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3 1924 090 301 874

EVOLUTION. OF TI COLORS

NORTH AMERICAN LAND BIRDS

CHARLES A. KEELER.

Committee of Publication:

H. W. HARKNESS, H. H. BEHR,
T. S. BRANDEGEE.

Editor: TOWNSHEND STITH BRANDEGEE.

PREFACE.

The present paper has been written more with the
hope of stimulating thought and inciting research in a
new and as yet almost untrodden field of ornithological
inquiry, than with the expectation of reaching definite
results. The subject is as yet too new and difficult to
be reduced to even the semblance of an exact science,
and accordingly all the views here set forth are more or
less provisional and tentative. I have constantly pro-
ceeded upon the assumption that a poor theory is better
than no theory, provided it be not considered as final,
since it affords an opening wedge for the further study of
a subject. Accordingly many of the views here set forth
are hardly to be considered as more than guesses, and it
is expected that future study will serve to show their
fallacy. If they lead to this further study, however,
and to more exact and comprehensive work by others, I
shall be glad to see them overturned and their places
filled by more worthy hypotheses.

In the preparation of the work I have received much
valuable assistance, which I here take pleasure in ac-
knowledging. Much of the examination of specimens
was done at the United States National Museum, the
authorities of which kindly placed their collection of
birds at my disposal. To Mr. Robert Ridgway I am in-
debted for innumerable personal favors during my stay
in Washington. His Manual of North American Birds
has formed the systematic basis for this paper. Dr. L.

al PREFACE.

Stejneger assisted me very materially by his criticism and
suggestions, he having devoted more attention to this
subject than any other American ornithologist. Dr.
Elliott Coues furnished me with several useful terms
(some of them new), as well as many suggestions which
have been of value. Mr. F.C. Test of Washington, Mr.
Witmer Stone of the Philadelphia Academy of Sciences,
and Dr. J. A. Allen and Mr. Frank M. Chapman of the
American Museum of Natural History, have aided my
work, both by placing their collections at my disposal
and by furnishing me with valuable facts and sugges-
tions. Dr. Allen’s important contributions to the sub-
ject here discussed have been of far more service than
the mere quotations indicate. To Mrs. Katharine Bran-
degee and Mr. Frank H. Vaslit I desire to return thanks
for their kind and painstaking revision of the proof,
also to Mr. Walter E. Bryant, who read the proof for
scientific terminology.
BERKELEY, Cal., December 21, 1892. C. A. K.

CONTENTS OUTLINE.

I. INTRODUCTION.

THE INHERITANCE OF ACQUIRED CHARACTERS.............
THEORETICAL ASPECT OF THE SUBJECT. ........ 00.00 cee eee cece tenes
Herepity. Introduction; 2; Lamarck, originator of the doctrine of
use and disuse, 2; Hilaire, originator of the doctrine of environ-
mental influences, 2-3; Darwin, doctrine of pangenesis, 3; Brooks
and Galton’s modifications of this, 5; Morgan, criticism of pan-
genesis, 5; Spencer, physiological units, 6; bearing of theories of
heredity upon transmission of acquired characters, 7; Weis-
mann, 7; value of his work, 8; immortality of protozoa, 8; mor-
tality of metazoa, 9; Naegeli’s nucleoplasm theory criticised, 10;
immortality of germplasm precluding the inheritance of acquired
characters, 10; Vines’ criticism of Weismann, 11; Weismann’s
reply to Vines, 13; Ryder, theory of heredity opposed to Weis-
mann, 17; Morgan on cellular continuity, 19; Himer on same, 20;
summary of argument between Weismann and his opponents, 21.
Pammixis. Morgan’s criticism of, 22; Cunningham’s criticism, 23;
Romanes on cessation of selection, 23; other theories to account
for degeneration, reversal of selection (Romanes), economy of
energy (Darwin), disuse (Lamarck), retardation (Cope), 24.
PRACTICAL ASPECT OF THE SUBJECT. 2... 16. eee cee eee eens
CONSEQUENCES OF DISPENSING WITH THE INHERITANCE OF ACQUIRED
CHARACTERS. Cunningham, general consequences, 27; inability
of accounting for origination of new characters, 27; inheritance
of acquired characters by protozoa admitted, 28; Osborn, why
should this factor become obsolete?, 28; Romanes, all variations
must be. directly acquired, 28.
EVIDENCE OF OBSERVED CASES OF TRANSMISSION. ABNORMAL TRANS-
MIssIons. Cunningham on why mutilations are not transmitted,
29; Morgan, the transmission of mutilations not a proof of the
inheritance of acquired characters, 29; Weismann on mutila-
tions, 30; experiments with white mice, 31; Cunningham’s criti-
cism of his observations on the feet of Chinese ladies, 31; Eimer,
instances of observed cases of transmission of mutilations, 32-34;
inheritance of artificially induced epilepsy in guinea pigs, 34.

