 

This being so, reproductive maturity may be defined as the period 
when the reproductive cells (bearing the inherited capital of germ- 
plasma) have established themselves to that degree that they can start 
fresh organisms, and have multiplied to an extent which in most cases 
makes their liberation a physiological necessity. In the lower animals, 
the maturity of the sexual functions is often as slightly marked as the 
liberation of the elements is passive and random. In slighriy differen- 
tiated organisms, like sponges, there is little reason to suppose that 
the distinction between cells preponderating in germ-plasma and the 
ordinary cells of the body is much marked. Nor in such cases is the 
anarchic opposition between body and reproductive cells at all 
emphatic, especially as regards the female cells. It is only as the 
differentiation increases, as the contrast between body-cells and sex- 
cells becomes emphasized, as the asexual mode of getting rid of surplus 
wanes, that the typical liberation of sex-elements which marks sexual 
maturity becomes a striking fact in the life. That the male cells are 



PHYSIOLOGY OF SEX AND REPRODUCTION. 



always more anarchic, usually mature before the female elements, and 
even in plants, and in such passive animals as a sponge or a hydra, 
burst from the organism, while the female cells remain in situ, is quite 
consonant with their predominantly katabolic character. 

II. Sexual Maturation. — The maturation of the sexes not only 
acquires increasing definiteness in the higher forms, but becomes asso- 
ciated with various characteristic accompaniments. The profound 
reaction of reproducti\'e maturity upon the whole system is best marked 
in birds and mammals, and perhaps most of all in man. Thus in a 
young male bird, the circulation in the testes is greatly increased, and 
these organs increase greatly in size and weight, and commence to 
develop spermatozoa. IVIeanwhile the ' ' secondary sexual characters ' ' 
of the adult — gayer plumage for alluring the female, or weapons for 
contest with other males — make their appearance, the voice and note 
may alter, and a marked increase of strength and courage may appear. 
Among mammals, the changes are of similar order, the secondary 
sexual characters of course differing in detail. The minor changes at 
puberty in man associated with the commencement of spermatogenesis, 
are (besides the reflex excitation of erection due to distension of the 
seminal vesicles, and the more or less periodic expulsion of their con- 
tents during sleep) the growth of hair on the pubic region and later on 
the lower part of the face, and the rapid modification of the laryngeal 
cartilages and the lengthening of the vocal chords, so rendering the 
voice harsh and broken during the change, and ultimately deepening 
it by about an octave. The marked strengthening of bones and mus- 
cles, and the profound psychical changes which accompany the whole 
series of processes, are also famihar. 

In hio-her vertebrates, the sexual maturity of the female is marked 
by a cellular activity within the ovary, not less remarkable than 
that in the testes. Associated therewith are minor but often very 
important characteristics, such as the increased mammary development 
in mammalia. In some of the lower animals, such as certain marine 
annelids, the ova become so numerous that their disruption or libera- 
tion is in great part a mechanical necessity. The same might be said 
of fishes, reptiles, and birds. At the same time the enlargement and 
escape of the ova are doubtless expressions of a normal cellular 
rhythm, of which hints are given in the frequent passage from an 
amceboid to an encysted phase, in the occasional relap.se to the former, 
and in the fatty degeneration or death of ova which have not accom- 
plished their destiny. 

The primitive ova of vertebrates lie in clusters in the substance or stroma of 
the origan, and are produced from the essential germinal epithelium. Only a 
minority, however, grow into genuine ova ; others, of smaller size, form a 



2 24 ^^-^ EVOLUTION OF SEX. 

nutritive sheatli or follicle around. In mammals, each follicle forms a cavity 
coiitaining a fluid. Into this the ovum, surrounded by a mass of follicle-cells, 
prt)jects. When mature, the follicle with its contained ovum has attained a 
superficial position. By the bursting of the ripe follicle the ovum is expelled, 
and passes into the approximated and ciliated upper end of the oviduct or 
Fallopian tube. The rupture of blood-vessels in the substance of the ovary 
fills up the Graafian follicle with blood. The white corpuscles form a frame- 
work resembling connecti\'e tissue, in which the solids and corpuscles of the 
blood-serum, with coloring matter derived from the haemoglobin of the latter, 
are retained. The whole constitutes the "corpus luteum," which, should 
pregnancy occur, may persist and undergo further retrogressive changes, or 
otherwise gradually disappear. 

As to the direct causes of this process of ovulation there is some difference 
of opinion. The congestion of the blood-vessels of the ovary, its own internal 
turgidity, a slight contractility of its stroma, have been regarded as determining 
factors. The process seems, however, rather to depend upon the growth and 
turgescence of the individual follicle. The question of the relation of ovulation 
to the process of copulation in the higher animals has also been much dis- 
cussed. Though we certainly know that ovulation is of regular occurrence 
M-hether fecundation takes place or not, it seems that in many cases copulation 
is speedily followed by the liberation of an ovum ; nor is it difficult to see how 
the profound nervous and circulatoi-y excitement associated with the former 
process might accelerate the bursting of a follicle. Leopold has conclusively 
shown, however, that ovulation may also long precede impregnation. 

Since the oviduct, unlike its male counterpart, is not, in the vast majority of 
vertebrates, continuous with its associated organ, it is often difficult to see how 
the ova once liberated into the body-cavity find their way safely into the small 
opening of the duct. In the frog, however, tracts of the peritoneal epithelium 
becomes ciliated, so propelling the ova in the right direction. In reptiles, 
birds, and mammals the open end of the oviduct is widened, fringed, and 
ciliated, and lies close to or even touching the ovarj- ; muscular fibers too are 
present, and more or less active movements of this cilated end over the ovarian 
surface have been alleged to occur. The oviduct once reached, the down- 
ward progress of the ovum is insured by the cilia of the epithelial lining, and 
probably also by peristaltic movements of its muscular coat. 

There is no doubt that the advent of sexual maturity varies with 
enviromental conditions of climate, food, and the like. Broadly- 
speaking, sexuality becomes pronounced as growth ceases. Especially 
in higher organisms, a distinction must obviously be drawn between 
the period at which it is possible for males and females to unite in 
fertile sexual union, and the period at which such union will naturally 
occur or will result in the fittest offspring. In the lower animals, where 
the individual life is usually shorter, sexual maturity is more rapidly 
attained, though we find cases such as that of the fluke [Polystomuni) 
so commonly present in the bladder of the frog, where maturity of 
the reproductive organs does not occur for several (three) years, and 
maturity of growth for some years afterwards. In cestode parasites, 
the bladder-worm stage remains indefinitely asexual, until in fact the 



PHYSIOLOGY OF SEX AND REPRODUCTION. 225 

stimulus of a new host admits of the development of the sexual tape- 
worm. In plants, reproductive maturity sets in at various ages; thus 
we have all gradations, at the one extreme our characteristically 
short-lived but magnificent annuals, then the biennials, and from 
these to a maturation at still longer date, as in the well-known case 
of the American aloe {Aloe americand), which even in Mexico takes 
from seven to twelve years to reach the floral climax in which it 
expires, and in our greenhouses as much as a generation or two, 
whence its name of ' ' century plant. ' ' 

In contrast to such cases, precocious reproductive maturity occa- 
sionally occurs. We have already referred to those dipterous midges 
(^Cecidom.yi(Z) , in which the larvae for successive generations become 
reproductive, though only parthenogenetically. Very striking, too, 
is the trematode worm Gyrodadylus, which recalls the mystical views 
of the preformationists, in exhibiting three generations of embryos, 
one within the other, while the oldest is yet unborn. The well-known 
axolotl of Mexican lakes, though with its persistent gills in a sense the 
^ larval form of Amblystoma, attains of course to sexual maturity. A 
more marked precocity has been observed in the Alpine salamander 
(^Triton alpestris). In higher organisms, it occasionally happens that 
long before growth has ceased or adolescence been reached sexuality 
sets in, especially in the male sex, but this is fortunately a compara- 
tively rare pathological occurrence. In one set of organisms pre- 
cocious reproductive maturity has been of paramount importance, 
namely, in the flowering plants. Here the prothallium stage, as 
contrasted with the vegetative, has been much reduced, and has 
remained associated with or been absorbed by the asexual genera- 
tion. This is to be in part explained by the accelerated reproduction 
of the prothallus, comparable to a similar process which has reduced 
the separate medusoid sexual persons of a hydroid colony to mere 
buds. 

III. Menstruation. — The process of menstruation (menses, catamenia), 
altliough from the earliest times the subject of medical inquiry, is by no means 
yet clearly understood. It occurs usually at intervals of a lunar month in all 
females during their period of potential fertilit)^ (fecundity), and so far from 
being confined to the human species, has been observed at , the period of 
"heat "in a large number of mammals. Though thus clearly a normal 
physiological process, it yet evidently lies on the borders of pathological change, 
as is evidenced not only by the pain which so frequently accompanies it, and 
the local and constitutional disorders which so frequently arise in this connection, 
but by the general systemic disturbance and local histological changes of 
which the discharge is merely the outward expression and result. In general 
terms, and apart from ovulation, menstruation may be described as a periodic 
discharge of blood, glandular secretion, and cellular detritus from the lining 
of the uterus. After from three to six days the blood ceases to appear, and 



226 T^iE EVOLUTION OF SEX. 

the lost epithelium is rapidly replaced, apparently by proliferation from the 
necks of the glands. By the ninth or tenth day the mucous coat is fully healed, 
and the beginnings of the next menstrual process recommence. 

The age at which the process commences varies with race and climate, with 
nutrition and growth,, with habit of life (for e.xample, with difi'erence between 
town and countr>' life), and with mental and moral characteristics. Of these, 
however, climate seems most important ; thus while in Northern Europe the 
age is reckoned at the beginning of the fifteenth year, in the tropics it com- 
mences earlier, in the ninth or tenth year, according to some. The cessation 
of menstruation usually takes place between the age of forty-five and fifty, and, 
somewhat aS the secondary characteristics of female puberty coincide with its 
appearance, a less distinct reduction of these is associated with its close ; in 
many cases secondary resemblances to the masculine tjpe may supervene. 

The old theories of menstruation were, that it served to rid the system of impure 
blood, that it simply corresponded to the period of "heat" obser\'ed in lower 
animals, or, later, that it was associated with ovulation, — which indeed seems 
broadly to correspond with the end of the menstrual period. And while it can 
not be maintained that either "heat" or ovulation are necessarily associated 
with menstruation in Homo, there can be little doubt of the general physio- 
logical parallelism of all three processes. At present there may be said to be 
two rival theories. According to the first of these, the process is viewed as a 
kind of surgical "freshening" of the uterus for the reception of the ovum, 
whereby the latter during the healing process can be attached safely to the 
uterine wall. The other view is exactly the reverse of this. Its upholders 
regard the growth of the mucous coat before this commencement of the flow 
as a preparation for the reception of an ovum if duly fertilized, and the men- 
strual process itself as the expression of the failure of these preparations, in 

short, as a consequence of the non-occurrence of pregnancy. A decided 
majority of gynaecologists appear to incline to the latter view. 

The process may, however, be expressed in more general, and at the same 
time more fundamental terms. If the female sex be indeed preponderatingly 
anabolic, we should expect this to show itself in distinctive functions. Men- 
struation is one of these, and is interpretable as a means of getting rid of the 
anabolic surplus, in absence of its consumption by the development of 
offspring,— just as it is intelligible that the process should stop after fertiliza- 
tion, when replaced by the demands of the practically parasitic ftetus. In the 
same way, the occurrence of lactation, after this internal parasitism has been 
terminated by birth, is seen to be reasonable. The young mammal is thus 
enabled to become what is practically a temporary ecto-parasite upon the 
unfailing maternal anabolic surplus ; and when lactation finally ceases, we have 
the return of menstruation, from which the whole cycle may start anew. So 
in the widely different yet deeply similar worid of flowers, the distinctly 
anabolic overflow of nectar ceases at fertilization, and the surplus of continual 
preponderant anabolism is drafted into the growing seed or fruit, 

IV. Sexual Union.— In a previous chapter we have noted the 
passive and random way in which the sex-elements of man)^ of the 
lower animals are liberated, and the chance manner in which they 
are brought together by water-currents and the like, though this may 
not be quite so common as our ignorance leads us to suppose, witness 
the recent observation of the sexual intertwining of Asterina and of 



PHYSIOLOGY OF SEX AND REPRODUCTION. 



227 



Antcdon. Yet more in plants is tlie liberation of male elements, 
and notably that of pollen-grains, a passive dehiscence, and fertiliza- 
tion a matter of chance, only reduced by the prodigal wealth of 
material. Secure as the methods of fertilization of flowers by the 
aid of insects often are, the margin of risk is wide; and this is yet 
more marked when the pollen is carried by the wind. It is true that, 
both in plants and animals, there are subtle attractions between the 
essential elements, but this is only at a close range; and the external 
union is in many cases none the less random. 

It must be allowed that the primary importance of the timely 
encounter of the ovum and spermatozoon has perpetuated in the 
various groups a varied series of adaptations securing fecundation. 
At the same time, the increasing differentiation of the sexes has in 
the higher animals been enhanced by psychial as well as physical 
attractions, thus more and more insuring the continuance of the 
species. ^ 




Fig. 86. — Male of Paper NautJlu.s (A rg:onauta), with its 
modified arm. — From Leunis. 

A not unfrequent mode of fecundation is by means of spermato- 
phores, or packets of spermatozoa. These may be seen at times 
attached to the earthworm, or found within the leech and snail. Even 
in newts spermatophores are formed, which are taken up by the 
females. 

In the spider the spermatoza are stored in a special receptacle on 
the palp, and hence hastily transferred to the fierce female. In cuttle- 
fishes this mode of impregnation is yet more marked. One of the 
"arms" of the male, much modified and laden with spermatophores, 
is thrust, or in many cases bodily discharged into the branchial cavity 
of the female, where it bursts. Such a discharged arm was, on first 
discovery, regarded as a parasite, and hence received the name of 
Hedocotylus. A curious aberration from the ordinary relations is 



2 28 THE EVOLUTION OF SEX. 



figured above, where two distinct animals {Diplozomi) join in almost 
lifelong union. 

In many cases again, especially in bony fishes, there is a sexual 
attraction between male and female, but without any copulation. The 
female, accompanied by her mate, deposits o\'a, which he thereupon 
fertilizes with spermatozoa. A slightly more advanced stage is seen 
in the frog. Fertilization is still outside the body of the mother, but 
the male, embracing the female, liberates spermatozoa upon the eggs, 
which are at the same time laid. 

In the majority of cases, however, special organs for emitting and 
for receiving spermatozoa are developed, and copulation occurs. The 
male organ is often an adaptation of some structure already existing, 
as in many crustaceans, where modified appendages form external 
canals for the seminal fluid. In skates and other gristly fishes, the 

remarkably complex copulatory organs, 
so-called "claspers," are in close con- 
nection with the hind limb. The penis 
of higher vertebrates is virtually a new 
organ. The copulation may be quite 
external, as in crustaceans, where the 
male seizing the female deposits sper- 
matozoa upon the already laid eggs. 
Fig. ZT.—DipiozoouparadojcHm,s.&oMh\e Oftcuer, howevcr, it is internal, and the 
organism formed from the intromittcut Organ is inserted into the 

union of two distinct herma- ^ 

phrodite individual trema- genital apcrture of the female. True 

todes {Diporpa) at an early ■% ,• - , . 

stage in tiieir life. copulatiou may occur without the pres- 

ence of special organs, — notably in 
the case of many birds, where the cloaca of the male is apposed to that 
of the female. The spermatozoa, forcibly expelled by the excited male 
organs, pass up the female ducts, probably, in part, as the result oi 
peristalsis, but chiefly at least by their own locomotor energy, and one 
of them may eventually fertilize an ovum. In addition to the intro- 
mittent organ, and the lower portion of the female duct which receives 
it during copulation, there may be auxiliary structures, such as true 
claspers for retaining hold of the females. The limy " cupid's dart" 
or " spiculum amoris " of the snail, is usually interpreted as a pre- 
liminary e.xcitant. 

Three further notes in regard to higher animals are requisite, (i) 
There is much reason to believe that the follicles tend to burst toward 
the end of menstruation ; that this may be accelerated by copulation ; 
successful fertilization may occur at any period, but most frequently- 
soon after menstruation, and most rarely during the relatively infertile 
period most distant from that process. (2) After conception, when 




PHYSIOLOGY OF SEX AND REPRODUCTION. 229 



IS 



the fertilized (tgg has begun to develop, the mouth of the uterus 
closed by a secretion, which prevents the entrance of other spermato- 
zoa should further copulation occur. (3) The period of gestation, that 
is, between the fertihzation of the ovum and the extrusion of the foetus, 
varies widely in mammals, from about 18 days in opossum, or 30 in 
rabbit, to about 280 days in Homo or 600 in the elephant, being 
longer in the more highly evolved types. But it also depends on size, 
being about 280 days in cow and 150 in sheep; on number of off- 
spring, being about 350 in mare and 60 in dog ; and on the degree of 
maturity at birth, being 420 in giraffe and 40 in kangaroo. 

V. Parturition. — In many cases, for e.xample, marine annelids, 
mature ova burst, as we have already noted, from the mother animal, 
who may thenceforth have nothing more to do with them. Liberation 
of ova from the ovary and from the organism may be almost coinci- 
dent, as in most bony fishes. In other cases, the ova are retained 
within the mother until fertilized, but are expelled not long after, 
before development has advanced to any marked degree. Such eggs 
are often furnished with the important capital of nutriment, so familiar 
in the case of birds, and may be also surrounded by chitinous, horny, 
membranous, or hmy shells. All such forms of birth are familiarly 
described as oviparous. 

In numerous invertebrates, fishes, amphibians, and reptiles, the ova 
develop within the mother, and the young are born more or less 
actively alive. To such cases, where there is no nutritive connection 
between parent and offspring, the term ovo-viviparous used to be 
applied. They were contrasted with oviparous birth, as in birds, on 
the one hand, and with the viviparous birth of mammals, on the other. 
It is the well-known characteristic of the latter that there is an intimate 
nutritive connection between mother and offspring. The term is of 
little use, however, for the cases to wh'ch it is applied shade off toward 
the two other forms of birth. Thus among gristly fishes {Mustelus 
IcBvis and Carcharias), in the curious bony fish Anableps, and in certain 
lizards (^Trachydosauras and Cyclodus), a somewhat placenta-like 
function is discharged by the yelk-sac and the wall of the oviduct; 
while in fishes, reptiles, &c. , oviparous and ovo-viviparous birth may 
occur in nearly related forms. The distinction involved in the term is 
therefore abandoned, and it must also be recognized that the difference 
between egg-laying and the production of young actively alive is only 
one of degree. Even in mammals, which are viviparous par excellence, 
the two lowest genera — the duck-mole and the echidna — are ovipar- 
ous. The common grass-snake, normally oviparous, has been induced, 
in artificial conditions, to bring forth its young alive, and this is prob- 
ably true of other forms. The parthenogenetic generations of 



230 THE El'OLUriON OF SEX. 



aphides are usually viviparous, while the fertilized eggs are laid as 
such. 

VI. Early Nutrition. — The early nutrition of the embryo, and 
even larva, is in most cases an absorption of the legacy ol yelk mate- 
rial, which is probably richest in the eggs of birds. The tadpole of 
the frog grows and exerts itself for some time before it begins to feed 
at the expense of this inheritance of yelk. Later on, in the frog 
di\ ision of amphibians, the growth of new structures appears to be 
pro-\'ided for by the nutritive absorption of the tail, the larva litei'ally 
living upon itself The same is true in the elaborate metamorphosis 
of echinoderm larvje. In many cases, the cells of the embryo, inde- 
pendently and actively, devour the yelk and other available material, 
doing so after the amceboid fashion technically known as intra-cellular. 
At the same time, osmotic currents may more passively effect the 
like result. In the whelk and related forms, a curious cannibalism is 
well known to occur among the crowd of embryos inclosed within a 
common capsule. The stronger and older devour the younger and 
weaker, — a struggle for existence happily of exceptional precocious- 
ness. In the higher vertebrates (above amphibians), foetal membranes 
— amnion and allantois — are developed, in addition to the yelk-sac 
which incloses the yelk. Of these the amnion is mainly protective, 
and the allantois at first almost wholly respiratory. But in birds (and 
probably to a slight extent in reptiles) the allantois begins to assume 
nutritive functions, assisting in the absorption of the yelk. In placental 
mammals, however, a nutritive function becomes paramount, the allan- 
tois forming the greater part of the embryonic side of the placenta. 
The yelk-sac is here virtually yelkless, but in lower orders may absorb 
nutriment as it did in birds, though from a different source — the 
maternal wall. In most cases, however, what was incipient on the part 
of the yelk-sac, in the exceptional elasmobranchs and lizards already 
mentioned, becomes the emphatic function of the allantois, — namely, 
the establishment of a vascular or nutritive connection with the wall ol 
the maternal uterus. By this means, though no drop of blood ever 
passes from mother to offspring, a very intimate osmotic transfusion is 
effected. 

VII. Lactation. — If menstruation be a means of getting rid ot 
anabohc surplus, in absence of the fcetal consumption, lactation is still 
more an anabolic overflow, adapted to, though not of course originally 
caused by the offspring's demands. It is at the same time evident 
enough, and easily verified by the histologist, that in actual occurrence 
both processes are katabolic, involving cellular disruption and death. 
That peculiar liability of these uterine and mammary tissues to disease, 



PHYSIOLOGY OF SEX AND REPRODUCTION. 231 



the most tragic possibilities of the lii'c of woman, 
becomes thus less mysterious. We can understand more readily the 
association of such diseases with much of what we are pleased to gen- 
eralize as civilization, and view more hopefully the possibilities of their 
enormous diminution by the rational hygiene of civilization properly 
so-called. 

The milk or mammary organs are modified skin-glands, probably 
most nearly allied to the ordinary sebaceous type, except in mono- 
tremes which appear to be divergent. Every one knows that they are 
exclusive characteristics of mammals, and are only normally functional 
in the female sex. Rudimentary in the males, they may even there 
produce milk ("witches' milk") at birth, puberty, and under patho- 
logical conditions, while cases have been put on record of men who 
have actually given suck. * They vary greatly in position and num- 
ber, a large number being doubtless the primitive condition. In 
function, after the birth of offspring, the surrounding tissue is specially 
rich in white blood-corpuscles, which probably form some of the 
structural elements of the milk. It has also been shown that the 
nuclei of the gland-cells undergo degeneration, disruption, and expul- 
sion, and that they in all likelihood form the casein elements of the 
nutritive fluid. 

Before birth, the mammalian embryo has been nourished through 
the placenta, by the transfusion already referred to. The alimentary 
canal has obviously had no experience in digestive function. Before 
it proceeds to digest the food of the parents, it is put through a course 
of what Sollas neatly terms "gastric education," by feeding upon the 
readily assimilated mother's milk. 

VIII. Other Secretions. — Every one has heard at least 01 
"pigeon's milk," and many are familiar with its administration to the 
young birds. This is produced by both sexes for a week or so after 
the hatching of the young, and is the result of a degeneration of the 
cells hning the crop. Some of the cells break up, others are discharged 
bodily. The result forms a milky emulsion-like fluid, which is regur- 
gitated by the parents into the mouth of the young bird. A similar 
substance is said to occur in some parrots. 

Of some interest, also, is the supra-salivation which occurs at the 
breeding season in the swiftlets {Conocalid),\\\\\c\i form the edible 
birds' nests, — the costly, though to us wofully insipid, luxury oj 
Chinese epicures. Certain salivary glands become peculiarly active 
in these birds when breeding, and the secretion, which, according to 



* Merriam (Hayden's U. S. Geol. Survey, VI., p. 666) gives a definite 
account of male lactation in Lepus bairdi. 



2,2 '^^E EVOLUTION OF SEX. 

Green, consists chiefly of a substance akin to mucin, is used to form 
the snow-white fibrous nest. 

Take only one other instance of peculiar secretion curiously linked 
to the above by one of those profound physiological unities which 
show how superficial after all are the utmost contrasts of organic form, 
— we refer to the viscid threads with which the male stickleback 
weaves his nest. Mobius has shown that the kidneys are greatly 
affected by the mature testes; that they produce, by a now normal 
pathological process, special waste or katabolic elements, in the form 
of mucous threads. The male gets rid of this uneasy encumbrance 
(which has a somewhat parallel pathological equivalent in higher ani- 
mals), by rubbing itself against objects, and thus almost mechanically 
has been evolved the familiar weaving of the aquatic nest. 




Fig 88, 



.—The nest of the Stickleback (Gastcrostcus). —?\oxn Thomas Bolton. 



IX. Incubation. — The physiological sacrifice of the female birds 
does not end with providing the large capital of nutritive material with 
which the germ is endowed, but is continued in all the patience of 
brooding. In passerine birds the male relieves the female in her task 
of love, and in the ostrich tribe takes the duty usually upon himself. 
In the cuckoos and cow-birds the parental care is shirked, and with 
varying degrees of deliberateness the eggs are foisted into foster-nests, 
and the young thus put out to nurse. After the fatigue of reproduc- 
tion it is perhaps natural enough that the female should rest a while 
upon the eggs in the shelter of the nest; and since there is observed to 
be an increased circulation in the skin of the abdominal region at this 
time, it has been argued that the bird merely sits to cool itself! This 
view has been supported by the cruel experiment of singeing off" th 
feathers from the same region in a cock, which then sat to cool the 



e 



PHYSIOLOGY OF SEX AND REPRODUCTION. 233 

irritated surface. Yet the increased circulation may also be viewed 
as increased by the sitting; in any case, the patience and solicitude of 
the brooding, and the subsequent diligence in feeding the hatched 
young, are obviously the expression of genuine parental affection. 

Here, too, one must include the retention of the young in skin- 
pouches, exhibited by the great majority of marsupial mammals and 
by the echidna. In the latter, the pouch is a simple and possibly 
periodic structure, arising from an insinking of the skin in the mam- 
mary region of the abdomen. Here the eggs are somehow or other 
•stowed away and the young developed. The milk-glands simply open 
on the surface of the depression. In most marsupials, the young, 




Fig. 89.— The female Surinam Toad, with young ones on its back.— From Leunis. 

which are born precociously after a very short uterine life, are shehered 
in similar, but more developed, pouches of the skin, within which the 
teats open. 

In oviparous reptiles, the eggs are usually left to hatch of them- 
selves, aided by the warmth of sun and soil. "The female python 
disposes herself in coils round her eggs, and incubates them for a 
prolonged period, during which the temperature has been observed to 
rise as high as 96° F. within the coils. 

Some exceedingly curious parental adaptations occur among 
amphibians, which seem to have made numerous experiments on the 
matter. Thus in the Surinam toad {Pipd), the male spreads the ova 



234 



THE EVOLUTION OF SEX. 



on the female's back, a sort of erysipelas sets in, and each ovum 
becomes surrounded by a skin-cavity in which the tadpole develops. 
After the process is over, the skin of the back is renewed. In other 
cases this mode of carrying the ova becomes somewhat more definite; 
thus in Notodclphys and NoMrema the eggs are stored in dorsal 
pouches. Nor are the males without their share in the task of parent^ 
age. In the obstetric frog {Afytes obsictricans), the male helps to 




Fig. go. — The female Notcirema jnarsnpiatimz^ — an amphibian, 
with eggs m a dorsal sac, which is shown partly un- 
covered. — From Carus Sterne, after Giinther. 

remove the eggs from the female, twists them in strings round his 
hind legs, and buries himself in the water till the tadpoles escape and 
relieve him of his burden. In Rhinodcrma darwinii, the croaking sacs, 
which were previously used for amatory calling, become enlarged as 
cradles for the young. 

Among fishes, parental care is largely in abeyance, and there are 
only slight hints of anything in the way of incubation. In a siluroid 
fish (Aspi-edd), the female deposits her ova and lies upon them till they 
become attached to the spongy skin of the belly, very much as hap- 
pens in the dorsal attachment of the Surinam toad. After hatching, 
the skin e.Tcrescence is smoothed away. In Solenostovia (allied to 
pipe-fish) the ventral fins unite with the skin to form a pouch in which 
the eggs are retained. In other cases, it is the male which incubates 
or cares for the ova. Not a few form nests, as in the stickleback, over 
which they keep a jealous guard. In some species oi Arms the eggs 
are carried about in the pharynx; \'i hile in the sea-horses a pouch is 
developed on the posterior abdomen. 

Among invertebrates, brood-chambers or cradles for the young 
are not uncommon. The capsules of hydroids, the tent of spines on a 



PHYSIOLOGY OF SEX AND REPRODUCTION'. 



235 



few sea-urchins, the depressions in the skin in one or two sea- 
cucumbers, the modified tentacles of some marine annehds, the dorsal 
shell-chamber in water-fleas, the incurved abdomen of higher crusta- 




FlG. 91. — The Sea-horse {Hippocavipjis guttiilatus). — From 
the Atlas of the Naples Aquarium. 



ceans, the gill-cavities of bivalves, the beautiful brood-shell of the 
argonaut, illustrate a habit even an outline of which is beyond our 
limits. 




Fig. 92. — The female of the " Paper Nautilus (.4 7-^^7?(2?/z'<z rt^x"^), 
with its brood-chamber. — After Leunis. 



X. Nemesis of Reproduction. — We ha\'e already shown how 
reproduction in its origin is linked to death. The primitive ruptures 
by which the protozoon reduces encumbering bulk, saves its own life, 
and multiplies its kind, are only a step or two from more diffuse 
dissolution which is death. 

The association of death and reproduction is indeed patent enough, 
but the connection is in popular language usually misstated. 



236 



THE E VOL UTION OF SEX. 



Organisms, one hears, have to die ; they must therefore reproduce, 
else the species would come to an end But such emphasis on 
posterior utilities is almost alwaj-s only an ai'terthought of our inven- 
tion. The true statement, as far as history furnishes an answer, is not 
that animals reproduce because they have to die, but that they die 
because they have to reproduce. As Goette says, "it is not death 
that makes reproduction necessary, but reproduction has death as its 
inevitable consequence." This, of course, refers primarily to the 
incipient forms of both these katabolic processes. 








Fig, 93. — A figure of cell-division suggesting the internal disruptions and rearrange- 
ments of the nucleus {a) and protoplasm. — From Rauber. 

It is necessary to give a few illustrations. Goette refers to 
Haeckel's Magosphara, a protozoon which just as it had formed for 
itself a multicellular body broke up into the component units. These 
lived on, and there was no corpse, but at the same time the 
multicellular colony was no more. Again he takes the case of the 
lowly and somewhat enigmatical orthonectids, which Van Beneden has 
classed as Mesozoa, between the single-celled and the stable many- 
celled animals. Here the mature female forms numerous germ -cells 
and terminates her individual life by bursting. The germs are 



PHYSIOLOGY OF SEX AND REPRODUCTION. 



237 



liberated, the mother animal has been sacrificed in reproduction. 
' ' The death is an altogether inevitable consequence of the reproduc- 
tion." 

Nor is this sacrifice confined to the incipient multicellular 
organisms. Thus in some species of the annelid Polygordius, the 
mature females break up and die in liberating their ova. This is 
approached, but suggestively avoided, in a genus of capitellid sea- 
worms {Clitomastus) . The whole organism is not sacrificed, but only 
an abdominal portion of the body. This is, in fact, one of the key- 
notes to reproductive differentiation, — the sacrifice is lessened, and the 
fatality thus warded off. 

But again, we find in some threadworms or nematodes (for 
example, Ascaris dadyluris) that the young live at the expense of the 





Fig. 94. — Orthonectids, showing the rupture of the female in hberating 
the germs. — From Goette, after Juiin. 

mother, until she is reduced to a mere husk. In fresh-water Polyzoa, 
KraepeUn notes that the ciliated embryo leaves the maternal body- 
cavity through a prolapsus uteri of the sacrificed mother. In the 
precocious reproduction of some midge larvse {Chironomus, &c.), the 
production of young is fatal through successive generations. 

Both Weismann and Goette, though with different interpretations, 
note how many insects (locusts, butterflies, ephemerids, &c.) die a few 
hours after the production of ova. The exhaustion is fatal, and the 
males are also involved. In fact, as we should expect from the 
katabolic temperament, it is the males which are especially liable to 



238 



THE EVOLUTION OF SEX. 



exhaustion. The males of some spiders normally die after fertilizing 
the female, a fact perhaps helping to throw light upon the sacrifice of 
others to their mates. The similarly tiny (ultra-katabolic) male 
rotifer — an ideal but too unpractical lover, with not even an alimentary 
canal — would seem usually to fail and expire prematurely, leaving the 
female to undisturbed parthenogenesis. Every one is familiar with the 
close association of love and death in the common mayflies. 
Emergence into winged liberty, the love-dance and the process of 
fertilization, the deposition of eggs and the death of both parents, are 
often the crowded events of a few hours. In higher animals, the 
fatality of the reproducti\'e sacrifice has been greatly lessened, yet 
death may tragically persist, even in human life, as the direct nemesis 
of love. 

The temporarily exhausting effect of even moderate sexual 
indulgence is well known, as well as the increased liability to all forms 
of disease while the individual energies are thus lowered. 

XI. Organic Immortality. — Comparatively httle is yet known 
about the length of life among lower animals, but there is no reason to 
doubt that all multicellular organisms die. We have just emphasized 
the view of Goette and other naturalists, that reproduction is the 
beginning of death; which is not inconsistent with the apparent para- 
dox that local death was the beginning of reproduction. Allowing, 
then, that multicellular organisms at any rate are mortal, and that the 
very blossoming of the life in reproduction is fated with a prophecy of 
death which lis its own fulfillment, we have to face two questions, — 
What of death in the Protozoa f and, In what sense is there an immor- 
tality throughout the organic series? 

Often enough already, in the preceding pages, we have had to 
reiterate the contrasts between the Protozoa and the higher animals. 
These firstlings are physiologically complete in themselves, and have 
at least very great, if not unlimited, powers of self-recuperation. They 
leave off where higher animal life begins, that is to say, in a unicellular 
state. They do not form bodies. Their reproduction, moreover, is 
in the majority simple cell-division into two. If there be lops of indi- 
viduality, there is hardly loss of life. Death is not so serious when 
there is nothing left to bury. Nor in most cases can one half of the 
divided unit be the mother individual, and the other the daughter, for 
the two appear indistinguishably the same. Thus an idea, broached 
long ago by Ehrenberg, has been revived and elaborated by several 
naturalists, and especially by Weismann, that the Protozoa are virtu- 
ally immortal. 

In Weismann's own words, "Natural death occurs only among 
multicellular organisms, the single-celled forms escape it. There is no 



PHYSIOLOGY OF SEX AND REPRODUCTION. 239 

end to their development which can be likened to death, nor is the 
rise of new individuals associated with the death of the old. In the 
division the two portions are equal, neither is the older nor the 
younger. Thus there arises an unending series of individuals, each as 
old as the species itself, each with the power of living on indefinitely, 
ever dividing but never dying." Ray Lankester puts the matter 
tersely," It results from the constitution of the protozoon body as a 
single cell, and its method of multiplication by fission, that death has 
no place as a natural recurrent phenomena among these organisms." 
Some limitations must be noticed, which make this idea of pristine 
immortality yet more emphatic. It is only asserted that the Protozoa 
escape " natural death," a violent fate maj' of course await them like 
any other organisms. They have no charmed life, being as liable to 
be devoured as those of higher degree. In relation to the environ- 
ment, however, their simplicity gives them a pecuUar power of avoiding 
impending destiny. The habit of forming protective cysts is very 
general, and thus enwrapped they can, like the ova and a few of the 
adults of some of the higher animals (see fig. p. 179), endure desicca- 
tion with successful patience, which is rewarded by a rejuvenescence 
when the rain revisits the pools. But the doctrine of the ' ' immor- 
tality of the Protozoa" refers to a defiance of natural, not violent, 
death. 

The psychological objection that the mother psyche is really extin- 
guished when she divides into two, intrudes a conception which is 
hardly applicable. The individualities are doubled, nothing is really 
lost. Most seriously difficult are those cases where the protozoon 
produces a series of buds, spores, or division units, and leaves a resid- 
ual core or unused remnant behind to die. But in regard to the 
gregarines, for instance, where such a remnant is left, it has been fairly 
answered that the residue is rather a kind of excretion than the parent 
left to perish after its reproductive sacrifice. Weismann is, however, 
willing to admit the possibility, that in the suctorial AcinetcB, and in 
the parasitic gregarines, which are both somewhat removed from the 
normal protozoon type, there may be cases of true mortality. 

Another point in regard to which experts differ, is whether the 
Protozoa are really quite self-recuperative. They suffer injuries, they 
necessarily waste, portions are used up and may be ejected. The 
question then arises. Are those acquired defects obliterated, or do they 
become intensified ? Is the wasting only a local death, or is it the 
beginning of a true senescence ? This is a question which can only 
be answered by observation ; a priori reasoning is here futile. The 
most serious criticism of Weismann' s view is due to Maupas. Already 
we have noted his important result, that conjugation is essential to 



2_i.o THE EVOLUTION OF SEX. 

the youth of the species. Without this incipient sexual reproduction, 
the individuals in the course of numerous successive asexual genera- 
tions grow old. The nucleus degenerates, the size diminishes, the 
entire energy wanes, the senility ends in death. Maupas believes that 
all organisms are fated to suffer decay and death, and protests strongly 
against Weismann's theory that death begins with the Metazoa. 

It must be noted, however, that in natural conditions the conjuga- 
tion, prohibited in Maupas' s experiments, occurs when it is wanted 
and the life flows on. Furthermore, conjugation has not been shown 
to occur in many Protozoa. It seems therefore more warrantable to 
insert Maupas's result as a saving clause to Weismann's doctrine 
than to regard it as contradictory. The conclusion at present justi- 
fiable, is that Protozoa not too highly differentiated, living in natural 
conditions where conjugation is possible, have a freedom from natural 
death. To this must then be added the demonstrated saving clause, 
that in ciliated infusorians, conjugation, which here means an exchange 
of nuclear elements, is the necessary condition of eternal youth and 
immortality. 

Accepting then, with an emphasized proviso, the general conclu- 
sion that most, if not all, unicellular organisms enjoy immortality, that 
in being without the bondage of a "body" they are necessarily freed 
from death, we pass to consider the second question, What does the 
death of the higher and multicellular organisms really involve ? 

If death do not naturally occur in the Protozoa, it is evident that it 
can not be an inherent characteristic of lining matter. Yet it is uni- 
versal among the multicellular animals. Death, we ma)' thus say is 
the price paid for a body, the penalty its attainment and possession 
sooner or later incurs. Now, by a body is meant a complex colony of 
cells, in which there is more or less division of labor, where the com- 
ponent units are no longer, like the Protozoa, in possession of all their 
faculties, but through division of labor have only restricted functions 
and limited powers of self- recuperation. Like Maupas's isolated family 
of infusorians, the cells of the body do not conjugate with one another- 
and though they divide and redi\'ide for a season, the life eventually 
runs itself out. 

A moment's consideration, however, will show that in most cases 
the organism does not wholly die. Some of the cells usually escape 
from the bondage of the body as reproducti\'e elements, — as, in fact 
Protozoa once more. The majority of these may indeed be lost; eggs 
which do not meet with male elements perish, and the latter have even 
less power of independent vitality. But when the ova are fertilized, 
and proceed to develop into other individuals, it is plain that the 
parent organisms have not wholly died, since two of their cells have 



PHYSIOLOGY OF SEX AND REPRODUCTION. 241 

united to start afresh as new plants or animals. In other words, what 
is new in the multicellular organism, namely, the " body," does indeed 
die, but the reproductive elements, which correspond to the Protozoa, 
live on. 

This may be made more definite in the following diagram. There 
it is seen that the organism starts like a protozoon, as a single cell, or 
usually as a union of two cells in the fertilized ovum. This divides, 
and its daughter-cells divide and redivide. They arrange themselves 
in layers, and are gradually mapped out into the various tissues or 
organs. In division of labor, they become restricted in their functions, 
and specialized in their structure. They become differentiated as 
muscle-cells, nerve-cells, gland-cells, and so on. The result is a more 
or less complex " body," unstable in its equilibrium because of its very 
complexity, composed moreover of competing cells far removed from 
the protozoon all-roundness of function, limited in their powers of 




Fig. 95. — The rel:ition between reproductive cells and the body- The continuous chain of dotted cells 
at first represents a succession Qi Protozoa; further on, it represents the ova from which 
the "bodies" (undotted) are produced. At each generation, a spermatozoon fertihzing 
the liberated ovum is also indicated. 

recuperation, and emphatically liable to local and periodic, or to gen- 
eral and final death. But the body is not all. At an early stage in 
some cases, sooner or later always, reproductive cells are set apart. 
These remain simple and undifferentiated, preserving the structural 
and functional traditions of the original germ-cell. These cells, and 
the results of their division, are but little implicated in the differen- 
tiation which makes the multicellular organism what it is; they remain 
simple primitive cells like the Protozoa, and in a sense they too share 
the protozoon immortahty. The diagram shows how one of these 
cells, separated from the parent organism (and uniting in most cases 
with a germ-cell of different origin) becomes the beginning of a new 
body, and, at the same time, necessarily the origin of a new chain, or 
rather of a continued chain of fresh reproductive cells. 