27

villi CALIFORNIA ACADEMY OF SCIENCES.

NoRMAL TRANSMISSIONS: observed instances of change due to environ-
ment, 34; examples of changes in goldfish and brine-shrimp, 34-35;
discussion of the validity of these examples as proofs, 35; after
effects in plants illustrative of an hereditary rhythm, 36; change
in environment not established in one generation, but only the
predisposition to change, 37; inheritance of acquired habits, 38;
Elliott’s examples, 38; these not proofs of inheritance, 39; Dar-
win’s instances criticised by Ball, 40; results which can be best
explained by transmission of acquired characters, 41; Romanes’
examples of instinct due to the transmission of acquired charac-
ters, 41-42; zsthetic faculties of man, 42; Spencer’s three forms
of evidence of transmission, 43; the first and third form untenable,
according to Romanes and Ball, 43-44; correlation as a proof, 44;
the giraffe as an illustration of correlation, 44; Ball’s criticism of
this illustration, 45; further discussion of correlation, 46; Osborn
on paleontological proofs of transmission, 46-48; Poulton’s reply
to Osborn’s evidencs, 48; summary of discussion, 49-50.

VARIATION AND NATURAL SELECTION................ 0.0.24
Scope and meaning of natural selection, 51; not creative (Schur-
man), 51; conditioned by variation, 52; variations quantitative
rather than qualitative, 52; how are new parts originated?, 53;
variations must be advantageous to be favored by selection, 53;
Romanes on useless variations favored by selection, 54.

THEORIES EXPLAINING VARIATION... 0.0.00... cece eee tee eee
Spencer’s physiological units, 55; Weismann on sexual combina-
tions, 55; variations according to this view must be within the ex-
tremes of ancestral modification, 55; Hartog’s objection, 56; Lloyd
Morgan’s theory of organic compounds, 58; objections to this
view, 59; Three views of the origin of variations, 59; spontaneous,
uninfluenced by environment, 59; due solely to environment, 60;
inherent tendency to vary in a specific direction, 60; conservative
variations, 61; progressive variations, 61; Brooks’ secondary
laws of variation, 62; conclusion concerning variations, 62.

LAWS CONDITIONING EVOLUTION

LAWS OF DEVELOEMENT pace dwaaacteticiitade Sa sada headiomme ee enee
Bathmism, or growth force, 65; law of extent and density; 66;
phylogenic extent and density, 67; metabolism, 67; law of sexual
intensification, 68; Gulick’s criticism of Brooks’ theory of male
progressiveness, 72; acceleration and retardation, 73; cause of ac-
celeration, 74; the originator of the fittest, 75; bearing of the law of
phylogenic extent and density upon acceleration and retardation,
76; law of concentration, 77.

AWS OF ‘STRUCTURES cnc vevne cova cy acne eee HRVEawWeS cee ceeded s
Homology, 77; successional relation, 77; parallelism, 78; adapta-
tion, 78; geratology, 78; bilateral symmetry, 79; correlation of
growth, 79.

CONTENTS OUTLINE.

Lawes OF HEREDITY cjccidiawe ooaa gaia wine oasis cb aaiswonmmaeteey news
Uninterrupted or continuous transmission, 79; interrupted or
latent transmission, 79; sexual transmission, 79; mixed or mutual
transmission, 79; abridged or simplified transmission, 80.

SEXUAL SELECTION GS. c662 seve 22 pases s soy 4 se ewawmary eames 284.0%
Law of battle, 81; preferential mating, 81; Darwin’s and ‘Wallace’ 8
views compared, 82; relation of color of male to vitality and vigor,
82; color as related to integumentary expanse and complexity, 83.