" The body or soma," Weismann says," thus appears to a certain 
extent as a subsidiary appendage of the true bearers of the life, — the 
reproductive cells. ' ' Ray Lankester has again well expressed this : — 
"Among the multicellular animals, certain cells are separated from the 
rest of the constituent units of the body, as egg-cells and sperm-cells; 



242 THE El'OLUTION OF SEX. 

these conjugate and continue to live, whilst the remaining cells, the 
mere carriers as it were of the immortal reproductive cells, die and dis- 
integrate. The bodies of the higher animals which die, may from this 
point of view be regarded as something temporary and non-essential, 
destined merely to carry for a time, to nurse, and to nourish the more 
important and deathless fission-products of the unicellular (t^'g-'" 

In most cases, as Weismann insists, it is more correct to speak of 
"the continuity of the germinal protoplasm" than of the continuity of 
the germ-cells; but, with this proviso, the diagram expresses a fact 
most important in understanding reproduction and heredity, that the 
chain of life is in a real sense continuous, and that the " bodies " which 
die are deciduous growths, which arise round about the real links. 
The bodies are but the torches which burn out, while the living flame 
has passed throughout the organic series unextinguished. The bodies 
are the leaves which fall in dying from the continuously growing 
branch. Thus although death take inexorable grasp of the individual, 
the continuance of the life is still in a deep sense unaffected ; the repro- 
ductive elements have already claimed their protozoon immortality, 
are already recreating a new body; so in the simplest physical, as in 
the highest psychic life, we may say that love is stronger than death. 



PHYSIOLOGY OF SEX AND REPRODUCTION. 



243 



SUMMARY. 

I. Sexual maturity generally occurs toward the limit of growth, is marked 
by liberation of reproductive elements and by secondary characteristics, due to 
the reaction of tiie reproductixe function on the general system. Precocious 
maturity may be due to constitutional or environmental conditions, and has 
been of much importance in the evolution of flowering plants. 

II. Menstruation is interpreted as a means of getting rid of the anabolic 
surplus of tlie female in absence of its fatal consumption. 

III. Sexual union, at first very passive and random, becomes active and 
definite witli the gradual evolution of sex and secondary sexual organs. 

IV. Birth is at first accomplished by rupture, but becomes a definite process 
usually effected through special ducts. Oviparous and viviparous birth only 
differ in degree. 

V. Early nutrition is usually an absorption of the yelk, but in mammals is 
accomplished by osmotic transfusion from the blood of the mother to that of the 
fcetus. 

VI. Lactation is interpreted as an anabolic overflow. 

VII. Besides milk, there are other secretions associated with the nutrition 
and sheltering of the young. Pigeon's milk, edible birds' nests, and the mucous 
threads of sticklebacks, are illustrations. 

VIII. Incubation, reaching a climax in birds, is paralleled in many other 
classes. 

IX. Reproduction and death both represent katabolic crises. Primitively, 
they are nearly akin. Reproduction may ward off" death from the Protozoon, 
but in the simplest Rletazoa it probably caused it. 

X. The Protozoa come nearer immortality than other organisms. The fact 
of germinal continuity involves an organic immortality. 



LITERATURE. 

For the special physiology of se.x and reproduction consult standard text- 
books such as those of Foster, Landois and Stirling, and especially Hensen's 
work already often cited. 

On the continuity of the germ-plasma, consult recent translation of Weis- 
mann's papers — "Heredity," O.xford, 1889; while a full bibliography will be 
found in " History and Theory of Heredity," by J. A. Thomson, Proc. Roy. 
Soc. Edin., 1888; and, since 1886, in the Zoological Record. 

On the nemesis of reproduction, and on organic immortality', see A. Goette, 
"Uber den Ursprung des Todes, " Hamburg and Leipzig, 1883 ; and A. Weis- 
mann, " Ueber die Dauer des Lebens, " Jena, 18S2 ; " Ueber Leben und Tod," 
Jena, 1884; E. Maupas, " Archives de Zoologie exp^rimentale, " 1888. 



244 '^^^ EVOLUTION OF SEX. 

CHAPTER XIX. 

PSYCHOLOGICAL AND ETHICAL ASPECTS. 

I. Common Ground Between Animals and Men. — 

Hitherto we have been justifying the orthodoxy of an anatomical 
training, by almost wholly ignoring the fact that animals have a 
psychic life, or only mentioning the mere neural aspect of functions. 
Only in discussing sexual selection, and the general facts of sexual 
union and of parentage, have we intruded words like " care," " sacri- 
fice," and "love." A purely physiological treatment of sex and 
reproduction is, however, obviously incomplete. It would be rejected 
with scorn in reference to human life; it must be equally rejected in 
regard to the higher animals, which, taken together, exhibit the 
analogues of almost every human emotion, and of all our less recon- 
dite intellectual processes. It is with emotions that we have here most 
to do; and without raising the difficult question whether animals 
exhibit any emotions exactly analogous to those which in man are 
associated with the "moral sense," "religion," and "the sublime," 
we accept the conclusion of Darwin, followed by Romanes and 
others, that all other emotions which we ourselves experience, are 
likewise recognizable in less perfect, or sometimes more perfect, 
expression in the higher animals. Those which are associated with 
sex and reproduction are indeed among the most patent; love of 
mates, love of offspring, lust, jealousy, family affection, social sympa- 
thies, are undeniable. 

II. The Love of Mates. — In the lowest animals, where two 
exhausted cells flow together in incipient sexual union, there is 
apparently only one component of that most complex musical chord 
in life which we call "love." There is physical attraction, and the 
whole process is very much a satisfaction of protoplasmic hunger. 

In multicellular animals, the liberation of sex-element is at first 
very passive. It concerns the individual alone. Fertilization is a 
random m.atter; and though sex exists, sexual attraction does not. 

A grade higher, true sexual union begins to appear. But at first 
this simply occurs between any male and any available female. The 
union is physiological, not psychological; there is no genuine paring, 
and it would be folly to use the word love in such cases. 

Gradually, however, for instance among insects, the sexes associate 
in pairs. There is some psychic sexual attraction, often accompanied 
with no httle courtship, but much more important is the occasional 



PSYCHOLOGICAL AND ETHICAL ASPECTS. 



245 



maintenance of the association for a lengthened period. There may 
e\ en be cooperation in work, as in dung-rolling beetles such as 
Ateuchus, where the two sexes pursue their somewhat disinterested 
labors together. The male and female of another lamellicorn beetle 
{Lethrus ephalotes) inhabit the same cavity, and the virtuous 
matron is said greatly to resent the intrusion of another male. As 
degenerate ofifshoots from the path of psychic progress, or as illustra- 
tions of the predominance of merely physical attraction, one must 
regard such prolonged associations of the two sexes as are seen in the 
formidable parasitic worm Bilharzia, where the male carries the 
female about, or in some parasitic crustaceans where the positions 
are reversed (see figs. pp. 15 and 67.) 

Amonar the cold-blooded fishes, the battles of the stickle-back 
with his rivals, his captivating manceuvers to lead the female to the 
nest which he has built, his mad dance of passion around her, and 
his subsequent jealous guarding of the nest, have often been observed 
and admired. In one of the sunfishes the male and female alternate 
in guarding the ova. The monogamous habits of the salmon, and the 
frequently fatal contests between rival males are well known. Car- 
bonnier has beautifully described the elaborateness of sexual display 
and the ardency of passion in the male butterfly-fish, and also in the 
rainbow-fish of the Ganges. 

The amatory croaking of frogs, the love-gambols of some newts, 
the curious paternal care of some male amphibians mentioned in the 
preceding chapter, and the like, illustrate the continuance of more 
than crude physical attraction between the sexes. It is indeed only 
in sexual and reproductive relations that the amphibians seem to wake 
up out of their constitutional sluggishness. 

In regard to reptiles, little is known beyond the exhibition of 
sexual passion and the jealous combats of rival males. Yet Romanes 
refers to the interesting fact that when a cobra is killed its mate is 
often found on the same spot a day or two afterwards. 

Among birds and mammals, the greater differentiation of the 
nervous system and the higher pitch of the whole lite is associated 
with the development of what pedantry alone can refuse to call love. 
Not only is there often partnership, cooperation, and evident affection 
beyond the limits of the breeding periods, but there are abundant 
illustrations of a high standard of morality, of all the familiar sexual 
crimes of mankind, and of every shade of flirtation, courtship, jealousy, 
and the like. There is no doubt that in the two highest classes of 
animals at least, the physical sympathies of sexuality have been 
enhanced by the emotional, if not also intellectual, sympathies of 
love. Those skeptical on this point should consult such a work as 



246 THE EVOLUTION OF SEX. 

Biichner's ^' Licbe und Uebcslcbcn in dcr ThicriveU'' which contains 
an overflowing wealth of instances. 

III. Sexual Attraction. — Mantegazza has written a work 
entitled " The Physiology of Love," in which he expounds the optim- 
istic doctrine that love is the unixersal dynamic; and from this 
Biichner quotes the sentence, that "the whole of Nature is one 
hymn of love." If the last word be used very widely, this often 
repeated utterance has more than poetic significance. But even in 
the most literal sense there is much truth in it, since so many anim.als 
are at one in the common habit of serenading their mates. The 
chirping of insects, the croaking of frogs, the calls of mammals, the 
song of birds, illustrate both the bathos and glory of the love-chorus. 
The works of Darwin and others have made us famihar with numerous 
ways, both gentle and violent, in which mammals woo one another. 
The display of decorations in which many male birds indulge, the 
amatory dances of others, the love-lights of glow-insects, the joyous 
tournaments or furious duels of rival suitors, the deliberate choice 
which not a few females exhibit, and the like, show how a process, 
at first crude enough, becomes enhanced by appeals to more than 
merely sexual appetite. But it is hardly necessary now to argue 
seriously in support of the thesis that love — in the sense of sexual 
sympathy, psychical as well as physical — exist among animals in 
many degrees of evolution. Our comparative psychology, too, has 
been too much influenced by our intellectual superiority; but while 
this, no doubt, has its correspondingly increased possibilities of emo- 
tional range, it does not necessarily imply a corresponding emotional 
intensity; and we have no means of measuring, much less limiting, 
that glow of organic emotion which so manifestly flushes the organism 
with color and floods the world with song. Who knows whether the 
song-bird be not beside the man what the child-musician is to the 
ordinary dullness of our daily toil and thought? The fact to be 
insisted upon is this — that the vague sexual attraction of the lowest 
organisms has been envolved into a definite reproductive impulse, 
into a desire often predominating over even that of self-preservation; 
that this again, enhanced by more and more subtle psychical additions, 
passes by a gentle gradient into the love of the highest animals, and 
of the average human individual. 

But the possibilities of evolution are not ended, and though some 
may shrink from that comparison of human love with its analogues in 
the oranic series, the theory of evolution offers the precise compensa- 
tion such natures require. Without recognizing the possibilities of indi- 
vidual and of racial evolution, we are shut up to the conventional view 
that the poet and his heroine alike are exceptional creations, hopelessly 



PSYCHOLOGICAL AND ETHICAL ASPECTS. 247 

beyond the everyday average of die race. Whereas, aJinitting the 
theory of evolution, we are not only entitled to the hope, but logically 
compelled to the assurance, that these rare fruits of an apparently more 
than earthly paradise of love, which only the forerunners of the race 
have been privileged to gather, or it may be to see from distant 
heights are yet the realities of a daily life toward which we and ours 
may journey. 

IV. Intellectual and Emotional Differences Between the 
Sexes. — We have seen that a deep difference in constitution expresses 
itself in the distinctions between male and female, whether these be 
physical or mental. The differences may be exaggerated or lessened, 
but to obliterate them it would be necessary to have all the evolution 
over again on a new basis. What was decided among the prehistoric 
Protozoa can not be annulled by Act of Parliament. In this mere 
outline we can not of course do more than indicate the relation of the 
biological differences between the sexes to the resulting psychological 
and social differentiations; for more than this neither space nor powers 
suffice. We must insist upon the biological considerations underlying 
the relation of the sexes, which have been too much discussed by 
contemporary writers of all schools, as if the known facts of sex did 
not exist at all, or almost as if these were a mere matter of muscular 
strength or weight of brain. Even a recent discussion which is pro- 
fessedly from the biological point of view, that of Mr. Romanes, 
sorely disappoints us in this regard. 

The reader need not be reminded of the oldest and most traditional 
views of the subjection of women inherited from the ancient European 
order; still less perhaps of the attitude of the ordinary politician, who 
supposes that the matter is one essentially to be settled by the giving 
or withholding of the franchise. The exclusi\-ely political view of the 
problem has in turn been to a large extent subordinated to that of 
economic laisses /aire, from which of course it consistently appeared 
that all things would be settled as soon as women were suiffciently 
plunged into the competitive industrial struggle for their own daily 
bread. While, as the complexly ruinous results of this intersexual 
competition for subsistence upon both sexes and upon family life 
have begun to become manifest, the more recent economic panacea of 
redistribution of wealth has naturally been invoked, and we have 
merely somehow to raise women's wages. 

All disputants have tolerably agreed in neglecting the historic, 
and still more the biological factors; while, so far as the past evolu- 
tion of the present state of things is taken into account at all, the 
position of women is regarded as having simply been that in which 
the stronger muscle and brain of man was able to place her. The 



24S 



THE EVOLUTION OF SEX. 



past of the race is thus depicted in the most sinister colors, and the 
whole view is supposed to be confirmed by appeal to the practice of 
the most degenerate races, and this again as described with the scanty 
sympathy or impartiality of the average white traveler, missionary, or 
settler. 

As we have already said, we can not attempt a full discussion of 
the question, but our book would be left, as biological books for the 
most part are, without point, and its essential thesis useless, if we did 
not, in conclusion, seek to call attention to the fundamental facts of 
organic difference, say rather divergent lines of differentiation, under- 
lying the whole problem of the sexes. We shall only suggest, as 
the best argument for the adoption of our standpoint, the way in 
■\vhich it becomes possible relatively to affiliate the most varied stand- 
points. We shall not so readily abuse the poor savage, who lies idle 
in the sun for days after his return from the hunting, while his heavy- 
laden wife toils and moils without complaint or cease; but bearino- 
in view the extreme bursts of exertion which such a life of incessant 
struggle with nature and his fellows for food and for life involves upon 
him, and the consequent necessity of correspondingly utilizing e\-ery 
opportunity of repose to recruit and eke out the short aud precarious 
life so indispensible to wife and weans, we shall see that this crude 
domestic economy is the best, the most moral, and the most kindlv, 
attainable under the circumstances. Again, the traveler from town 
who thinks the agricultural laborer a greedy brute for eating the 
morsel of bacon and leaving his wife and children only the bread, 
does not see that by acting othenvise the total ration would soon be 
still further lowered, by diminished earnings, loss of employment, or 
loss of health. 

The actual relations of fisherman and fishwife, of the smallest farmer 
and his wife, seem to us to give a truer as well as a healthier picture of 
antique industrial society than those we find in current literature; and 
if we admit that such life is deficient in refinement (although, on all 
deeper grounds, from religion to ballad poetry, we might even largely 
dispute this), it has still much to teach in respect of simplicity and 
health. 

The old view of the subjection of women was not, in fact, so much 
of tyranny as it seemed, but roughly tended to express the average 
division of labor; of course hardships were frequent, but these have 
been exaggerated. The absolute ratification of this by law and religion 
was merely of a piece with the whole order of belief and practice, in 
which men crushed themselves still more than their mates. Being 
absolute, however, such theories had to be overthrown, and the appli- 
cation of the idea of equality, which had done such good service in 



PSYCHOLOGICAL AND ETHICAL ASPECTS. 249 



demolishing the established castes, was a natural and serviceable one. 
We have above traced the development of this, however, and it is now 
full time to re-emphasize, this time of course with all scientific rela- 
tivity instead of a dogmatic authority, the biological factors of the case, 
and to suggest their possible service in destroying the economic falla- 
cies at present so prevalent, and still more toward reconstituting that 
complex and sympathetic cooperation between the differentiated sexes 
in and around which all progress past or future must depend. Instead 
of men and women merely laboring to produce things as the past 
economic theories insisted, or competing over the distribution of them, 
as we at present think so important, a further swing of economic 
theory will lead us round upon a higher spiral to the direct organic 
facts. So it is not for the sake of production or chstribution, of self- 
interest or mechanism, or any other idol of the economists, that the 
male organism organizes the climax of his life's struggle and labor, 
but for his mate; as she, and then he, also for their little ones. Pro- 
duction is for consumption; the species is its own highest, its sole 
essential product. The social order will clear itself, as it comes more 
in touch with biology. 

It is equally certain that the two sexes are complementary and 
mutually dependent. Virtually asexual organisms, like Bacteria, 
occupy no high place in Nature's roll of honor; virtually unisexual 
organisms, like many rotifers, are great rarities. Parthenogenesis 
may be an organic ideal, but it is one which has failed to realize itself. 
Males and females, like the sex-elements, are mutually dependent, and 
that not merely because they are males and females, but also in func- 
tions not directly associated with those of sex. But to dispute whether 
males or females are the higher, is like disputing the relative superior- 
ity of animals or plants. Each is higher in its own way, and the two 
are complementary. 

While there are broad general distinctions between the intellectual, 
and especially the emotional, characteristics of males and females 
among the higher animals, these not unfrequently tend to become 
mingled. There is, however, no evidence that they might be gradually 
obliterated. The seahorse, the obstetric frog, many male birds, are 
certainly maternal; while a few females fight for the males, and are 
stronger, or more passionate than their mates. But these are rarities. 
It is generally true that the males are more active, energetic, eager, 
passionate, and variable; the females more passive, conservative, slug- 
gish, and stable. The males, or to return to the terms of our thesis, 
the more katabolic organisms, are more variable, and therefore, as 
Brooks has especially emphasized, are \'ery frequently the leaders in 
evolutionary progress, while the more anabolic females tend rather to 



250 THE EVOLUTION OF SEX. 

preserve the constancy and integrity of the species; thus, in a word, 
the general heredity is perpetuated primarily by the female, while 
variations are introduced by the male. Yet along paths where the 
reproductive sacrifice was one of the determinants of progress, we shall 
see later that they must have the credit of leading the way. The 
more active males, with a consequently wider range of experience, 
may have bigger brains and more intelligence ; but the females, espe- 
cially as mothers, have indubitably a larger and more habitual share of 
the altruistic emotions. The males being usually stronger, have 
greater independence and courage; the females excel in constancy of 
affection and in sympathy. The spasmodic bursts of activity charac- 
teristic of males contrast with the continuous patience of the females, 
which we take to be an expression of constitutional contrast, and by 
no means, as some would have us believe, a mere product of masculine 
bullying. The stronger lust and passion of males is likewise the obverse 
of predominant katabolism. 

That men should have greater cerebral variability and therefore 
more originality, while women have greater stability and therefore 
more " common sense," are facts both consistent with the general 
theory of sex and verifiable in common experience. The woman, 
conserving the effects of past variations, has what may be called the 
greater integrating intelligence; the man, introducing new variations, 
is stronger in differentiation. The feminine passivity is expressed in 
greater patience, more open-mindedness, greater appreciation of subtle 
details, and consequently what we call more rapid intuition. The 
mascuHne activity lends a greater power of maximum effort, of scientific 
insight, or cerebral experiment with impressions, and is associated 
with an unobservant or impatient disregard of minute details, but with 
a stronger grasp of generalities. Man thinks more, woman feels more. 
He discovers more, but remembers less; she is more receptive, and 
less forgetful. 

V. The Love for Offspring.— Just as it is impossible to point 
to the stage where psychical syn.pathles enhance the reproductive 
impulse into the love of mates, so we can not tell where parental care 
becomes disinterested enough to warrant our calling it love of off- 
spring. For, as no one can be foolish enough deliberately to ignore 
the sexual or physical basis of ' ' love ' ' in the higher and highest 
organisms, so it must be allowed that even maternal care has its selfish 
side. To take only one example, that of lactation. The unrelieved 
pressure in the mammary glands of a mother animal robbed of her 
young is no doubt largely concerned in prompting her to adopt young 
ones not her own, yet we soon see these established in her affections. 
So in normal cases, there naturally remains an alloy which prevents us 



PSYCHOLOGICAL AND ETHICAL ASPECTS. 



251 



from regarding even maternal care as altogether disinterested. In all 
such cases, our interpretations risk an undue materialism on the one 
hand, and an undue transcendentalism on the other; and while our 




Fig. 96.— a Sea-cucumber, or Holothurian yCucumaria crocea), with numerous young attached to the 
skin.—From Carus Sterne, after "Challenger" Narrative. 



modern temper may habitually incline us to the former, we must not be 
too fond of taking for granted that all the common sense is on that 
side, for we must remember that the course of evolution not only has 
been, but must be toward the other. 

Among animals low down in the organic series there often occurs, 
as we have already noticed, a close association between mother and 



THE EVOLUTiON OF SEX. 



oflspriiig. Even in some ccelenterates, worms, and echinodcrms, the 
offspring cling about the mother animals, and may be protected in 
various kinds of brood-chambers. In some lowly crustaceans, the 
young may return to the shell-cavity ot the mother after hatching, and 
even after they have undergone a moulting. The young crayfish are 
said to return to the maternal shelter after they have been set adrift. 
The care of the nurse-bees for their charge, though not exactly 
maternal, deserves to be recalled; and the way in which ants save 




Fig. 97.— a Male "Seaspider," or Pycnogonid, carrying the ova. 

After Carus Sterne. 



the cocoons when danger threatens is well known. De Geer describes 
how one of the insects infesting plants behaves to her young brood 
exactly like a hen with her chickens; and Bonnet vividly describes a 
case where a mother spider, at the mercy of an ant-lion, fought for 
her eggs at the sacrifice of her own life. Some spiders, too, carry 
their young; and some crustaceans, like Gammarus, swim along with 
their young ones, like a hen among her chickens. Some cuttlefishes 
take pains in keeping their egg-clusters clean and safe; while even 
the headless fresh-water mussel retains her young, when there is no 



PSYCHOLOGICAL AND ETHICAL ASPECTS. 253 

fish present to which they ma_y attach themsehes. In fishes, it 
must be allowed that the care, il at all evident, is usually paternal; in 
amphibians, it is rare; in reptiles, somewhat more marked. In birds 
and mammals, however, paternal care is general, and unquestionably 
grows into love for offspring.> 




Fig. 98. — Egg-clusters of a species of Cuttlefish — From Von Hayek. 

VI. The Habits of the Curkoo. — As animals exhibit the 
analogues of the human virtues, it is not surprising to find the occur- 
rence of parallel vices. Those of much magnitude, such as paternal 
negligence or cruelty, are however rare, for the conditions of life are 
too simple to admit of such developed evils as in human society, 
while the crimes of sexuality are also lessened by the limitations 
of definite breeding-seasons. Without exposing the details of the 
crime-list, it will be instructive, as a concrete illustration, to discuss 
at some length the parasitic instinct of the cuckoo. 

Every schoolboy knows that the female cuckoo shirks the brood- 
ing sacrifice usually associated with bird maternity. But though 
as the Scriptures say, somewhat too severeh', of the ostrich, "she is 
hardened against her young ones, as though they were not hers," 
she is not ' ' deprived of wisdom " ; by an elaborate and well-executed 
trick she foists her several eggs, at intervals of a few days, into the 
nests of various birds, which are usually insectivorous and suited 
for the upbringing of the intruder. The foster-parents, all uncon- 



2c,^ THE EVOLUTION OF SEX. 

scious of being fooled, hatch the cuckoo egg among their own. 
The nestling grows rapidly, and is a dog in the manger by birth. 
Greedy and jealous, he (the pronoun is oftenest correct) soon asserts 
his monopoly of nest and food and care, by the summary eviction 
of the rightful tenants, whether they be still passive in ova or more 
awkwardly assertive as nestlings. The result is the success of the 
stronger. 

Of this habit there are various explanations, but the prevalent one 
regards it as only a special case of a universal method which favors 
selfishness. Jenner was the first to emphasize what he regarded 
as obvious advantages of the trick. The bird has but a short time 
to stay in its breeding area, and much to do in that short time. 
"Nature," he said, "has a call upon it to produce a numerous pro- 
gency," and as it is at the same time advantageous to migrate 
early, the gain of leaving the eggs to a succession of other birds 
to incubate is manifest. Darwin supposed the habit to crop up as 
a mere fortuitous variation, as it occasionally does in the normally 
nesting American cuckoo. The result was an advantage to the 
parent and also to the offspring; the former got away sooner, the 
latter were better cared for. Those that learned the trick prospered, 
those that did not were eliminated; and so, in virtue of its natural 
or unnatural success, the device passed from being exceptional to 
become universal, became in fact an inherited specific instinct. Com- 
menting upon this, Romanes, in a surely somewhat sanguine passage, 
says : ' ' We have here a sufficiently probable explanation of the raison 
d' etre of this curious instinct; and whether it is the true reason, or 
the only reason, we are justified in setting down the instinct to the 
creating influence of natural selection." 

But against the supposition that a mere freak has been fostered 
b)' selection into a habit, it must be noticed that the trick, to be 
successful, must be played with some care. It is hardly on a par 
with the casual use made by a partridge of a pheasant's nest, or 
by a gull of an eider duck's. Again, the advantages to the parent, 
apart from that of trouble saved, are somewhat dubious. Food, 
Macgilivray says, remains abundant, and the climate which does not 
injure the young for two months longer could hardly incommode the 
parents. Nor is the case improved outside the British area. To 
suppose, on the other hand, that the advantage to the young has 
formed the utilitarian basis, is involved in difficulties. We can 
not suppose that the mother bird had or has a careful forethought 
of the best for her offspring in sending them out to nurse. Nor 
is it easy to see how the comfort of fostered youth will remain as 
an impulse to the adult to do the like for her young in turn. The 



PSYCHCLOCICAL AND ETHICAL ASPECTS. 



255 



difficulty as to the inheritance of such a freak, especially with the 
preponderant majority of males, is certainly appreciable. The com- 
mon difficulty of the combination of happy circumstances required 
to insure incipent success is unusually great; the young bird has 
its part to play as well as the parent; the habit is not generic, yet 
obtains in related genera, and also in the widely separated starling- 
like cowbirds. 

A truer view of the habit is that which considers it as a deliberate 
expression of the whole constitution of the bird. 

1. The general character of the cuckoo is very significant. Brehm 
describes it as a " discontented, ill-conditioned, passionate, in short 
decidedly unamiable bird. ' ' ' The note itself, and the manner in 
which it is emitted, are typical of the bird's habits and character. 
The same abruptness, insatiability, eagerness, the same rage, are notice- 
able in its whole conduct." The cuckoos are notoriously unsociable, 
even in migration individualistic. They jealously guard their terri- 
torial "preserves," and verify in many ways the old myth that they 
are sparrow-hawks in disguise. The parasitic habit is consonant with 
their general character. 

2. The species consists predominantly of males. The prepon- 
derance is probably about five to one, though one observer makes it 
five times greater. In so male a species, it is not surprising to find 
degenerate maternal instincts. 

3. Reproduction and nutrition, we have seen, vary inversely. The 
love-impulses wane before those of hunger. Now there is no doubt 
that even among greedy birds the cuckoos hold a very high rank. 
They are remarkably insatiable, hungry, gluttonous. Even the ana- 
tomical conditions asserted by some to be important, the swollen 
low-set stomach, may have their influence in the cuckoo, which has 
certain other peculiarities, though the same conditions may be over- 
come in other birds which remain perfectly natural. It might almost 
be suggested, that the habit of feeding so largely as cuckoos do on 
hairy caterpillars, whose indigestible hairs form a fretwork in the giz- 
zard, may also have its irritant, gizzard-fretting, dyspeptic influence. 
But the main point is, that in a bird with so strong nutritive impulses, 
it is little wonder the reproductive emotions are degenerate. There is 
too much hunger and gluttony for the higher development of love. 

4. The reproductive relations of the sexes are at a lower level than 
polygamy, or rather polyandry. The males and females do not pair 
in the strict sense, there is no keeping company, though the males are 
said to be passionate during the breeding-season. Nor is the female in 
its adult state externally distinguishable from the male. 



256 



THE EVOLUTION OF SEX. 



5. The reproductive organs of both sexes are very small for the 
size of the bird. There is said to be a diminished blood supply. Little 
wonder, then, that the reproductive emotions are in degree slightly 
developed. The sluggish parturition, at intervals of six to eight days, 
is also striking and significant. 

6. The eggs are remarkably small. While the adult cuckoo is 
some four times the size of an adult skylark, the eggs are about the 
same size. The American cuckoo, which is only occasionally parasitic, 
lays full-sized eggs. It is true that the size of an ttg'g is not always 
proportionate to the size of the bird; but it is reasonable to beheve 
that when a bird for constitutional conditions seems to require all it 
can for itself then it will have less to spare for its reproductive sacri- 
fice. To say that the small size of the cuckoo's &gg is " an adaptation 
in order to deceive the small birds," seems to strain the natural selec- 
tion theory to the breaking point. 

7. It has been usual in discussing beginnings to take some cue 
from the young stages. It is noteworthy, in this light, to emphasize 
the jealous cruelty of the young form, — a fit prophecy of the adult 
character. In the restlessness of rapid growth, the nestling expresses 
the constitution of the species in its selfish monopolizing greed and 
insatiable appetite. Observations are recorded of the persistence of 
the cruel disposition into adolescence, though it usually wanes with the 
anatomical peculiarity of the back, not very long after birth. The 
young form at any rate exhibits the essential character of the species. 

8. Some corroboration is obtained from the character of the 
American cuckoo. There seems no doubt that it is occasionally para- 
sitic, and it is interesting to note that observers speak of its unnaturally 
careless indifference for the fate of its young. The character, in fact, 
is less markedly evil; the occasional parasitism is just as intelligible as 
the occasional " reversion " of our cuckoo to ancestral habits, even in 
some cases to apparent affection for the young. 

9. In the cowbirds, again, where the habit occurs in different 
species in different degrees of perfection (if the term be admissible), 
the character is strikingly immoral. In one species {Molothrus cadius), 
a nest may be simply stolen, or the rightful nestlings may be thrown 
out, or actual parasitism may occur as an exception. In M. canariejisis, 
the eggs may be dropped on the bare ground, or fifteen to twenty 
from different parents may be lazily and of course fatally huddled 
together in one nest. Two cuckoo-eggs are sometimes found in one 
nest. In M. pecoi-is, which is polygamous, the crime has been evolved, 
and the habit is that of our cuckoo, one egg being laid in each foster- 
nest. The important point is the general immorahty and reproductive 



PSYCHOLOGICAL AND ETHICAL ASPECTS. 257 

carelessness, which in one species finds expression in an organized 
device. 

Conclusion. — The general character of the birds ^ — the unsocial 
life, the selfish cruelty of the nestlings, and the lazy parasitic habit — 
ha\'e a common basis in the constitution. The insatiable appetite, the 
small size of the reproductive organs, the smallness of the eggs, the 
sluggish parturition, the rapid growth of the young, the great prepon- 
derance of males, the absence of true pairing, the degeneration of 
maternal affection, are all correlated, and largely explicable, in terms 
of the fundamental contrast between nutrition and reproduction, 
between hunger and love. Similar unnatural or immoral instincts in 
other birds, in mammals, and even in the lower animals, are explicable ' 
in similar terms. The cuckoo's habit is a natural outcrop of the 
general character or constitution, only one expression of a dominant 
diathesis. 

In his recent important work on the " Origin of Species," Professor 
Eimer maintains a similar view. He briefly criticises the Darwinian 
explanation, which appears to him to postulate too many happy com- 
binations. He maintains that the ancestral cuckoo acted deliberately in 
the trick, and some of this dehberateness of device may still persist. 
The explanation of the unnatural habit is to be found in the bird's 
whole character and mode of life. In this connection Eimer empha- 
sizes (a) the vagabond, restless habit; {b) the looseness of the sex- 
relations, strong in passion, weak in love; (c) the irregular and 
gluttonous nutrition considered in relation to reproductive stimulus; 
{d') the slow laying of the eggs, itself dependent upon nutrition, and 
pointing to physiological conditions which modify even the deeply- 
rooted impulse and instinct to brood; (e) the degeneration of social 
instincts, and the preponderance of the egoistic. 

VII. Egoism and Altruism. — The optimism which finds in I 
animal life only ' ' one hymn of love ' ' is inaccurate, like the pessimism 
which sees throughout nothing but selfishness. Littr6, Leconte, and J: 
some others less definitely, have more reasonably recognized the coex- 
istence of twin streams of egoism and altruism, which often merge for 
a space without losing their distinctness, and are traceable to a com- 
mon origin in the simplest forms of life. In the hunger and reproductive 
attractions of the lowest organisms, the self- regarding and other- 
regarding activities of the higher find their starting-point. Though 
some vague consciousness is perhaps coexistent with life itself, we can 
only speak with confidence of psychical egoism and altruism after a 
central nervous sj'stem has been definitely estabHshed. At the same 
time, the activities of e-\'en the lowest organisms are often distinctly 
referable to either category. 



258 



THE EVOLUTION OF SEX. 



A simple organism, which merely feeds and grows, and liberates 
superfluous portions of its substance to start new existences, is plainly- 
living an egoistic and individualistic life. But whenever we find the 
occurrence of close association with another form, we find the first 
rude hints of love. It may still be almost wholly an organic hunger 
which prompts the union, but it is the beginning of life not wholly 
individualistic. Hardly distinguishable at the outset, the primitive 
hunger and lo\'e become the startii 4-points of divergent lines of egois- 
tic and altruistic emotion and activity. 



Ideal unity, 




society, 



family. 



offspring. 



mates. 



Fig. 99.— Protoplasmic Identity. Diagrammatic Representation of the Relations 
between Nutritive, Self-Maintaining, or Egoistic, and Reproductive, 
Species-Regarding, or Altruistic Activities. 



The differentiation of separate sexes; the production of offspring 
which remain associated with the parents; the occurrence of genuine 
pairing beyond the hmits of the sexual period; the estabhshment of 
distinct famiUes, with unmistakable affection between parents, offspring. 



PSYCHOLOGICAL AND ETHICAL ASPECTS. 259 

and relatives; and lastly, the occurrence of animal varieties wider than 
the family, — mark important steps in the evolution of both egoism and 
altruism. 

The diagram sums up the important facts. There are two diver- 
gent lines of emotional and practical activity, — hunger, self- regarding, 
egoism, on the one hand; love, other-regarding, altruism, on the other. 
These find a basal unity in the primitively close association between 
hunger and love, between nutritive and reproductive needs. Each 
plane of ascent marks a widening and ennobling of the activities; but 
each has its corresponding bathos, when either side unduly prepon- 
derates over the other. The actual path of progress is represented by 
action and reaction between the two complementary functions, the 
mingling becoming more and more intricate. Sexual attraction ceases 
to be wholly seliish; hunger may be overcome by love; love of mates 
is enhanced by love for offspring; love for offspring broadens out into 
love of kind. Finally, the ideal before us is a more harmonious blend- 
ing of the two streams. 



26o THE EVOLUTION OF SEX. 



SUMMARY. 

I. In most of the emotions, and in the simpler intellectual processes, there is 
common ground between animals and men. This is especially true of the 
emotions associated with sex and reproduction. 

II. The love of mates has its roots in physical sexual attraction, but has been 
gradually enhanced by psychical sympathies. 

III. The means of sexual attraction rise from the crude and physical to the 
subtle and psychical, with the growth of love. 

IV. The intellectual and emotional differences between the sexes are 
correlated with the deep-seated constitutional differences. Males and females 
are complementary, each higher in its own way. 

V. The love for offspring has grown as gradually as the love for mates. 
Even lactation and maternal care may be in part egoistic. Except in a few 
precociously tender animals, genuine love for offspring is only emphatic in birds 
and mammals, where the reproductive sacrifice of the mother has also been 
increased. 

VI. The cuckoo illustrates the evolution of a criminal habit, mainly due to 
constitutional conditions. 

VII. Egoism and altruism have their roots in the primary hunger and love, 
or nutritive and reproductive activities. The divergent streams of emotion and 
activity have a common origin, subtly mingle at various turning-points, and 
ought to blend more and more in one. 



LITERATURE. 

See works on Sexual Selection cited at Chapter I. 

EiMER, G. H. T. — Die Entstehung der Arten auf Grund von Vererben 

Erworbener Eigenschaften nach den Gesetzen Organischen Wachsens. 

Jena, 1888. 
BucHNER, L. — Liebe und Leibesleben in der Thierwelt. Berlin, 1879. 
ROLPH, W. H.— Op. cit. 
Romanes, G. J. — Animal Intelligence. Intemat. Sci. Series. Fourth edition, 

1S86 ; and Mental Evolution in Animals, by the same. 
Thomson, J. A.— A Theory of the Parasitic Habit of the Cuckoo. Proc. Roy. 

Phys. Soc. Edin. 1888. 
See sXio Carus Sterne's most admirable of general natural history books — 

Werden und Vergehen. Third edition. Berlin, 1886. 
Ploss.— Das Weib in der Natur und Volkerkunde. Second edition. Leipzig, 

1887. 
Mantegazza, p.— Die Physiologie der Liebe ; Die Hygiene der Liebe ; 

Anthropologisch-Kulturhistorische Studien uber die Geschlechtsverhaltnisse 

des Menschen. Jena. 



LA WS OF MUL TIPLICA TION. 26 1 



CHAPTER XX. 
LAWS OF MULTIPLICATION. 

I. Rate of Reproduction and Rate of Increase. — We know 
much more about the rate at which organisms reproduce, than about 
the rate at which the number of adults in reality increases or decreases. 
The one fact may be ascertained by observation; the other involves 
comparative statistics, which are difficult enough to obtain, even 
for the human species. The rate of reproduction depends upon 
the constitution of the individual and its immediate environment, 
including, above all, its nutrition. The rate of increase or decrease 
depends upon the wide and complex conditions of the entire animate 
and inanimate environment, or upon the degree of success in the 
struggle for existence. 

That there are enormous differences in the rates of reproduction 
is very evident. Maupas tells us how a single infusorian becomes 
in a week the ancestor of a progeny only computable in millions, — 
of numbers which the progeny of a pair of elephants, supposing they 
all lived their natural term of years, would not attain to in five cen- 
turies. Again, Huxley calculates that the progeny of a singe par- 
thenogenetic plant-louse — supposed again to live a charmed life — 
would in a few months literally outweigh the population of China. 
The geometrical ratio of reproduction, so often emphasized, would 
indeed have starthng results if it involved real, and not merely 
potential, increase. 

That it does sometimes realize itself for short periods or special 
areas of favorable conditions is well known; for instance, in the peri- 
odic plagues of insects, or in the still unmastered rabbit pest of 
Australia. But in the established fauna and flora of a country, 
without intruded importations or marked climatic changes, the rise 
and fall of population is seldom emphatic. The rate of reproduction 
is only one factor in the numerical strength of the species or in 
its increase. The common tapeworm produces myriads of embryos, 
but these have only one chance in eighty-five millions (it is said) of 
succeeding. Many common and numerous animals reproduce very 
slowly. That some species are on the increase, for example, bac- 
teria, under the unprecedentedly favorable conditions which our 
recent ' ' industrial progress ' ' affords, while other species are on the 
decrease, for example, many birds, is certain; but the rate of repro- 
duction is not a direct condition in either case. 