EVIDENCE OF SELECTION BY THE FEMALE. ............ 000.00 0cee eee
Peckham on sexual selection among spiders, 84; Wallace’s ex-
planation of sexual display by male birds, 85; the house-finch
as an illustration of selection, 86; objection that each bird finds
a mate, 87.

DIFFICULTIES IN THE THEORY OF SEXUAL SELECTION AND ALTERNATIVE

TIN POTHESES::: Saacuateove deste se. tatgam aedanina eae d ine Adaya
Beddard on sexual dichromatism without selection, 89; difficulty
of believing in a highly developed a«sthetic sense in birds, 90;
Stolzman’s view of bright colors and appendages of male birds,
91; Romanes’ reply to Wallace’s objections, 92; Wallace on the
esthetic sense in relation to sexual selection, 93; Poulton on the
same, 94; Morgan on the esthetic taste of birds, 95; the beautiful
in inorganic nature contrasted with the beauty of birds and in-
sects, 96; Weismann on novelty as the cause of sexual selection,
97; Romanes on the beautiful among the lower forms of life, 97;
coloration viewed from a general standpoint, 98; colors which
have been evolved with reference to some percipient being, 99;
implications of the theory of sexual selection, 100; Grant Allen
on the mechanical explanation of the pleasure derived from color
combinations, 100; the pleasure given to birds by color combina-
tions is of this lower order, 101; synthesis of factors involved in
the production of the sexual character of birds, 102.

THE NATURE OF SPECIES. ...cs0ssucwces cuece os puae eee menian de

On THE MEANING AND LIMITATIONS OF THE TERM................-5
Natural and artificial modes of classification, 103; the basis of a
natural system, 103; relativity of the term species, 104; definition
of species as used in science, 105.

On THE RELATION OF EVOLUTION TO SPECIES............. 00c ccc eee
Species necessary for rationality in evolutionary progression, 106;
Romanes on the preservation of the species by natural selection,
106; objections to Romanes’ view, 107; reproduction as growth
beyond the individual as an explanation of the origination of
altruistic characters and traits, 108; necessity for natural selection
to preserve family, and in some instances the tribe, but not the
species, 109.

80

84

88

103
103

106

x

CALIFORNIA ACADEMY OF SCIENCES.

ISOLATION AS A FACTOR IN THE EVOLUTION OF SPECIES. 110
PHYSIOLOGICAL SELECTION. 20.0.0. 00 0.0 cece ee cece cen ee cee eee ees

Romanes’ three difficulties in natural selection considered as a
theory of the origin of species, 110; explanation of physiological
selection, 111; sterility of species, 111; variations in the repro-
ductive system as correlated with other variations, 113; on the
swamping effects of interbreeding and the inutility of specific
characters, 114; objections to physiological selection by Meldola
and Galton, 115; Romanes’ reply to these objections, 116; Wal-
lace’s objections, 117; Romanes’ reply to Wallace, 118; Seebohm’s
criticisms, 121; Romanes’ answer, 122; Dyer’s criticisms, and
Romanes’ refutation of his objections, 123; conclusions concern-
physiological selection, 124, —

GULLICK ON ASOLATION je. an42 +a ngusedn enn wee esa sens AE RE

II.

ON

Resemblance and difference between the view of Romanes and
Gulick, 125; criticism of his views, as ignoring the origin of varia-
tions, 126; necessity for isolation established, 127; forms of isola-
tion, 128 ef seg.

THE COLORS OF NORTH AMERICAN
BIRDS.

MODES OF PLUMAGE CHANGES............. 0000.0 0ce ee ee
By the feather itself becoming altered in color, 133; by the wear-
ing off of the lengthened lighter-colored tips of the feather, 135;
classification of different modes of plumage change, 137.

GENERAL PRINCIPLES OF COLOR IN BIRDS.................

On the physical and mechanical causes of color effects, 137; state-
ment of the theory of bird colors to be elaborated, 139; law of as-
sortment of pigments, 140; examples illustrating this law, 140;
on generalized and specialized colors, 142.

THE PROPORTION AND DISTRIBUTION OF THE COLORS IN

THE PATTERN OF MARKINGS

On

NORTH AMERICAN GENERA... 2.0.0.0... 00.00.0200 0004
On the general distribution of the fundamental colors among
North American genera, 143; the bearing of these facts on the law
of the assortment of pigments, 145; the proportion and distribu-
tion of black, 146; white, 151; black and white as cognate colors,
152; the proportion and distribution of red, 153; red and yellow
as correlative colors, 154; examples showing this interdependence,
155; the proportion and distribution of yellow, 157; blue, 157;
generalizations from foregoing facts, 158.