262 THE EVOLUTION OE SEX. 

II. History of Discussion on Rate of Reproduction. — In 

this, as in not a few other cases, the biologist is profoundly indebted 
to the student of social questions, for no adequate attention was paid 
to the laws of multiplication before the appearance of the epoch- 
making "theory of population" of Malthus, nor is it yet possible or 
profitable to isolate the human question from the general one. 
Malthus' s fundamental proposition is indeed usually softened from 
its earliest form — the population tends to increase in geometrical, 
subsistence only in arithmetical ratio — into the simple statement that 
population tends to outrun subsistence, but has none the less served 
as a base of weighty deductions for both the naturalist and the 
economist. From Darwin's standpoint, the "positive checks" to 
population (disease, stan'ation, war, infanticide), and the "pruden- 
tial" (moral or birth-restricting) checks, come to be viewed as 
special forms of natural or artificial selection, while the fundamental 
induction has been extended throughout Nature as the essential 
condition of the struggle for existence. After long dispute, the 
induction of Malthus gained acceptance, followed by wide deductive 
use and abuse, among economists. Yet, fundamentally important as 
the subject thus is to naturalist and economist alike, the former has 
not as yet effected any thorough investigation of the conditions of 
multiplication, or even usually incorporated the keen analysis which 
we owe to Spencer, while the economic theorist or disputant frequently 
still employs the doctrine even in its pre-Darwinian form. It is thus 
doubly needful to summarize, as briefly as maybe, Spencer's elaborate 
statement of the laws of multiplication. 

III. Summary of Spencer's Analysis. — Different species exhibit different 
degrees of fertility, wliich have become established in process of evolution like 
the organisms themselves. To understand this particular adaptation of function 
to conditions of existence, of organism to environment, we may analyze these 
into their respective factors. It is evident that in the environment of any species 
there are many conditions with which its individuals establish a moving 
equilibrium, sooner or later overthiown in death. To prevent extinction, the 
organism meets these environing actions in two distinct ways, — (i) by individual 
adaptations, active thrusts or passive parries ; (2) by the production of new 
individuals to replace those overthrown, — in other words, hy genesis. The 
latter may occur, as we have seen, in varied forms, sexual or asexual, and at 
various rates, which depend upon age, frequency, fertility, and duration of 
reproduction, together with amount and nature of parental aid. These actions 
and reactions of environment and organism admit of another grouping in more 
familiar terms, into two conflicting sets, — (a) the forces destructive of race ; (b) 
the forces preservative of race. 

Leaving aside cases in which permanent predominance of destructive forces 
causes extinction, and also, as infinitely improbable, cases of perfectly 
stationary numbers, the inquiry is : In races that continue to exist, what laws 
of numerical variation result from these variable conflicting forces that are 



LAWS OF MULTIPLICATION. 263 

respectively destructive or preservative of race? How is tlie alternate excess of 
one or other rectified? A self-sustaining balance must exist; the alternate 
predominance of each force must initiate a compensatory excess of the other • 
how is this to be explained ? 

When favorable circumstances cause any species to become unusually 
numerous, an immediate increase of destructive influences, passive as well as 
active, takes place ; competition becomes keener and enemies more abundant, 
and conversely. Yet this is not the sole, much less the permanent, means of 
establishing a balance ; nor does it explain either the differences in the rate of 
fertility and mortality, or the adaptation of one to the other. This minor adjust- 
ment in fact implies a major one. 

The forces preservative of race were seen above to be two, — power to 
maintain individual Hfe, and power to generate the species. Now, in a species 
which survives, given the forces destructive of race as a constant quantity, those 
preservative of race must be a constant quantity also ; and, since the latter are 
two, the individual plus the reproductive, these must vary inversely, one must 
decrease as the other increases. To this law every species must conform, or 
cease to exist. Let us restate this at greater length. A species in which self- 
preser\'ative life is low, and in which the individuals are accordingly rapidly 
overthrown in the struggle with the destructive forces, must become extinct, 
unless the other race-preservative factor be proportionally strengthened,— 
unless, that is to say, its reproductive power become proportionally great. On 
the other hand, if both preservative factors be increased, if a species of high 
self-preservative power were also endowed with powers of rnultiplication beyond 
what is needful, such success of fertility, if extreme, would cause sudden 
extinction of the species by starvation, and if less extreme, and so effecting a 
permanent increase of the numbers of the species, would next bring about such 
intenser competition, such increased dangers to individual life, that the great 
self-preservative power would not be more than sufficient to cope with them. 

In short, then, we have reached the a priori principle, that in races which | 
continuously survive, in which the destructive forces are balanced bv the 
preservative ones, there must be an inverse proportion between the power to | 
sustain individual life and the power to produce new individuals. But what is ^ 
the physiological explanation of this adjustment, and how has it arisen in 
process of evolution ? Spencer has elsewhere enlarged upon the proposition, 
which we have already illustrated, that genesis in all its forms is a process of 
disintegration, and is thus essentially opposed to that process of integration 
which is one element of individual evolution. The matter and energy supplied 
for the young organism represent so much loss for the parent ; while, conversely, 
the larger the amount of matter and energy consumed by the functional actions 
of the parent, the less must be the amount remaining for those of the offspring. 
The disintegration which constitutes genesis may be complete or partial, and in 
the latter case the parent, having reached considerable bulk and complexity 
before reproduction sets in, may survive the process. In the same way, 
individual evolution may be expressed in bulk, in structure, in amount or 
variety of action, or in combinations of these ; yet, in any case, this progress of 
each individuality must correspondingly retard the establishment of the new 
ones. 

While in the first portion of the argument, then, it was shown that a species 
can not be maintained unless self-preser\'ative and reproductive power vary 
inversely, it is now evident that, irrespecti\'e of an end to be subserved, these 
powers can not do other than vary inversely, and the one a priori principle is 



264 



THE EVOLUTION OF SEX. 



thus seen to be the obverse of the other. And if we group under the term 
individuation all those race-preservative processes by which individual life is 
completed and maintained, and extend the term genesis to include all those 
processes aiding the formation and perfecting of new individuals, the result of 
the whole argument may be tersely expressed in the formula — Individuation 
and Genesis vary inversely. And from this conception im- 
portant coroUeries open ; thus, other things equal, advancing 
evolution must be accompanied by declining fertility ; again, 
if the difficulties of self-preservation permanently diminish, 
there will be a permanent increase in the rate of multiplica- 
tion, and conversely. 

In attempting the inductive verification of these a priori 
inferences, practical difficulties arise, owing to the high com- 
plexity of each of our two sets of factors and the independent 
variability of their details, and thus the total cost of indi- 
viduation and of genesis alike is hard of estimation and com- 
parison. For this purpose, however, there are successively 
to be investigated, — (i) the antagonism between growth and 
genesis, sexual and asexual ; (2) that between development 
and genesis ; (3) that between expenditure and genesis ; and 
(4) the coincidence between high nutrition and genesis. It is 
impossible to summarize the wealth of evidence drawn from 
a wide survey of the animal and vegetable world contained 
in the chapters devoted to those various heads, but attention 
maj' be called to the last and most obscure of these. It is 
indeed evident a priori that, if the cost of individuation be 
once provided for, a higher nutrition will render possible a 
greater propagation, sexual or asexual, and this may be 
abundantly verified by observation and experiment. \\'itness 
tlie case of aphides, in which the rate of parthenogenetic 
reproduction is found to be directly proportional to tempera- 
ture and food-supply ; or, again, that of domestic animals, 
such as the sheep, whose fertility is in direct relation to rich- 
ness of pasture and warmth of climate ; or, finally, and most 
obviously of all, tliat of field or fruit crops, upon which the 
influence of increased liberality of manuring will not be dis- 
puted. Yet it is sometimes maintained, for both plants and 
animals, that overfeeding checks increase, while limited nutri- 
ment stimulates it ; and to support this view there are cited 
such cases as that of the barrenness of a ver>' luxuriant plant, 
and the fruitfulness which appears on its depletion. But if 
this objection really held, manuring would in all cases be 
inexpedient, instead of only in plants where the growth of 
sexless axes is still too luxuriant ; and a tree which has borne 
a heavy crop should, by this depletion, bear again yet more 
heavily, instead of being more or less barren next year unless manured. Or the 
difficult)' may also be met by interpreting such vegetative luxuriance, not as a 
case of higher individuation at all, but simply as a case of asexual multipli- 
cation of secondary axes ; or again, and perhaps most simply, by regarding the 
appearance of sexual reproduction on depletion simply as a case of the previ- 
ously demonstrated antagonism between genesis and growth. 



Fig. 100. — A species 
of Onion with asex- 
ual vegetative bul- 
bils {/') among tiie 
flowers (a). 



LAWS OF MULTIPLICATION. 265 

But again, since fatness is associated vvitli sterility, it is often argued that higli 
feeding is unfavorable to genesis. Obesity, however, is now known to be 
associated with imperfect assimilation, with physiological impoverishment or 
degeneration,— by no means with that constitutional wealth which is favorable 
to fertility. If, in short, we bear in mind that truly high nutrition means only 
due abundance of, and due proportion among, all the substances which the 
organism requires, and that their perfect assimilation by the organism is also 
needful, such objections to the generalization not only disappear, but such a 
phenomenon as the coincidence of returning fertility with disappearing obesity 
affords a confirmatory argument. 

Organisms having aberrant modes of life are next appealed to for crucial 
evidence bearing on these general doctrines. Thus, turning to vegetable and 
animal parasites, which combine superabundant nutrition with greatly diminished 
e.xpenditure, the enormous fertility exhibited by all such forms is seen to be the 
necessary correlative of such a state of nutrition and expenditure, and not 
merely an acquired adaptation to their peculiar difficulties of survival. The 
reversion exhibited by so many species (especially among the higher arthropods, 
for example. Aphis, Cecidomyia) from sexual reproduction to primitive forms 
of genesis, is explained by pointing out that such species are peculiarly situated 
in obtaining abundant food with little exertion. Among bees, ants, and termites 
alike, the enormous fertility of the inactive and highly nourished queen-mother 
are obviously also cases in point. 

The inverse variation of genesis with individuation has no\v been demon- 
strated inductively as well as deductively, and that for each element of the latter 
(growth, development, or activity). Yet before discussing its application to the 
problems of the multiplication of the human species, two points remain, — a 
question has to be answered, and a qualification made. The question, only 
partially answered in course of the preceding argument, is, How is the ratio 
between individuation and genesis established in each special case? and the 
answer is. By natural selection. This may determine whether the quantity of 
matter spared from individuation for genesis be divided into many small ova or 
a few larger ones ; whether there shall be small broods at short intervals, or 
larger broods at longer intervals ; or whether there shall be many unprotected 
offspring, or a few carefully protected by the parent. Again, sun.-ival of the 
fittest has a share in determining the proportion of matter subtracted from 
individuation for genesis. Yet this operation of natural selection goes on strictly 
under the limits of the antogonism above traced. 

The needed qualification arises on introducing the conception of evolutionary 
change. If time be left out of account as hitherto, — or, what is the same thing, 
if all the species be viewed as permanent, — the inverse ratio between individua- 
tion and genesis holds absolutely. But each advance in individual evolution (it 
matters not whether in bulk, in structure, or in activities) implies an economy ; 
the advantage must exceed the cost, else it would not be perpetuated. The 
animal thus becomes physiologically richer ; it has an augmentation of total 
wealth to share between its individuation and its genesis. And thus, though the 
increment of individuation tends to produce a corresponding decrement of 
genesis, this latter will be somewhat less than accurately proportionate. The 
product of the two factors is greater than before ; the forces presen.'ative of race 
become greater than the forces destructive of race, and the species spreads. In 
short, genesis decreases as individuation increases, yet not quite so fast. 



266 THE EVOLUTION OE SEX. 



Hence e\'ery type that is best adapted to its conditions — every liigher 
type _ has a rate of multiplication that insures a tendency to predominate. For 
thousfh the more e\'olved organism is the less fertile absolutely, it is the more 
fertile relatively. 

The whole generahzation admits of the simplest graphic illustra- 
tion. For if the line AB represents the aggregate matter or energies. 

C 

A \ B 

the structures or the functions, of the organism, of which AC denotes 
the amount devoted to individuation and CB to reproduction, the 
inverse variation of AC to CB is obvious, as also if AC and CB 
represent the psychological obverse of these two classes of function. 
Nor does an increase in total energy modify this, as when the stronger 
members of a species frequently also exhibit greater reproductive 
power; for if in one case AB = 20, of which CB = 4, and in another 
AB :^ 25, CB may become 5 without any rise of reproductive ratio, 
since ^ij ^ ^^. But if the species be evolving, the advance in individ- 
uation implies a certain economy, of which a share may go to 
diminish the decrement to genesis, as above explained. 

IV. Spencer's Application of his Results to Man. — In 
extending this hard-won generalization to the case of man, the con- 
comitance of all but highest total individuation with all but lowest 
rate of multiplication (the enormous bulk of the elephant involving a 
yet greater deduction from genesis) is at once apparent. Comparing 
different races of nations, or even different social castes or occupa- 
tions, the same holds good; while the prevalence of high multiplica- 
tion in races of which the nutrition is in obvious excess over the 
expenditure is also evident, witness the Boers or French Canadians. 
Such an apparent difficulty as that of the Irish, in whom rapid multi- 
plication occurs despise poor food, is accounted for by the relatively 
low expenditure in obtaining it (since the "law of diminishing return" 
implies its converse for diminishing labor), though, no doubt, also in 
part by the habit of early marriage, if not by some measure of 
lowered individuation as well. The mam position being established, 
Spencer proceeds to discuss the question of human population in the 
future, and insists strongly on the importance of pressure of popula- 
tion, which he regards as the main incentive to progress alike in past, 
present, and future. Reviewing the possibilities of progress in bulk, 
complexity of structure, multiplication and variation of function, he 
concludes that the more complete moving equilibrium, and more 
perfect correspondence between organism and environment, which 
such evolution involves, must take place mainly in the direction of 
psychical development. Yet this development, while stimulated by 



LAIVS OF MULTIPLICATION. 



267 



pressure of population, constantly tends to diminish the rate of fer- 
tility; in other words, this cause of progress tends to disappear as it 
achieves its full effect. The acute pressure of population, with its atten- 
dant evils, thus tends to cease as a more and more highly individuated 
race busies itself with its increasingly complex yet normal and 
pleasurable activities, its rate of reproduction meanwhile descending 
toward that minimum required to make good its inevitable losses. 
V. Summary of the Population Question. — The general 
question, so far as yet developed, may now be conveniently sum- 
marized in the accompanying tabular form. Here the stage of 
knowledge reached by each author, together with any practical 
applications therefrom deduced, may be read horizontally, while ' 
the historic development of each separate line of conceptions may 
be traced vertically. 



Author. 


Development of Theory of Population. 


Practical Action 

Deduced. 


I. 

Non-biological 
writers (pre- 
decessors and 
opponents of 
Malthus). 


Increase of population does 
not tend to outrun subsist- 
ence. 








II. 

Malthus. 

1798. 


Increase of population tends 
to outrun that of subsist- 
ence. 


But meets checks: 

A. Positive. 

B. Preventive. 




To avoid A, adopt 
B. 


11 L 
Darwin. 

1859. 


Do. 


Hence struggle for 
existence : 

A. Natural se- 
lection. 

\\. Artificial se- 
lection. 


Leading to 
evolution. 

Do. 
Also leading to 
evolution of 
species. 


Laissez-faire, that 
is, on account of 
advantage to 
species from A, 
avo.d R, 


IV. 
Spencer. 

1852-66 


Do. 
Rate of multiplication inves- 
tigated for different spe- 
cies, and shown to vary in- 
versely as individuation. 


Do. 


Do. 
[ [udividiinte. ] 



From such a summary, brief as it is, the main steps in the develop- 
ment of our knowledge are clear enough, but a deeper analysis is 
required before final exposition or complete application is possible. 
Nor, when we note how vast the progress of science through the 
advance in precision and extension effected upon the conception of 
Malthus* by Darwin, will the utility of such increasing ekiboration be 



* It is also interesting to compare Malthus's view of population, tending to 
increase in geometrical proportion and substance only in arithmetical, with 
Spencer's demonstration of the limit of growth already summarized (see p. 
204) the more so when we bear in mind that reproduction is discontinuous 
growth. The precise statement of Malthus becomes confirmed, as regards the 
cell, if not the cell-aggregate. 



268 



THE EVOLUTION OF SEX. 



/ 



A 



\ 



I 



disputed. Thus the full inductive verification of Spencer's law 
involves a detailed comparison of the rates of reproduction of each 
g;roup of organic species, with their observed degree of individuation 
(first in each of its factors, and finally in their sum), deviations from 
the inverted symmetry of the theoretic curves (see fig. below) 
having to be separately discussed. Natural selection also requires a 
yet deeper analysis; the limits and possibilities of artificial selection 
are but little known, while a theory of variation is still far from agreed 
upon. If, however, we bear in mind that the amount of evolution in 
given time is but small, our knowledge seems not insufficient for the 
practical deductions which are so pressingly 
demanded; yet it is here that the most serious 
disagreement has prevailed. Thus the Mal- 
thusian position is obviously inadequate, in 
not allowing for the Darwinian one; yet the 
converse also is undeniable, for the position of 
laisscs-faire, upon which Darwin and Spencer 
alike take their stand, not only almost ignores 
the wellbeing of the individual in considering 
the advancement of the species, but is even 
then too optimistic, since it not only fails to 
accelerate the progressive evolution which is 
alone considered, but also fails to provide 
against the equal possibility of degenerative 
change. Are we then simply to return to the 
somewhat crude proposals and excessive hopes 
for the increase of individual wellbeing due to 
Malthus or his followers, — based, too, as these 
have been on imperfect pre-Spencerian knowl- 
edge. 

The answer is not far to seek, — it lies in the 
generalization above established ; yet it is 
remarkable that Mr. Spencer, after not only 
establishing the inverse variation of individua- 
tion and genesis among species in general, but even showing for the 
human species in particular that it is essentially upon increase of the 
psychical activities that the increased individuation and diminished 
genesis of the future must depend, should not have proceeded to a 
fuller application. For unless the main generalization be abandoned, 
it is obvious that the progress of the species and of the individual 
alike is secured and accelerated whenever action is transferred from the 
negati^'e side of merely seeking directly to repress genesis, to the posi- 
tive yet indirect side of proper; ionally increasing individuation. This 



tik 



& 



Fig. ioi. — Let the perpendicu- 
lars above the line A B denote 
the increasing degree of total 
individuation of a series of 
forms I, 2, 3, 4, 5, 6 (say- 
Worm, Fish, Frog, Bird, Man, 
Elephant), and similarly let 
the perpendiculars to C D rep- 
resent the rate of multiplica- 
tion of the same forms; the 
curves joining these two scries 
of points respectively illustrate 
by their inverted symmetry 
the inverse ratio of individu- 
ation and genesis. 



LAIVS OF MULTIPLICATION. 269 



holds true of all species, yet most fully of man, since that modification 
of psychical activities in which his evolution essentially lies, is par 
excellence and increasingly the respect in which artificial comes in to 
replace natural selection. Without therefore ignoring the latter, or 
hoping ever wholly to escape from the iron grasp of Nature, we yet 
have within our power more and more to mitigate the pressure of 
population, and that without any sacrifice of progress, but actually 
by hastening it. Since, then, the remedy of pressure and the hope 
of progress alike he .in advancing individuation, the course for practical 
action is clear, — it is in the organization of these alternate reactions 
between bettered environment (material, mental, social, moral) and 
better organism in which the whole evolution of life is defined, in 
the conscious and rational adjustment of the struggle into the culture 
of existence. 

The practical corollaries of the Malthusian view are celibacy, late 
marriage, and moral control ; the objections are vice, increased 
mortality in childbirth, and the present low evolution of our moral 
nature. The practical corollary of the Darwinian doctrine is virtually 
nil; the objection, that the survival of what we consider the best types 
is doubtful, and that the survival of the fit is apt to be cruel. The 
practical corollaries of the Spencerian principle, although Mr. Spencer 
can hardly be said to have insisted upon these, are individuate and 
educate. The objection is, that the pressure of population is already 
felt, and that individuation is a matter of centuries. Furthermore, the 
effect of education, for instance in reducing se.xuality, will tell most 
where it is least wanted, namely, among the best types. 

We are therefore bound to include, as a continuation of the above 
table, the amendment, of some of the most thoughtful exponents of 
what is generahy called neo- Malthusian doctrine. This advocates the 
use of artificial preventive checks to fertilization. Discussion of this 
proposal is at present difficult, because of the comparative absence of 
distinctly expressed opinion on the part of medical experts, and because 
of strong superficial prejudices, not only against the scheme, but 
against its discussion. These prejudices are, however, dying out; and 
that is well, for they do nothing but obscure appreciation alike of the 
merits and demerits of the doctrine. An increasing realization of the 
plain facts of reproduction and population must rapidly exterminate the 
persistently theological absurdities which people utter, if they do not 
believe on the subject. The vague feeling that control of fertilization 
is "interfering with Nature," in some utterly unwarrantable fashion, 
can not be consistently stated by those who live in the midst of our 
highly artificial civilization. The strongest prejudice seems to be 
based in a moral cowardice, which gauges a scheme by its ' ' respecta- 



270 THE EVOLUTION OF SEX. 



-f- 



bility," while even more culpable is that consciously or unconsciously 
derived from the profitableness to the capitalist classes of unlimited 
competition of cheap unskilled labor. For never did the proletariat 
more literally deserve its name than since the advent of the factory 
period, their rapid and degenerative increase, indeed, primarily repre- 
senting ' ' the progress of investments. 

The general attitude of the modern Malthusian may first of all be 
roughly indicated bv quoting the mottoes which head the organ of 
their league. ' ' To a rational being, the prudential check to popula- 
tion ought to be considered as equally natural with the check from 
poverty and premature mortality " (Malthus, 1806). " Little improve- 
ment can be expected in morality until the production of large 
families is regarded in the same light as drunkenness, or any other 
physicial excess"^ (John Stuart Mill, 1872). "Surely it is better to 
have thirty-five millions of human beings leading useful and intelligent 
lives, rather than forty millions struggling painfully for a bare 
subsistence" (Lord Derby, 1879). Starting from the familiar induc- 
tion that ' ' population has a constant tendency to outrun the means of 
subsistence," they recognize in this over-population " the most fruitful 
source of pauperism, ignorance, crime, and disease." To counteract 
this there are checks, positive or life-destroying on the one hand, 
prudential or birth-preventing on the other. ' ' The positive or life- 
destroying checks comprehend the premature death of children and 
adults by disease, starvation, war, and infanticide." As these positive 
checks are happily reduced with the progress of society, attention 
must be concentrated on the other side. ' ' This consists in the 
limitation of offspring by abstention from marriage, or by prudence 
after marriage." But as to the first, prolonged abstention from 
marriage, as advocated by Malthus, this is ' ' productive of many 
diseases, and of much sexual vice," while "early marriage, on the 
contrary, tends to secure sexual purity, domestic comfort, social 
happiness, and individual health." The check that remains to be 
advocated is thus ' ' prudence after marriage, ' ' and by this the neo- 
Malthusians most distinctly mean attention to methods which will 
secure that sexual intercourse be not followed by fertilization. For the 
details of the various methods, we must refer to the Malthusian 
literature; but a brief outline is imperative, even for an approximate 
understanding of the problem. 

(a) Thus we have the suggestion that intercourse should be 
limited to the relatively infertile period most remote from menstruation 
when conception may indeed occur, but with less probability than at 
other periods. Although gynecologists are disagreed as to the 
degree of this probability, there can be litde doubt that such 



LAIVS OF MULTIPLICATION. 271 

limitation would have a useful influence, although in itself confessedly 
incomplete. The so-called artificiality of control is here reduced to a 
minimum, and the suggestion is obviously in harmony with that 
increased temperance which all must allow to be desirable. 

(/') In the second place, there are methods employed by the males, 
such as that of withdrawal before the emission of the seminal fluid, a 
habit common enough both in' savage and civilized communities. 
Fertilization is in this way absolutely prevented, but apart from a more 
general objection to be afterwards emphasized, such a practice is 
maintained by some to be injurious to the male, and yet more to the 
female. Moreover, although the risks of over-population and female 
exhaustion by child-bearing are here minimized, there is still risk of 
male exhaustion. 

if) Thirdly, although again under the severe criticism of some of 
the medical experts, there are means employed by the females for 
securing by means of pessaries that the spermatozoa do not come into 
contact with the ovum, or by means of washes that the male elements 
are rendered ineffectual. In reply to the medical objections to both 
these methods of artificial check, it is answered (a) that it may in 
many cases be necessary to choose between two evils, of which the risk 
involved in the artificial check may be much less than that involved in 
continued child-bearing; {U) that it is hardly a fair argument as yet to 
urge that the proposed checks of neo-Malthusianism are fraught with 
danger. As to the popularly supposed preventive check of prolonged 
nursing one baby in the hope of thereby preventing a new conception, 
it is necessary to emphasize that nursing does not effect this, and that 
the prolongation of the lacteal function and diet beyond their natural 
limits is seriously injurious alike to mother and offspring. 

Even recognizing some of these objections, the neo-Malthusians 
urge the number of distinct advantages, — the reduction of the present 
rapid rate of increase; the possibility of earlier marriages, and a 
probable diminution of vice; an increase in the fitness oftheraceby 
lessening the propagation of unfit types and the exhaustion of the 
mothers by too frequent child-bearing. Supposing, again, the 
general adoption of the proposal, the neo-Malthusians insist upon the 
possibility of a heightened standard of comfort among the poorer 
members of the community, and the removal of obstacles to marriage 
which stands in the way of those who ought to marry but ought not to 
be parents. 

Without urging medical objections above referred to, — for in 
regard to the discussion of these, professional experts must bear the 
responsibility, — we must emphasize several counter-arguments. Thus 
it has been maintained, though with no great degree of certitude, that 



272 



THE EVOLUTION OF SEX. 



a ]jroposal involving some deliberate and controlled action would tend 
to be adopted most where least wanted, namely, among the more 
indix'iduated types, whose numbers would in consequence be propor- 
tionately reduced. The diminished rate of increase, which is the most 
obvious social result of the extensive adoption of neo-Malthusian 
practices, has long been known to the student of population; and in 
some countries, particularly France, — although here, no doubt, to 
some extent the result of peculiarly high individuation, — is a recog- 
nized national danger, especially since the diminished population, in 
being largely freed from the normal acuteness of the struggle for exis- 
tence, loses many of the advantages of this as well. 

The statistician will doubtless long continue his fashion of confi- 
dently estimating the importance and predicting the survival of 
populations from their quantity and rate of reproduction alone; but at 
all this, as naturalists we can only scoff. Even the most conventional 
exponent of the struggle for existence among us knows, with the bar- 
barian conquerors of old, that " the thicker the grass, the easier it is 
mown;" that "the wolf cares not how many the sheep may be." It 
is the most individuated type that prevails in spite, nay, in another 
sense, positively because of its slower increase; in a word, the survival 
of a species or family depends not primarily upon quantity, but upon 
quality. The future is not to the most numerous populations, but to 
the most individuated. And as we increasingly see that natural 
history must be treated primarily from the standpoint of the species- 
regarding sacrifice raAer than from that of the individual struggle, we 
see the importance of the general neo-Malthusian position, despite the 
risks which the particular modes of its practice may involve. 

Apart from the pressure of population, it is time to be learning (i) 
that the annual childbearing still so common, is cruelly exhaustive to 
the maternal life, and this often in actual duration as well as quahty; 

(2) that it is similarly injurious to the standard of offspring; and hence 

(3) that an interval of two clear years between births (some gynzecolo- 
gists even go as far as three) is due alike to mother and ofTsprino-. It 
is time therefore, as we heard a brave parson tell his flock lately, " to 
]ia\e done with that blasphemous whining which constantly tries to 
look at a motherless" (ay, or sometimes even fatherless) " crowd of 
puny infants as a dispensation of mysterious providence." Let us 
frankly face the biological facts, and admit that such cases usually 
illustrate only the extreme organic nemesis of intemperance and 
improvidence, and these of a kind far more reprehensible than those 
actions to which common custom applies the names, since they are 
species-regarding vices, and not merely self-regarding ones, as the 
others at least primarily are. To realize the social consequences of 



LAWS OF MULTIPLICATION. 273 

sexual intemperance is enough to obviate any hasty criticism of neo- 
Mahhusianism, whatever conclusion may be arrived at as to its 
sufficiency. 

It is time, however, to point out the chief weakness in neo-Malthu- 
sian proposals, which are at one in allowing the gratification of sexual 
appetites to continue, aiming only at the prevention of the naturally 
ensuing parentage. To many, doubtless, the adoption of a method 
which admits of the egoistic sexual pleasures, without the responsi- 
bilities of childbirth, would multiply temptations. Sexuality would 
tend to increase if its responsibilities were annulled; the proportion of 
unchastity before marriage, in both sexes, could hardly but be aug- 
mented; while married life would be in exaggerated danger of sinking 
into ' ' monogamic prostitution. ' ' On the other hand, it seems probable 
that the very transition from unconscious animalism to deliberate pre- 
vention of fertilization would tend in some to decrease rather than 
increase sexual appetite. 

It seems to us, however, essential to recognize that the ideal to be 
sought after is not merely a controlled rate of increase, but regulated 
married lives. Neo-Malthusianism might secure the former by its 
more or less mechanical methods, and there is no doubt that a limita- 
tion of the family would often increase the happiness of the home; but 
there is danger lest, in removing its result, sexual intemperance become 
increasingly organic. We would urge, in fact, the necessity of an 
ethical rather than of a mechanical "prudence after marriage," of a 
temperance recognized to be as binding on husband and wife as chas- 
tity on the unmarried. When we consider the inevitable consequences 
of intemperance, even if the dangers of too large families be a\'oided, 
and the possibility of exaggerated sexuality becoming cumulative by 
inheritance, we can not help recognizing that the intemperate pair are 
falling toward the ethical level of the harlots and profligates of our 
streets. 

Just as we would protest against the dictum of false physicians who 
preach indulgence rather than restraint, so we must protest against 
reo-arding artificial means of preventing fertilization as adequate solu- 
tions of sexual responsibility. After all, the solution is primarily one 
of temperance. It is no new nor unattainable ideal to retain, through- 
out married life, a large measure of that self-control which must always 
form the organic basis of the enthusiasm and idealism of lovers. But 
as old attempts at the regulation of sexual life have constantly fallen 
from a glowing idealism into pallor or morbidness, it need hardly be 
said that the same fate will ever more or less befall the endeavor after 
temnerance, so long as that lacks the collaboration of other necessary 
reforms. We need a new ethic of the sexes; and this not merely, or 



274 THE EVOLUTION OF SEX. 



even mainly, as an intellectual construction, but as a discipline of life; 
and we need more. We need an increasing education and civism of 
women, — in fact, an economic of the sexes very different from that 
nowadays so common, which, while attacking the old cooperation of 
men and women because of its manifest imperfections, only offers us 
an unlimited and far more mutually destructive industrial competition 
between them instead. The practical problems of reproduction become 
in fact, to a large extent, those of improved function and evoh-ed 
environment; and limitation of population, just as we are beginning to 
see the cure of the more individual forms of intemperance, is primarily 
to be reached, not solely by individual restraint, but by a not merely 
isolated and individual, but aggregate and social, reorganization of life, 
work, and surroundings. And while our biological studies of course, 
for the most part only point the way toward deeper social ones, they 
afford also one luminous principle toward their prosecution, — that 
thorough parallelism and coincidence of psychical and material consid- 
erations, upon which moralist and economist have been too much wont 
respectively to specialize. 

VI. Rate of Reproduction " Nil." — Sterility. — When we view 
reproduction in terms of discontinuous growth, — that is, as a phenom- 
enon of disintegration, — it is obvious that complete integration of the 
matter acquired by the organism into its bulk, and for its own devel- 
opment, precludes reproduction, — that is, involves sterility, — and 
similarly as regards the energies of the organism. This is only a 
restatement of Spencer's generahzation above discussed; for it is 
evident that, if genesis vary inversely as individuation, it must be 
suppressed altogether if individuation becomes complete. The actual 
phenomena, however, by no means usually admit of explanation as 
such realizations of the ideal of evolution, and hence the cause and 
treatment of sterility mainly pass into the provinces of the experimental 
naturalist and the physiological physician. From the earliest times, 
indeed, physician and naturalist, priest and legislator, ahke devoted 
attention to the subject; and it was probably in this way, as a recent 
monographer remarks, that research became directed to the larger 
problem of reproduction in general. The general biological questions 
— for example, the relations between sterility within the limits of a 
species to changes in the environment, or that of sterility among 
hybrids — are extensively discussed in the copious literature which 
centers around Darwin's Variation of Animals a7id Platits under 
Domestication ; while with regard to the human species, an extensive 
medical literature of course exists, to which any encyclopedia of med- 
icine, or conveniently the recent monograph of P. Mliller {Die 
Unfruchtbarkeit der Ehe, Stuttgart, 1885) will furnish bibliographical 
details. 



LAWS OF MULTIPLICATION. 275 



SUMMARY. 

I. The rate of reproduction is ciiiefly determined by the constitution of the 
organism ; the rate of increase, by its relations to the animate and inanimate 
environment. 

II. The naturalist has to thank the sociologist for directing emphatic atten- 
tion to the laws of multiplication. 

III. Summary of Spencer's analysis. Individuation and genesis vary 
inversely. 

rv. In regard to man, Spencer urges the importance of pressure of popula- 
tion as an incentive to progress, and concludes that man's future evolution must 
continue mainly in the direction of psychical development, and predicts with the 
increase of individuation a diminution of fertility. 

V. Predecessors and opponents of Malthus denied that increase of popula- 
tion tended to outrun subsistence ; Malthus successfully demonstrated his thesis, 
and noted the checks which curbed the increase ; Darwin emphasized the 
advantage of the pressure and checks ; Spencer shows the inverse ratio of 
degree of development and rate of reproduction ; neo-Malthusians advocate the 
use of artificial preventive checks to fertilization. Discussion of these various 
generalizations and proposals. 

VI. Completed individuation, were that possible, would be theoretically 
associated with sterility. 



LITERATURE. 

Malthus. — Theory of Population. 1806. 

Spencer. — Principles of Biology. Lond. 1866. 

Geddes. — "Reproduction," Ency. Brit.; and Lecture on Claims of Labor. 

Edin. 1886. 
Drysdale. — The Population Question. Lond. 1878. 
Besant. — The Law of Population. Lond. n.d. 
Clapperton. — Scientific Meliorism. Lond. 1885. 



276 



THE EVOLUTION OF SEX. 



CHAPTER XXI. 
THE REPRODUCTIVE FACTOR IN EVOLUTION. 

I. General History of Evolution. — The history of the doctrine 
of evolution is essentially modern; for though the idea glimmered 
before the minds of many ancient philosophers from Empedocles to 
Lucretius, it was not till the eighteenth century that naturalists began 
seriously to apply the conception to the problem of the origin of our 
fauna and flora. In thinking of the history, it is necessary to distin- 
guish, on the one hand, the gradual demonstration of the fact that 
evolution is a modal explanation of the origin of organisms; and, on 
the other, the deeper problem of the real mechanism of the pro- 
cess. The former, the empirical fact of evolution, may be said to 
have been virtually demonstrated, soon after the middle of this 
century, by the labors of Spencer, Darwin, Wallace, Haeckel, and 
others; the latter — the real aetiology of organisms, the "how" of 
the process — is still the subject of searching inquiry and keen 
debate. 

The idea of evolution, for so many centuries a latent germ, first 
took definite shape, so far as biology is concerned, in the mind ol 
Buffon (1749), who not only urged the general conception with 
diplomatic skill and powerful irony, but sought to elucidate the 
working out of the process. He illustrated the influence of new 
conditions in evoking new functions; showed how these in turn 
reacted upon the structure of the organism; and how, most directly 
of all, altered climate, food, and other elements of the environment, 
were external factors in internal change, whether for progress or 
for degeneration. 

Contrasted with Buffon in many ways, both in his mode of treat- 
ment and in his view of the factors, was Erasmus Darwin (1794), the 
grandfather of the author of ' ' The Origin of Species. ' ' In rhyme and 
reason, with all the humor and common-sense of a true Englishman, 
and with a really Hving conception of Nature, he urged the general 
conception of evolution, and emphasized the organism's inherent 
power of self-improvement, the molding influence of new needs, 
desires, and exertions, and the indirect action of the environment in 
evoking these. 

To Treviranus (writing in 1802-31) — a biologist too much 
neglected both in his lifetime and since — organisms appeared almost 
indefinitely plastic, especially however under the direct influence of 



THE REPRODUCTIVE FACTOR IN EVOLUTION. 



277 



external forces, His keen analysis of possible factors did not fail 
to recognize — what Brooks, Galton, Weismann, and others have 
since elaborated — that the union of diverse sexual elements in fertili- 
zation was in itself a fountain of change. ' ' Every form of life, ' ' he 
says, " may have been produced by physical forces in either of two 
ways, either from formless matter, or by the continuous modification 
of form. In the latter case, the cause of change may be either in the 
injiuence of the hetrogeneous male reproductive matter on the female 
germ., or in the influence of other potencies after generation." 

His contemporary Lamarck Twriting in 1801-9) — of greater post- 
humous fame — fought in poverty like a hero for the evolutionary 
conceptions of his later years. He is well known to have emphasized 
the importance of changed conditions in evoking new needs, desires, 
and activities, urging at the same time the perfection wrought upon 
organs by increased practice, and conversely the degeneration which 
follows as the nemesis of disuse. Evolution seemed to him to be 
due to the interaction of two fates, — an internal progressive power of 
life; and the external force of circumstances, encountered in the 
twofold struggle with the inanimate environment and with living 
competitors. 

Among the philosophers, too, and especially in the minds of 
those who have been disciplined in physical or historical investigations, 
the speculations of the ancients were ever taking fresh form, gaining 
moreover in concreteness. Thus Kant viewed the evolution of species 
mainly in terms of the mechanical laws of the organism itself, but 
allowed also for the influence of environment, noted the importance 
of selection in artificial breeding, and, like such ancients as Empe- 
docles and Aristotle, had glimpses of the notion of the struggle for 
existence. The same idea is more distinct in Herder's " Philosophy of 
History," where, probably under Goethe's influence, he speaks of the 
' ' struggle, each one for itself, as if it were the only one, ' ' of the limits 
of space, and of the gain to the whole from the competition of individ- 
uals. Oken (i 809) saw the light of the evolution idea dancing like a will- 
o'-the-wisp in the mist of his " Urschleim " speculations, and seemed 
chiefly to interpret the organic progress in terms of action and reac- 
tion between the organism and its surroundings; while in the noble 
epic of evolution which we owe to his contemporary Goethe, the 
adaptive influence of the environment is clearly recognized. 

Wells in 1813, and Patrick Matthew in 1831, forestalled Darwin 
in suggesting the importance of natural selection; but their virtually 
buried doctrines, however interesting historically, were of less prac- 
tical importance than those of Robert Chambers, the long unknown 
author of the "Vestiges of Creation " (1844-53). His hypothesis of 



278 THE EVOLUTION OF SEX. 

evolution emphasized the growing or evolving powers of the organ- 
isms themselves, which developed in rhythmic impulses through 
ascending grades of organization, modified at the same time by- 
external circumstances, which acted with most effect on the generative 
system. It is difficult indeed to refrain from amusement or irritation 
at the naive simplicity with which he evolves a mammal from a bird, 
by the short and easy method of prolonging the period of uterine 
life in favorable nutritive conditions; but though a goose could not 
so simply give rise to a rat, the emphasis laid on the influence of pro- 
longed gestation is full of suggestiveness, especially in relation to the 
evolution of mammals. Apart from his common-sense view of evo- 
lution as a process of continued growing, Chambers deserves to be 
remembered as one of the first to appreciate ' ' the force of certain 
external conditions operating upon the parturient system." 

In France, Geoffroy and Isdore St. Hilaire — father and son — 
denied indefinite variations, regarded function as of secondary 
importance, and laid special stress upon the direct influence of the 
environment. To them it seemed not so much the effort to fly, as 
the (supposed) diminished proportion of carbonic acid in the atmos- 
phere, which had determined the evolution of birds from ancient 
reptiles. A complete history of evolution theories, up to the publica- 
tion of " The Origin of Species " (1859), would have to take account 
further of the opinions of the geographer Von Buch and the embry- 
ologist Von Baer, of Schleiden and Naudin, Owen, and Carus, and 
many others; but no such survey is here our purpose. 