FN DIVIDUAT: PWATHERS) 35.000 tec cme aa cae nd + aleden Sem paws GOA
Feather markings as affected by the general laws of growth, 159;

x
Eimer and Kerschner on feather markings, 162; secondary pig-

110

125

132

137

143

159
159

CONTENTS OUTLINE.’ xl

mentation to be accounted for by metabolism, 163; examples illus-
trating the’ method. of pigment assumption, 163; successional
taxology, 164; further examples of successional taxology, 164;
study of the pattern of markings on the wing of Falco sparverius,
165; the significance of successional taxology; 169; natural selec-
tion as a factor in the origination of feather marks, 170; repeti-
tive marks, 171; hybrid feathers, 172; examples of hybrid feathers,
173; classification of the different forms of hybrid feathers, 174;
explanation of the different forms of hybrid feathers, 174; pseudo-
hybrids, 177. ; $3 ,

ON THE GENERAL PATTERNS OF Bred COLORS...;... ..... feed eaten caw 17D
Eimer’s law of posterior-anterior progression of ‘markings, 179;
markings of the primitive plumage, 180; characteristic types of
markings, and those which seldom or never occur, 180; Dr. Har-
rison Allen on the relation between large masses of muscular and
nervous tissue and color patches, 182; on the location of color
patches on the most conspicuous parts of the body, 183; the
superciliary stripe, 185; on the markings of the head,‘ 186; rela-
tion between laws of growth and selection in the production of
these markings, 190; wing markings, 190; tail: markings, 191.

RECOGNITION MARRS ij ocj carne ye sence gece gE ds oak eyes Gase 193.
Markings considered from the standpoint of their utility, 193;
Poulton’s classification on this basis, 194; protective colors (pro-
cryptic), 195; aggressive colors (anticryptic), 197; false warning
colors (pseudosematic), 198; warning colors (aposematic), 200;
recognition marks proper (episematic), 200; Wallace on recogni-
tion marks, 200; Todd on directive colors, 201; classification of
recognition markings, 201; recognition at a distance and at close
range, 203; recognition markings among the grouse, pigeons and
hawks, 204; tail markings of the Caprimulgids, 207; law of sexual
recognition, 209; socialistic markings, 210.

VARIATION OF COLOR WITH SEX, AGE AND SEASON........ 211
Individual variation in bird markings, 211; epigamic colors, 211;
Darwin’s classification of birds according to the variations in
plumage with sex, age, and season, 212; additional classes, 213;
revised table of classification, 214; male like female, young like
adult, 214; male like female, young differ from adult, 216; re-
semblance of sexes differing with the seasons, or both sexes alike
and varying with the seasons, 218; male more conspicuously
colored, young like female, 221; male more conspicuously col-
ored, young with peculiar first plumage, 222; male more con-
spicuously colored, young of each sex resembles its respective
adult, 223; adult male more conspicuously colored, young male
unlike adults of either sex, 224.

xii CALIFORNIA ACADEMY OF SCIENCES.

THE DIRECT INFLUENCES OF THE ENVIRONMENT.......... 225
Beddard on the general effects of food on color, 226; Stejneger’s
theory of melanism and albinism, 226; Sauermann’s experiments
in changing color by food, 227; influence of temperature, moist-
ureand sunlight on food, 229; instances where intensifying and
bleaching has been the result of the presence or absence of sun-
light, 2830; Garman on the bleaching power of sunlight, 231; ob-
jections to Garman’s theory, 231; pale colors of the under parts
of animals due to the absence of sunlight, 232; reconciliation
of this view with the bleaching effect of sunlight, 233.