For it must be already evident from the above brief sketch of 
representative opinions that successive naturalists have emphasized 
now one factor and now another in the evolutionary process. To one 
it seemed as if the organism had a motor power of development — 
often a metaphysical one, it must be allowed — - within itself, and that 
evolution was to be explained, in Topsian fashion, ' ' according to the 
laws of organic growth' ' ; to another, function appeared all-important, 
perfecting organs on the one hand, allowing them to wane in disuse on 
the other; to a third, organisms were seen under the hammers of 
external forces and circumstances, being continuously welded in more 
and more perfectly adapted forms. The organism, its function, and 
its environment, on each of the three factors in the problem emphasis 
was in turn laid. 

At this juncture Darwin elaborated his theory of "The Origin of 
Species by means of Natural Selection and the Preservation of 
Favored Races in the Struggle for Life, ' ' and was independendy and 
simultaneously corroborated by Alfred Russel Wallace. They did not 
indeed deny a spontaneous power of change in the organism itself, nor 



THE REPRODUCTIVE FACTOR IN EVOLUTION. 279 



the influence of function and environment; but, without definitely- 
discussing the origin of variations, sought to show how the destructive 
or eliminating, and the conservative or selecting agency of the 
animate and inanimate environment, were the principal factors in 
evolution. Given a sufficient crop of indefinite variations, — 
unanalyzed or unanalyzable as to their origin, — the struggle for 
existence separated the minority of wheat-ears from the majority of 
tares, and secured a finer and finer harvest. 

So much had Darwin in his magistral labors to do with making the 
general conception of evolution current coin, that we can readily 
understand how not only the educated laity, but the majority of 
professed naturalists, identified their adherence to the general doctrine 
with a subscripdon to the specific principle of natural selection, and in 
becoming evolutionists became at the same time Darwinians, that is to 
say, natural selectionists. Of late years, however, as conflict has 
passed from the outworks to the very citadel of evolution, — has come, 
that is to say, to center round the problem of the origin of varia- 
tions, — history has repeated itself Naturalists such as Nageli, 
Mivart, and Eimer have championed the cause of internal organismal 
variations, of evolution in terms of the constitution of the organism, of 
progress according to the definite laws of organic growth. An active 
school of neo-Lamarckians, such as Cope and Packard, has arisen in 
America; while Spencer has re-emphasized the importance both of 
function and of environment as factors in organic evolution, supported 
moreover in this position by the experimental work of Semper and 
others. The last published essays of Spencer may be referred to in 
illustration of the unended state of the controversy, but at the same 
time of the growing tendency to limit the importance of natural 
selection, and as a good instance of successful endeavor to recognize 
the measure of truth in the different theories. Wallace remains 
staunchest am.ong the upholders of the theory of natural selection, for 
his share in which he seems ever to refuse to take to himself sufficient > 
credit; but it is interesting to notice, that in his recent valuable work, 
in re-inforcing his old objections against the importance which Darwin 
attached to sexual selection, he has made admissions welcome to those 
of us who believe that the shoulders of natural selection have also been 
overburdened. As we have already noticed, the phenomena of male 
ornament are discussed and summed up as being ' ' due to the general 
laws of growth and development, ' ' and as such that it is " unnecessary 
to call to our aid so hypothetical a cause as the cumulative action of 
female preference. ' ' Again ' ' if ornament is the natural product and 
direct outcome of superabundant health and vigor," — a view to which 
the reader of the preceding pages can be no stranger, — "then no 



THE EVOLUTION OF SEX. 



other mode of selection is needed to account for the presence of such 
ornament." Granted, but does not the author see that if the origin of 
characters so important as those often possessed by males is to be 
ascribed to internal constitution rather than to external selection, the 
origin of this, that, and the other set of characters will next be 
explained in the same way, as the heretics are in fact now doing. In 
pulling down the theory of sexual selection in favor of that of natural 
selection, Mr. Wallace has really handed over Mr. Darwin's elaborate 
outwork to the enemy, who will not fail to see its value for a new 
assault. 

Before we conclude this necessary historical sketch, we must how- 
ever refer to the subject of debate recently reopened by Weismann, to 
whom, as one of the foremost of European naturalists, the reader's 
attention has already been so frequently directed. To a very large 




Fig. I02. — Two adjacent animal cells, showing communications through adjacent 
intercellular substance: also the protoplasmic network, and the 
nucleus. — After Pfitzner. 



extent at least, we and our fathers have believed that characters 
acquired by the individual organism from functional or environmental 
conditions might be transmitted as a legacy to the offspring. Accord- 
ing to Weismann, and not a few others independent of and dependent 
on him, this has been a delusion. Not only is positive proof of such 
transmission of acquired characters, that is, other than those of 
constitutional, congenital, or germinal origin, so scanty and unsatis- 
factory that His has not hesitated to call the catalogue of cases a mere 
"handful of anecdotes," but the connection between the body-cells 
and the sex-elements seems to Weismann and his school so far from 
close or dependent, that there is a great probability against any 
"somatic" dint or modification directly affecting the reproductive 



THE REPRODUCTIVE FACTOR IN EVOLUTION. 2S1 



elements, — that is to say, affecting the offspring. If the reproductive 
elements, in spite of the close connection between all parts of the 
body, or even between cell and cell (see fig. p. 280), do lead such a 
charmed physiological life within the organism that they are unaffected 
directly by changes in the other parts of the body, then an optimism 
of heredity is demonstrable. How far we believe it from being so can 
not be here discussed, but the consequences of Weismann's conclusion 
to the general theory of evolution must be re-emphasized. If ' 
individually acquired characters are of importance only to the 
individual body, they are obviously of no account in the evolution of 
the species, — above the level of the Protozoa at least; and, as Weis- 
mann himself says, the ground is thus taken from under the feet of 
Buffonians, Lamarckians, neo-Lamarckians, &c. The ground is left 
clear for natural selectionists, and the struggle for existence acting on 
variations thus becomes the exclusive factor in the mechanism of 1 
evolution. But what then starts these variations which natural 
selection eliminates or fosters, as the case may be? Weismann's 
answer is clear and definite, the interminghng of the sexual elements in 
fertilization is the sole fountain of variation; a view which certainly 
accents the " Reproductive Factor in Evolution," though it seems to 
us hardly to conform with the author's previously expounded opinion, 
that the action of the sperm upon the ovum is quantitative rather than 
qualitative. But, even if none but constitutional or germinal variations""' 
are transmissible, we are not shut up to the exclusive adoption of the 
natural selectionist position. It is still open to the naturalist to 
demonstrate that many adaptations at least are not explicable as the 
result of a long process of fostering and eliminating selection among a 
host of sporadic results of sexual interminglings, but are rather the 
direct and necessary results of " laws of growth," of " constitutional 
tendencies," or of the precise chemical nature of the protoplasmic 
metabolism in the organisms in question. If constitutional variations 
occur along a few definite lines, as Eimer, Geddes, and others have 
shown to be true in certain cases, then we can understand the origin, 
though not perhaps the distribution, of species apart from any long 
process of selection, for which indeed, if variations be strictly definite, 
the material must be vastly reduced. In other words, we can think of 
the organism not merely under the molding influence of its functions, 
nor solely as the product of environmental hammering, least of all as 
the survivor from a crowd of unsuccessful competitors, but as the 
expression of an internal fate, no longer mystical, but expressible in 
terms of the dominant chemical constitution. ^/ 

II. The Reproductive Factor. — Without further discussion of 
the still open controversy as to the various factors of evolution, 



282 ^^-^ EVOLUTION OF SEX. 

which would not be relevant to such a work as this, we must 
summarily collate the more prominent opinions as to the share repro- 
duction has in the process. To most of these we have already 
alluded in the body of the book. 

{a) First of all, as to the origin of variations, we find that what 
Treviranus recognized in the first years of this century, — namely, the 
influence of fertilization in evoking change, — has been emphasized by 
several, such as Brooks and Galton, and has been especially elaborated 
by Weismann. As we have just seen, Weismann finds in the inter- 
mingling of two " germ -plasmas," which is the essence of fertilization, 
the sole origin of variations of any account in the evolution of the 
species. Whether this be consistent with Weismann' s theory of fertili- 
zation or not is matter for debate, but there is no doubt that his 
emphasis on the evolutionary value of sexual reproduction is a most 
important contribution to the general theory. In somewhat marked 
contrast is the view recently advocated by Hatschek, who sees in the 
intermingling essential to fertilization a counteractive of idiosyn- 
cracies, a means of controlling and checking disadvantageous individual 
peculiarities. The two positions are not antagonistic, but rather 
complementary to one another. 

{U) No impartial student of Darwinism can fail to admit that in the 
"struggle for existence " stress is laid upon the nutritive and self- 
maintaining functions and strivings, while the reproductive and 
species-maintaining activities are regarded as of secondary importance. 
One can not forget, indeed, how much Darwin insisted upon the role 
of ' ' sexual selection ' ' ; yet it has been already shown that this recog- 
nition of the reproductive factor was, after all, very external; that 
sexual selection is only a special case of natural selection; that it seeks 
to explain the elaboration, not the origin of sexual peculiarities; and 
lastly, that Darwin's arguments in favor of the mechanism which he 
emphasized, have been seriously impugned by Wallace in an attack 
which reacts strongly upon the critic's own position. 

(c) Romanes has recently elaborated, what others seem also to 
have suggested, the importance of mutual sterility in splitting up one 
species into several. ' ' Whenever any variation in the highly variable 
reproductive system occurs, tending to sterility with the parent form 
without impairing fertility with the varietal form, a physiological bar- 
rier must interpose, dividing the species into two parts, free to develop 
distinct histories, without mutual intercrossing, or by independent 
variation." The reproductive system is very apt to vary, — why, he 
does not say; the consequence might readily be, that among the 
progeny of a parent stock some were fertile inter se, but infertile with 
the consistent members of the parent stock; these will be isolated by a 



THE REPRODUCTIVE FACTOR IN EVOLUTION. 283 

physiological barrier, just as they might be insulated by a geographical 
one, and left free to develop along divergent paths of their own. Here 
again there is recognition of the reproductive factor in evolution; but 
how far, and in what cases species have so originated, is obviously a 
question which would involve discussion of each individual instance. 

id') Worthy of reiteration is the suggestion of Robert Chambers, 
crudely illustrated as it may have been, that environmental influences 
acted with special power upon the generative system, and that the 
prolongation of gestation was a maternal sacrifice which brought its * 
own reward in the higher evolution of the offspring. Miss Buckley, 
along a similar line of thought, has well pointed out how the increase 
of parental care was a factor in, as well as a result of, the general \ ^ 
ascent; how the success of birds and mammals especially must in part 
be interpreted in reference to the noteworthy deepening of parental 
affection, and strengthening of the organic and emotional links between 
mother and offspring. In emphasizing the progressive value of pro- 
longed infancy, especially in the evolution of the emotions, Fiske has 
also recognized the importance of the reproductive factor. 

III. Further Construction. — The general tendency of all theo- | 
ries of evolution has been to start with the individual organism as the | 
unit, and to consider the self-maintaining and nutritive activities as 
primary, the reproductive and species-regarding as only secondary. ; 
But along many lines of research, such as those indicated in the pre- 
ceding paragraph, the importance of the reproductive factor has been 
recognized, and the center of gravity of the evolutionary inquiry has 
already been to some degree shifted. Recent investigations on 
heredity, for instance, forbid that attention should any longer be 
concentrated on the individual type, or reproduction regarded as a 
mere repetition process; the living continuity of the species is seen 
to be of more importance than the individualities of the separate 
links. Physiologists and evolutionists are coming to see the most 
complex individual lives, in Foster's phrase, as "but the bye-play 
of ovum-bearing organisms." The species is a continuous undying""] 
chain of unicelluar reproductive units, which indeed build out of and J 
around themselves transient multicellular bodies, but the processes of 
nutritive differentiation, and other individual developments, are 
secondary, not primary. _ 

Thus it is the central generalization of botany that, despite the 
individual differentiation of fern, selaginella, cycad, conifer, and flower, 
these turn out on deepest analysis, to be but the surviving phases of 
a continuous and definite increase in the subordination of the sexual 
parents to their asexual offspring (see pp. 185, 195). 



284 THE E VOL UTION OF SEX. 

Or if we take in particular the origin of the flower, which all 
botanists agree in regarding as a shortened branch, the natural selec- 
tionist explanation (did the theory trouble itself with such questions) 
would seem to be, that the flower had arisen by selection from the 
two other alternatives of lengthened and unshortened axes. But this 
is at once excluded by the physiological explanation that shortening 
of the axes was inevitable, since the expense of the reproductive 
functions necessarily checks the vegetative ones, for it is evident that 
we can not speak of selection where the imaginable alternatives are 
physically impossible. So, too, the shortening of the inflorescence 
from raceme to spike or flowerhead, or still further into the hollowed 
form of a fig, with the corresponding reduction in the size of the 
flowers, is again the result of the check imposed by reproduction on 
the growth of axis and appendages. 

The same simple conception of a continuous checking of vegeta- 
tion by reproduction, unlocks innumerable problems of floral structure, 
large and small ahke, from the inevitable development of gymnosperm 
into angiosperm by the continuous subordination of the reproductive 
carpellary leaf, to the variations of cabbages as seen in the transitions 
between leafy kale and cauliflower. Or again, the origin of floral 
color, as primarily an inevitable consequence of the same principle of 
vegetative subordination through reproductive sacrifices, was long ago 
pointed out by Spencer, and admits of detailed elaboration without 
attaching more than secondary importance to selection by insects. 

In another way the antithesis between reproduction and nutrition 
may be illustrated among the existing orders and species of flowering 
plants. Just as the lilies, for instance, range on the one side toward 
the characteristically vegetative grass, or on the other toward the 
reproductive orchid, so it is with the main variations of every natural 
alliance. Thus, the Rayiunculace/z have their grassy and their orchid- 
like types in meadow-rue and larkspur respectively, while the species 
of these very genera show, within narrow limits, similar swings of 
variation. What we call higher or lower species are thus the leaders 
or the laggards along one or other of these two lines of variation. 
Among animals, the importance of the reproductive factor may 
be illustrated in the most diverse series. Thus the greatest step in 
organic Nature, that between the single-celled and many-celled 
animals, bridged as it is by loose colonies some of which are at a 
very low morphological level, is not due to the selection of the more 
individuated and highly adapted forms, but to the union of relatively 
unindividuated cells into an aggregate, in which each becomes dimin- 
ishingly competitive and increasingly subordinated to the social whole. 
The colonial or multicellular forms, originating pathologically in all 



THE REPRODUCTIVE FACTOR IN EVOLUTION. 285 

probability, may of course have rapidly justified their existence in 
the struggle for existence, just as unions of many kinds do in human 
society, but the Protozoa can not be accused of any prevision of 
future advantage in remaining clubbed together in cooperation, nor 
indeed credited with much primitive altruism in so doing. None the 
less is it clear that this greatest of morphological steps was directly 
due, not to any struggle, but rather to an organic sociality, or at any 
rate to a process which is not interpretable in terms of individual 
advantage. 

No structure is more emphatically nutritive in its adult results than 
the gut-cavity of the embryonic gastrula. It is worth inquiring 




" [^) 



Fig. 103.— Formation of the Gastrula.— From Haeckel. 

whether this important step in differentiation was attained in history 
in response to nutritive needs. The usual supposition is certainly 
that the gastrula cavity, by whatever peculiarities of growth it may 
have arisen, justified itself from the first in an additional nutritive 
advantage. But Salensky, in his studies on the primitive form of the 
Metazoa, has given strong arguments in favor of the theory that the 
primitive cavity, arising in a volvox-like form, was originally a brood- 
cavity or "genitoccel," and that it only secondarily acquired nutritive 
significance. It would be indeed striking if this important morpho- 
logical step in the establishment of the nutritive system was reached 
along the road of reproductive modification; for if this most funda- 
mental of nutritive and self-maintaining advantages, the belly itself, 



286 THE EVOLUTION OF SEX. 

be but a secondary resultant of an originally reproductive and species- 
regardino; progress, that lower Utilitarianism, which has so long been 
arguing from economics to biology and back again, is evidently a step 
nearer exposure. 

Or again, that increase of reproductive sacrifice, which at once 
makes the mammal and marks its essential stages of further progress 
through oviparous monotreme, prematurely-bearing marsupial, and 
various grades of placental; that increase of paternal care; that fre- 
quent appearance of sociality or cooperation, which even in its rudest 
forms so surely secures the success of the species attaining it, be it 
mammal or bird, insect or even worm, — all these phenomena of sur- 
vival of the truly fittest, through love, sacrifice, and cooperation, 
need far other prominence than they could possibly receive on the 
hypothesis of the essential progress of the species through intqrnecine 
struggle of its individuals at the margin of subsistence. Each of the 
greater steps of progress is in fact associated with an increased measure 
of subordination of individual competition to reproductive or social 
ends, and of interspecific competition to cooperative association. 

The corresponding progress in the historic and individual world, 
from sex and family up to tribe or city, nation and race, and ultimately 
to the conception of humanity itself, also becomes increasingly appar- 
ent. Competition and survival of the fittest are never wholly eliminated, 
but reappear on each new plane to work out the predominance of the 
higher, that is, more integrated and associated type, the phalanx being 
victorious till in turn it meets the legion. But this service no longer 
compels us to regard these agencies as the essential mechanism of 
progress, to the practical exclusion of the associative factor upon which 
the victory depends, as economist and biologist have too long misled 
each other into doing. For we see that it is possible to interpret the 
ideals of ethical progress, through love and sociality, cooperation and 
sacrifice, not as mere Utopias contradicted by experience but as the 
highest expressions of the central evolutionary process of the natural 
world. The ideal of evolution is indeed an Eden; and although com- 
petition can never be wholly eliminated, and progress must thus 
approach without ever completely reaching its ideal, it is much for our 
pure natural history to recognize that " creation's final law " is not 
struggle but love. The fuller working out of this thesis, however, 
would lead us far beyond our present limits, toward a restatement of 
the entire theory of organic evolution. Leaving this to other papers, 
but specially to a future work, suffice it here, in conclusion, to indicate 
an important change in the general point of view. The older biologists 
have been primarily anatomists, analyzing and comparing the form of 
the organism, separate and dead; however incompletely, we have 



THE REPRODUCTIVE FACTOR IN EVOLUTION. 287 

sought rather to be physiologists, studying and interpreting the high- 
est and intensest activity of things Hving. From the study of indi- 
vidual structure they were wont to pass, indeed, to that of reproductive 
structures, and thence even functions; hence, too, the pair and the 
totality of the species did at length come successively into view; but 
this with the individualistic theory of natural selection bulking as 
practically all-important in the foreground, to which even sexual selec- 
tion was a mere harmonious corollary. For us, however, this 
perspective has become entirely reversed. The individual is a mere 
link in the species, and its reproductive processes are thus of funda- 
mental importance to the interpretation even of its self-maintaining 
ones. Hence we no longer regard, with Darwin and the majority of 
our brother naturalists, the operation of natural selection upon indi- 
vidual characters as the simplest of problems, looking for residual 




Fig. 104. — An Opossum {Didelphys dorsigera carrying its young on its back.- 
From Cams Steme. 



explanation to sexual selection, and only in extreme difficulty invoicing 
the aid of " principles of correlation," " laws of growth," and the like, 
viewed as almost inscrutably mysterious. On the contrary, it is the con- 
tinual correlation, yet antithesis — the action and reaction — of vegetative 
and reproductive processes in alternate preponderance, which seems to 
us of fundamental importance, since to this the general rhythm of indi- 
vidual and racial life runs fully parallel. Hence it is that we have the 
primeval lily developing on the one hand the ideally vegetative grass, 
yet also the supremely specialized reproductive orchid; and that we 
can trace (as we hold) the same swing of divergent evolution, of 
definite variation, in every natural order, nay, in every genus, often 
even in the very varities of a species. Hence, too, it is that the rhythm 
of hydroid and medusoid in the individual Hfe of the typical forms 



2SS THE EVOLUTION OF SEX. 

becomes fixed in coral or ctenophore as a racial temperament. This 
preponderance of passivity or activity (which we can read throughout, 
in barnacle and insect, as well as in tortoise and swallow) once set up, 
goes on accumulating till it meets reversal through environment or 
other causes, and limitation or extinction through the agency of natural 
selection, which, however, is more frequently a retarding force than an 
accelerant of evolution. The problem of organic progress is thus to 
be interpreted not merely as on conventional lines, by help of an 
analogy derived from an age of mechanical progress which gives us 
the watch, or sewing-machine, or tricycle, — by the cumulative patent- 
ing, as it were, of useful improvements in detail. The essential 
problem is not one of mechanism but of character, to which 
incident is accessory but not fundamental, — not of details put 
together, but of aggregate organic life or temperament. The life of 
the individual or the species is essentially a unity, of which the specific 
characters are but the symptoms, be their subsequent measure of 
importance and utility in adaptation, their modification by environ- 
ment, their enhancement or diminution by natural selection, what it 
may. Our special study of the reproductive process has thus brought 
us to the threshhold of a larger inquiry, the primary one of the organic 
sciences, that of the factors of organic evolution. For it is in Nature, 
as Schiller saw long ago in the human life, which this foreshadows: 
' ' While philosophers are disputing about the government of the 
world, Hunger and Love are performing the task." 