GEOGRAPHICAL DISTRIBUTION AS A FACTOR IN THE

EVOLUTION OF COLORS... 2. .cccue cise see sneer eee 234
General survey of the faunal areas of North America, in their relation
to extralimital faunas, 234; birds of the Boreal area, 235; absence
of brilliantly colored genera among boreal forms, 236; reasons for
this lack of specialization among northern forms, 237; brilliancy
of tropical species, 238; Beddard on special relations of color to
geographical distribution, 238; on the resemblance of the colors
of Sturnella and Macronyx, 239; Ridgway on the relation between
color and geographical distribution, 241; Spinus psaltria as an
instance of melanism as related to distribution, 241; other ex-
amples of melanism toward the south, 243; intensification in
yellow toward the tropics, 243; intensification of red with change
in climate, 244; increase in blue from north to south, 245; Cyano-
citta stelleri studied with reference to the connection between
color marks and geographical distribution, 245; J. A. Allen on the
relation between climate and geographical races, 247; Melospiza
Jasciata as an instance of the result of climatic influence, 248; the
ice age as a factor in isolation, 249; examples of forms produced
by an east and west isolation, 250; examples of north and south
isolation, 252; varieties or closely related species whose ranges
overlap, 253; particular instances where the different races cannot
have been produced solely by climatic influences, 255; Myiarchus
as an example of species which have been produced by climatic
influences alone, 256.

ORDERS, FAMILIES, AND GENERA OF NORTH AMERICAN
BIRDS, CONSIDERED FROM THE STANDPOINT OF THEIR

PY OTs WELTON onssocareisiont sited Wei Nah Ro eewteath-o2 s suawle niga niaed: AeA 257
BIBLIOGRA BEY: ccmdea saae wei uens see wate 6 ee’ aaa wh BETO 337
EXPLANATION, OF PARES Gs 45 4 sig Saeaede vadohe 4444 paeawe Manes 345

THE EVOLUTION OF THE COLORS OF NORTH AMER-
ICAN LAND BIRDS.

I. Inrropuction.

A few words of explanation may be said with regard to
the greatly disproportionate length of the introductory
portion of this work. To the uninitiated the most perfect
harmony is commonly thought to prevail concerning
the great problems of evolutionary philosophy, but once
within the circle of current scientific thought and the
reverse is found to be true. Instead of harmony, discord
is discovered. There is hardly one of the important
doctrines concerning which a consensus of scientific
opinion has been attained. To be sure, all maintain that
Darwinism or natural selection is a factor in evolution,
but while some hold it to be the only factor, and all-suffi-
cient in the creation of species, others believe it to be a
very minor agency, and relegate it to the post of inspec-
tor-general of the army of life. With regard to sexual se-
lection the same diversity of opinion prevails, one school
advocating sexual selection as the sole agent in produc-
ing the brilliant colors and varied plumes of male birds,
etc., the other extreme asserting that sexual selection as
a factor in evolution is a myth. Still greater is the di-
versity of opinion and more intense the feeling in regard
to that momentous question which is at present agitating
the troubled sea of scientific thought—the transmission
of acquired character.

In view of all this disagreement, it is quite impossible
to undertake any general scientific investigations in the
field of evolution without a tolerably thorough survey of
the whole ground. With this end in view, and merely
as a preparation for the more particular investigations
of the work, these preliminary pages have been written.
The tenability’ of the theories here advanced need not

2 CALIFORNIA ACADEMY OF SCIENCES.

materially affect the work of the second part, although
the theories concerning the special investigations in the
evolution of color in birds are largely founded upon the
principles laid down in Part I. Although this first part
is necessarily of a general character, nothing has been
inserted which has not some bearing upon the investiga-
tions which follow. An attempt has been made to dis-
cuss the general principles of evolution according to a
logical system, the subject of the inheritance of acquired
characters being treated first, as it is the most funda-
mental question in dispute.

The doctrine of evolution is by no means modern in
its conception, having been dimly foreshadowed from
the days of Aristotle; but it was first suggested in a
plausible scientific form by Jean Lamarck, who, in 1809,
published his Philosophie Zoologique. He attempted
to account for the changes in organic forms almost ex-
clusively by the principles of the use and disuse of parts
of which doctrine he was the originator. His views were
hardly noticed at the time they were announced, but a
little later Geoffroy St. Hilaire was more successful in
calling the attention of the scientific world to his own
closely related theory of the action of the environment
in producing the changes in organic beings; although
his views were not generally accepted by the naturalists
of the day The nature and extent of his theory is ex-
plained in the following words of Haeckel.* ‘‘ He con-
ceives the organism as passive, In regard to the vital
conditions of the outer world, while Lamarck, on the
contrary, regards it as active. Geoffroy thinks, for ex-
ample, that birds originated from lizard-like reptiles,
simply by a diminution of the carbonic acid in the at-
mosphere, in consequence of which the breathing pro-

*History of Creation, I., p. 117.

EVOLUTION OF THE COLORS OF BIRDS. 3

cess became more animated and energetic through the
increased proportion of oxygen in the atmosphere. Thus
there arose a higher temperature of the blood, an in-
creased activity of the nerves and muscles, and the scales
of the reptiles became the feathers of birds,” etc.

The principle of use and disuse, and the transmission
of acquired characters, are very closely related and stand
or fall together. Darwin, in his Origin of Species, and
throughout all his subsequent writings, accepted them
both as supplementary to his own doctrine of natural selec-
tion, in accounting for the origin of species. It was, of
course, generally admitted that an individual could, dur-
ing the course of its life, acquire characters peculiar to
itself, or could modify its parts by use or disuse. More-
over, it was commonly held that such modifications could
be transmitted by the individual to its offspring. To
this view Darwin was committed, and in order to ex-
plain it in a rational way he formulated the provisional
hypothesis of pangenesis, which he describes in the fol-
lowing language: ‘‘ This important distinction between
transmission and development will be best kept in mind
by the aid of the hypothesis of pangenesis. According
to this hypothesis, every unit or cell of the body throws
off gemmules or undeveloped atoms, which are trans-
mitted to the offspring of both sexes, and are multiplied
by self division. They may remain undeveloped dur-
ing the early years of life or during successive genera-
tions; and their development into units or cells, like
those from which they were derived, depends on their
-affinity for, and union with other units or cells pre-
viously developed in the due order of growth.’’*

Mr. Romanes has presented Darwin’s hypothesis of
pangenesist in so able and comprehensive a manner

*Descent of Man, p. 228.

tWeismann’s Theory of Heredity by George J. Romanes. Contempo-
rary Review. May 1890, pp. 686-699.

4 CALIFORNIA ACADEMY OF SCIENCES.

that in order to convey a proper understanding of the
subject I cannot do better than quote his words in full.
It is stated in the following seven assumptions:

“1. That all the component cells of a multicellular or-
ganism throw off inconceivably minute germs or ‘‘ gem-
mules,” which are then dispersed throughout the whole
system.

2. That these gemmules, when so dispersed and sup-
plied with proper nutriment, multiply by self division,
and, under suitable conditions, are capable of develop-
ing into physiological cells like those from which they
were originally and severally derived.

3. That while still in this gemmular condition, these
cell seeds have one for another a mutual affinity, which
leads to their being collected from all parts of the system
by the reproductive glands of the organism; and that,
when so collected, they go to constitute the essential
material of the sexual elements—ova and spermatozoa
being thus nothing more than aggregated packets of
gemmules, which have emanated from all the cells of all
the tissues of the organism.

4, That the development of a new organism, out of
the fusion of two such packets of gemmules, is due to a
summation of all the developments of some of the gem-
mules which those two packets contain.

5. Thata large proportional number of the gemmules
in each packet, however, fail to develop, and are then
transmitted in a dormant state to future generations, in
any of which they may be developed subsequently—thus
giving rise to the phenomena of reversion or atavism.

6. That in all cases the development of gemmules
into the form of their parent cells depends on their suit-
able union with other partially developed gemmules,
which precede them in the regular course of their
growth.

EVOLUTION OF THE COLORS OF BIRDS. 5
7. That gemmules are thrown off by all physiolog-
ical cells, not only during the adult state of the organ-
ism, but during all stages of development. Or, in other
words, that the production of these cell-seeds depends
upon the adult condition of parent cells; not upon that
of the multicellular organism.”

This theory has subsequently been varied in its details
by Brooks, Galton, Herdman, and others. According
to Brooks the ovary is passive and does not aid in the
transmission of acquired characters, but the sperm cells
contain gemmules which are thrown off from such parts
as are undergoing change. Galton believed to a con-
siderable degree in the continuity of the germ plasm,
although also holding that acquired characters are in-
heritable. For example, he says, ‘‘ From the well-
known circumstance that an individual may transmit to
his descendents ancestral qualities which he does not
himself possess, we are assured that they could not have
been altogether destroyed in him, but must have main-
tained their existence in a latent form. Therefore each
individual may properly be considered as consisting of
two parts, one of which is latent and only known to us by
its effects on his posterity, while the other is patent, and
constitutes the person manifest to our senses.”* These
latent characters he considered to be transmitted from
generation to generation by means of a portion of the
gemmules of the fertilized ovum which remained unde-
veloped. Although at first adopting an hypothesis of
pangenesis he afterwards abandoned this for a theory of
the continuity of the germ-plasin not unlike that of
Weismann.