THE REPRODUCTIVE FACTOR IN EVOLUTION. 28c 



SUMMARY. 

I. A brief review of the history of evolution theories, and of the present state 
01 the question. 

II. A reproductive factor in evolution has been hinted at by a few 
naturalists. 

III. Further indications of the importance in evolution of reproductive 
and species-regarding, as opposed to the nutritive and self-maintaining 
activities. 



LITERATURE. 

See articles by the writers in "Chambers's Encyclopaedia," especially 
"Biology," "Botany," "Environment," "Evolution," and minor articles, 
such as "Ccelenterates," "Flower," "Fruit," &c.; also "Encyclopaedia 
Britannica," "Sex," and "Variation" and "Selection"; also Geddes, 
"A Restatement of the Theory of Organic Evolution " (Summary in Proc. 
Roy. Soc, Edin., 1888-89, still unpublished). 

Spencer, Mivart, Eimer, Wallace, Weismann, &c.— Op. cit. 



INDEX. 



Age of parents influencing sex, 29, 30 

Algce, reproduction in, 119, 120 

Aliantois, 230 

Aloe, 225 

Alternation of generations, 185-199, 

212, 213 
Alternation of generations in plants, 

1S5, 19.1, 195 
Altruism, development of, 257-259 
Amnion, 230 
Amphibians, parental care of, 233 

234 ; amatory emotion of 245 
Anah)olism, sec Protoplasm 
Angiostomum, 67, 192 
Animalculists, 79, Si, 147, 14S 
Anther cells, crj-stals in, 123 
Aphides, 40, 158, i5i, 163, 20S, 261 
Aposporj', 190 
Archoplasm, 92 
Argonauta, 227, 228 
Arthropods, hermaphroditism among, 

68 
Ascaris, ovum of 136, 137 
Asexual reproduction, 175-184 
Aura seminalis, 14S 
Aurelia, alternation of generations, 

186, 1S7 

Baer von, discovery of mammalian 
ovum, 81 

Balbiani on isolation of sex-cells, 86, 
87 ; conjugation of Protozoa, 140 

Balfour on polar bodies, 100 ; on 
parthenogenesis, 168 

Bariy, Jlartin, 134 

Bary, De, on fertilization, 150 

Bees, sex characters, 37; partial par- 
thenogenesis, 160, 161 ; reproduc- 
tion in, 173 

Beneden, Van, on origin of sex-cells, 
86; on polar bodies, 100; on fer- 
tilization, 136 

Bichat's analysis, 81 

Bilharzia, 67, 245 

Bird of Paradise, 4 

Birds, sexual characters, 6 ; sexual 
selection in, 9, 10 ; eggs, 97, 98 ; 
love among, 246, 247 

Blackcock, 6 

Blochmann on polar globules, 99, 168 

Body versus reproductive cells, 87, 88, 
240-242 

Bonellia, 17, 18 

Boveri on archoplasm, 92 ; on fertiliza- 
tion, 136-139, 150 

Brood-chambers, 234, 235 



Brooding, 56, 57 

Brooks on sexual selection, 9-12 ; 
theory of sex, 112, 125 ; on fertiliza- 
tion, 155 

Biichner on love among animals, 246 

Biitschli on extrusion of polar globules, 
99, 100 

Butfon, 276 

Butterflies, 23, 24, 40 

Canestrini, theory of sex, 29 

Carnoy on fertilization, 136, 137 

Castration, effects of, 19 

Cecidomyia, 225 

Cell-cycle, 112 

Cell-division, 205, 206, 215, 216, 236 

Cell-theory, 81, 82 

Chara, reproductive organs of, 123 

Chironomus, separation of sex-cells 
in, 86, 87, 162 

Chambers, Robert, on prolonged ges- 
tation, 277, 278, 283 

Claspers, 56, 228 

Coccus insects, 14, 17 

Ccelebogyne, 164 

Ccelenterates, reproductive organs, 
53 ; hermaphroditism, 66 ; origin of 
sex-cells, 86 ; asexual reproduction, 
179, 180; alternation of generations, 

193 

Color, animal, 20 

Color, floral, 284 

Comparative vigor, theory of, 31 

Conjugation, 142, 150, 151 

Conjugation, multiple, 141 

Continuity of cells, 280 

Continuity of germ-plasma, see Wcis- 
mann 

Copulation, 226-228 

Co-operation, 286 

Crime among animals, 253. Sec 
Cuckoo 

Crustaceans, sex in, 41 ; partheno- 
genesis in, 162 

Cuckoo, habits of 253-257 

Cupid's dart, 228 

Cutleria, reproduction in, 120 

Cuttlefishes, organs of 55 ; fertiliza- 
tion in, 56 ; egg-clusters of, 253 

Darwin on sexual selection, 7, 12, 21, 
23, 24; on determination of sex, 32, 
33 ; on cross and self-fertilization, 
130 ; on population question, 267, 
268 ; natural selection, 278, 279 

Darwin, Erasmus, 276 



2Q2 



INDEX. 



Darwinism, 2S6-28S 

Death, origin of, 235, 239-241 

Dichogamy, 69 

Diplozoon, 228 

Division of labor, 179 

Drones, nature of, 16 

Ducts of reproductive organs, 54 

Diising on the proportions of the 

sexes, 33 ; on sex-determination, 

41, 42 

Echinodermata, hermaphroditism 
among, 68 

Ectocarpus, reproduction in, 120 

Edible bird's nest, 231 

Egg-cells, see Ovum 

Egg, chemistry of the, 98 

Egg-envelopes, 97 

Egg-laying organs, 56 

Egg-shell, 97, 98 

Egoism, 257-259 

Eimer on humble bees, 38; on cuckoo, 
257 ; on evolution, 281 

Embryology, 78-81, 85-87, 92-99, 106, 
108 

Encystation, 179, 239 

Englemann on dimorphism in Proto- 
zoa, 121 

Environment, influence of, 217-219 ; 
transmission of acquired characters, 
280, 281 

Epigenesis, 78-81 

Ethical aspects of nutrition and repro- 
duction, 257-259 

" Evolution " in old sense, 78-81 

Evolution, theories of, 276-281 ; re- 
productive factor in, 276-288; essen- 
tial problems of, 287, 288 

Fallopian tube, 224 

Females, i-/^6passim; passivity, 14, 15; 
size, 18; preponderant anabolism, 
22 ; psychical characters of, 247-250 

Fern, alternation of generations, 189 

Fertilization, time of, 29 ; in plants, 
130 ; details of, 135-139 ; nuclear 
union in, 136 ; duality of, 139 ; in 
Protozoa, 139 ; origin of, 141-143 ; 
theory of, 147-157 ; uses of, 152-156; 
a source of variation, 277 

Fish, sex differences in, 5, 25 ; paren- 
tal care of, 234, 235 ; amatory 
emotions of, 245 

Flower, position of, 209; origin of, 284 

Fluke, alternation of generations, 189, 
191 

Follicular cells, 97 

Free-martin, 34 

Fungi, parthenogenesis in, 164 

Gall-wasps, parthenogenesis in, 163 
Galton, contrast between body and 
sex cells, 88; on fertilization, 152 



Gastrula, 80, 285 

Gegenbaur on origin of yelk gland, 

55 ; on hermaphroditism, 74 
Gemmules of (resh-water sponge, 193 
Genesis, 262 

Gentry on sex in moths, 40 
Germiparity, 27, 60 
Germ-plasma, see Weismann 
Gestation, length of, 229 ; influence of 

prolonged, 278 
Girou on sex, 29, 41 
Goette on origin of death, 217; on 

nemesis of reproduction, 236 
Gonads, or essential reproductive 

organs, 51-58 
Graaf, 79 

Graafian follicle, 224 
Growth, 203-214 
Gruber on reproduction of Protozoa, 

211 
Gyrodactylus, 192, 225 



Haeckel on continuity of reproduc- 
tion, 88; on Frotomyxa, 113; on 
alternation of generations, 198 ; on 
Magosphsera, 236 

Haller, 79, 80 

Hamm, discovery of spermatozoon, 
103 

Harvey, 78 

Hatschek on fertilization, 156; on 
nutrition and reproduction, 210 

Hectocotylus, 55, 227 

Hensen, theory of sex, 29 ; on fertiliza- 
tion, 153 

Heredity, see Weismann ; in alternat- 
ing generations, 195, 196 ; acquired 
characters, 280-281 

Hermaphroditism, 60-76 ; embryonic, 
60 ; casual, 61 ; partial, 62 ; normal, 
65; degrees of, 69; conditions of, 
73 ; origin of, 74 

Hertwig on hermaphroditism, 63 ; on 
fertilization, 149 

Heyer on sex in plants, 42 

Hofacker and Sadler's law, 29, 31 

Hofacker on determination of sex, 29 

Hunger and love, 257-259 

Hybridization, 143-145 

Hydra, 53, 175, 208 

Hydractinia, 179 

Hydroids, alternation of generations, 
186 



Immortality, organic, 238-242 

Incubation, 232-235 

Individuation, 262-264, 268, 269 

Insects, sex characters of, 5, 10, 14, 
16; determination of sex in, 37-40; 
hermaphroditism of, 65 ; partheno- 
genesis in, 163 

Tsophagy, 150 



INDEX. 



293 



Jager on continuitj' of germ-proto- 
plasm, 88 

Katabolism, see Protoplasm 

Knight, experiments on water-melons, 

43 
KoUiker on origin of spermatozoa, 
103 

Lactation, 230, 231 
Lamarck, 277 

Laulani^ on embryonic hermaphrodit- 
ism, 27, 28, 60, 61 
Leeuwenhoek, 79, 104, 147 
Limit of growth, 204-207 
Liverwort, 210 
Love, 244-259 
Love for offspring, 250-253 
Luciola, 21 



Magosphaera, 236 

Males, 3-47 /ai'wm,- variability, 10-12; 
activity, 14, 15 ; size, 17 ; pigmy, 
15, 17, 71, 73; color, 20; predomin- 
ant katabolism, 21 ; complemental, 
71. 73 ; psychical characters, 280 

Male-cell, see Spermatozoan 

Malpighi, 78 

Malthus, 262, 267, 268 

Malthusianism, i(>']-'2.6i) 

Mammals, love among, 246, 247 

Mammary glands, 230, 231 

Mantegazza on sexual characters, 
9-12 

Marsupials, 233, 234 

Maternal sacrifice, 236, 237, 238 

Maupas on conjugation and multipli- 
cation of infusorians, 140, 141, 210, 
211, 217, 218; on uses of fertiliza- 
tion, 152-155 ; on theory of organic 
immortality, 240 

Mayflies, 238 

Meehan, investigations on sex of 
plants, 42, 43 ; on cross-fertilization, 

Menstruation, 225, 226 

Mesozoa, liberation ol germs in, 54, 
236 

Metabolism, see Protoplasm 

Micropyle, 97 

Microstomum, iSi, 208 

Miescher on the milt of salmon, 109 

Migration of sex-cells, 53 

Milk, 231 

Minot on polar globules, 100 ; theory 
of genoblasts, in, 124; on parthe- 
nogenesis, 168, 190 

Mivart on sexual selection, 12 

Molluscs, hermaphroditism among, 
68 

Molothrus, 256 

Moonwort, 209, 210 



Moss, alternation of generations, 185, 

189, 190 
Myzostomata, hermaphroditism of, 

73 

Natural selection, a check on diver- 
gence of sexes, 25 ; scope of, 279, 
282, 285, 286, 288 

Nematodes, alternation of sexual 
generations in, 192 

Neo-Malthusianism, 268-274 

Notodelphys, 234 

Nototrema, 234 

Nucleus, behavior in fertilization, 136, 
137 ; essential elements of 222 

Nussbaum, 88, 149 

Nutrition, influence on sex, 36 

Nutrition versus reproduction, 207- 
209 

Nutrition of young, 230 

Obstetric frog, 234, 

Organs of reproduction, 51-58 

Orthonectids, bursting of female, 237 

Oviparous animals, 229 

Oviparous mammals, 229 

Ovipositors, 56 

Ovists, 79, 80, 147 

Ovo-viviparous animals, 229 

Ovulation, 224 

Ovum, 92-102 ; envelopes of, 97; 

maturation of 98-101 ■ liberation of, 

224 
Ovum theory, 77-78 
Owen on somatic and reproductive 

cells, 88 ; on residual spermatic 

force, 196 



Paedogenesis or juvenile partheno 
genesis, 161, 162 

Pairing, early forms of, 244, 245 

Paper nautilus, brood-shell of female, 
235 

Pararaaecium, conjugation of, 140 

Parental care, 250-253, 286 

Parental sacrifice, 236, 237 

Parthenogenesis, 158-174 

Parturition, 229, 230 

Penis, 55, 228 

Pfliiger on polyspermy, 29 ; on sex in 
tadpoles, 36 

Pigments, expressions of disruptive 
processes, 20 

Pigeon's milk, 231 

Placenta, 229 

Planarians, multiplication of 211 

Plants, determination of sex in, 42, 
43; origin of sex among, 119, 120 
conjugation in, 135 ; fertilization in, 
132, 149; parthenogenesis in, 164, 
165 ; asexual reproduction of, 177, 
178 



294 



INDEX. 



Plasmodium, 141, 142 

Plainer on sex-cells in snail, 118 

Ploss on embryonic hermaphroditism, 
28, 61 

Polar globules, 99-101, 167-171 

Pollen-grain, 212, 213 

Polyspermy, 29 

Polystomum, 224 

Population question, 267-274 

Precocious reproduction, 191, 192, 225 

Preformation, theory of, 79, 80 

Primary sexual characters, 3 

Protococcus, 120 

Protomyxa, 141 

Protoplasm, metabolism of, 82, 83, 
115, 116; in fertilization, 149, 150; 
mechanics of, 206, 207 ; in relation 
to cell-division, 206, 207 

Protozoa contrasted with Metazoa, 51, 
84, 85; illustrating cell-phases, 112- 
115; conjugation of, 139-142; asex- 
ual multiplication, 178 ; immortality 
of, 85, 155, 238, 239; alternation of 
generations in, 193 

Puberty, 223 

Purkinje, Si 

Pycnogonid, male with young, 252 

Rate of increase, 261, 263 

Rate of reproduction, 261-275 

Rauber on somatic and reproductive 
cells, 88 

Regeneration, 176 

Reproduction, processes of 129, 199; 
theory of, 203, 243 ; origin of, 215, 
216 ; in relation to environment, 217- 
219; nemesis of, 217, 235 

Reproductive cells, •]j-i2^ passim ; in 
relation to the "body," 241 

Reptiles, amatory emotions of, 245 

Richarz, theory of sex, 32 

Rhinoderma, 234 

Rhythms of life, 208, 209 

Rolph on sex characters, 21 ; on sex 
in wasps, 38, 39 ; on crustaceans, 41 ; 
theory of sex, in ; on sex-cells, 118, 
125 ; on fertilization, 150, 151 ; on 
parthenogenesis, 168, 169 

Romanes on cuckoo, 254 ; on physio- 
logical selection, 282, 283 

Rotifers, sexes of, 17 ; parthenogene- 
sis in, 162 ; male, 238 



Sabatier, theory of polarities, 100 
Sacrifice, 285 

Sachs, origin of fertilization, 143 ; na- 
ture of fertilization, 150 
Sadler on determination of sex, 29 
Salensky on primitive Metazoa, 285 
Schizogenes, multiplication of 215 
Schlechter on sex in horses, 43 
Schleiden, 81 



Schultze, hypothesis of male and fe- 
male ova, 29 

Schwann, 82 • 

Sea-anemones, asexual multiplication, 
175 

Sea-cucumber and offspring, 251 

Sea-horse, parental care of male, 234, 

Secondary sexual characters, 3-7, 1&-21 

Self-fertilization, 70 

Seminal vesicles, 223 

Sex, determination of, 27-48 ; theory 
of, 111-126; origin of, 118, 119 

Sexes, characters of 14-21 ; different 
habits, 14-16; sizes of 16-18; intel- 
lectual and emotional differences, 
247-250 

Sex elements, 77-91 ; early separation 
of, 86 ; compared with Protozoaf 
112 

Sexual attraction, 246-247 

Sexual maturation, 223-225 

Sexual organs and tissues, 51-57 

Sexual reproduction, 129-145 

Sexual selection, 7-12, 282 

Sexual union, 226-229 

Siebold, Von, experiments on wasps, 
38, 39; parthenogenesis, 159 

Silkmoth, parthenogenesis of, 158 

Simon on fertilization, 150 

Siphonophora, 180 

Snail, reproductive specialization in 
the, 56 

Spencer, theory of growth, 204-207 ; 
laws of multiplication, 262-267 \ pop- 
ulation question, 266-269 ; factors of 
evolution, 279 

Spermatogenesis, theory of, 107 

Spermatozoon, 103-109 ; discovery of, 
103 ; structure, 103, 104 ; forms, 105 ; 
physiology of, 105 ; resemblance to 
pollen, 108; chemistry of, 108, 109; 
influence of 277 

Spiculum amoris, 56 

Sponge, reproductive cells, 52, 53 ; 
hermaphroditism, 66 ; asexual re- 
production, 175, 176, 178, 179; alter- 
nation of generations in Spongilla, 

193 

Spores, 190-198 

Sprengel on fertilization in flowers, 
130 

Starfish, reproduction of parts, 182 

Starkweather's law of sex, 31, 32 

Statistics on male and female births, 
30 

Steenstrup on alternation of genera- 
tions, 185 

Stickleback, habits of 20 ; secretion 
of 231, 232 

Sterility, 274 

Strasburger on fertilization, 149, 150 ; 
on parthenogenesis, 169 

Summer eggs, 162, 163 



INDEX. 



295 



Surinam toad, 56, 233, 234 
Sutton on embryonic hermaphrodit- 
ism, 2S, 61 
Syllids, asexual multiphcation, 182 

Tadpoles, 36, 230 

Tapeworm, life-history of, 194 

Temperance, 273, 274 

Temperature, influence of, on sex, 43 

Thury, theorj' of sex, 29 

Tiger-lily, 209 

Treviranus, 276, 277 ; on influence of 

sperm, 155, 2S2 
Tunicates, asexual multiplication, 183; 

alternation of generations, 187 
Twins. 194; sex of, 33, 34 

Ulothrix, reproduction in, 120 

Variation, 11, 12, 155, 156, 277, 281 
Vines on reproduction in plants, 120 
Viviparous animals, 229 
Vorticella, reproduction of, 121, 140 
Voh'ox, 52, 120, 122 

Wallace, sexual selection, 9-12 ; nat- 
ural selection, 24, 278, 279 

Ward, Marshall, on parasitic fungi, 
212 



Water-fleas, parthenogenesis in, 162 

Weeping willows, 175 

Weismann on fertilization, 151 ; on 
polar bodies, 99, 100 ; on use of fer- 
tilization, 153 ; on parthenogenetic 
ova, 169-173 ; hydroids, 86, 195, 196 ; 
alternation of generations, 198; con- 
tinuity of germ-plasma, 88, 89, 221- 
223; on organic immortality, 238-241; 
inheritance of acquired characters, 
45, 2S0, 281 ; variation, 282 

Whelk, cannibalism of young, 230 

Winter eggs, 163 

Wolff, reassertion of epigenesis, 81 

Women, subjection, rights, functions 
of, 247-250 

Worms, hermaphroditism in, 67; asex- 
ual reproduction, 180-182 ; alterna- 
tion of generations, 193 

Yolk glands, 55 

Yolk-sac, 230 

Yung on sex in tadpoles, 36 

Zacharias on male and female ele- 
ments, 109 ; on asexual multiplica- 
tion, 208 

Zona pellucida, 97 

Zona radiata, 97