Lloyd Morgan criticises the pangenetic hypothesis in
the following well chosen words: ‘‘ The existence of

* On Blood Relationship, Proc. Roy. Soc., 1872, p. 394.

6 CALIFORNIA ACADEMY OF SCIENCES.

gemmules, then, is unproven, and their supposed mode
of origin not in altogether satisfactory accordance with
organic analogies. Furthermore, the whole machinery
of the scheme of heredity is complicated and hyper-
hypothetical. It is difficult to read Darwin’s account of
reversion, the inheritance of functionally acquired char-
acters and the non-inheritance of mutilation, or to fol-
low his skillful manipulation of the invisible army of
gemmules, without being tempted to exclaim—What
cannot be explained, if this be explanation? and to ask
whether an honest confession of ignorance, of which we
are all so terribly afraid, be not, after all, a more satis-
factory position.””*

Haeckel’s plastidule theory and Spencer’s theory of
physiological units do not differ very essentially from
Darwin’s hypothesis of pangenesis, although Spencer’s
idea is a much less crude one. He finds that the units
of which an organism is composed have the property of
arranging themselves in a definite form or sequence,
and then proceeds to enquire into the nature of these
units. He first shows that they cannot be chemical, for
the chemical composition of the various organic bodies
which arrange themselves in such diverse shapes is
essentially alike in all cases. Neither can morphologi-
cal units be accepted as final. The simple cell is the
morphological unit, but certain tissues arise directly
out of the formative substance without the intervention
of acellular stage. Moreover, certain non-cellular or-
ganisms, such as Rhizopods, are capable of transmitting
peculiar specific characters. From these exceptions it
is evident that this formative power does not reside in
cells; and consequently both chemical and morphologi-
cal units are disposed of. Spencer then argues for

= Animal Life and Intelligence, p. 137.

EVOLUTION OF THE COLORS OF BIRDS. ve

physiological units in the following terms:* ‘If, then,
this organic polarity can be possessed neither by the
chemical units nor the morphological units, we must
conceive it as possessed by certain intermediate units,
which we may term physiologicul. There seems no alter-
native but to suppose, that the chemical units combine
into units immensely more complex than themselves,
complex as they are; and that in each organism, the
physiological units produced by this further compound-
ing of highly compound atoms, have a more or less dis-
tinctive character.”

Before considering the subject of heredity in further
detail, it may be well to pause a moment to consider its
bearing upon the question in hand—the inheritance of
acquired characters. Obviously the nature of the me-
chanical process by which heredity is made possible
must most decisively determine what the possibilities of
heredity are—just what characters can be inherited, and
what characters cannot (if any such exist). Darwin be-
gan with the assumption that all characters could be in-
herited and framed his theory of heredity upon this
assumption. In this he was followed by the various
subsequent writers on the subject, with the exception of
Spencer, whose theory does not appear to be designed
with the express view of accounting for the inheritance
of acquired characters, but rather to have been con-
structed inductively.

Such was the state of the case when, in 1885, Mr. A.
E. Shipley in an article in ‘‘ The Nineteenth Century,”
called the attention of English and American scientists
to the views of Prof. August Weismann, of Freiburg.
Since then two editions of an English translation of the
collected essays of Prof. Weismann on the subject of

*Principles of Biology, I, 183.

5 CALIFORNIA ACADEMY OF SCIENCES.

heredity have appeared, the second in 1891. Prof.
Weismann’s views, although not entirely original, are
stated with such a rigid consistency to all the conse-
quences which they involve, are fraught with such un-
usual and daring speculations, and so vitally affect many
of our scientific dogmas, that they have caused a re-
markable revolution in the scientific world. By many
English biologists these new views have been received
with great favor. Wallace has thrown the weight of his
approval with them. With such avidity have they been
accepted that one is almost tempted to feel that a reac-
tion must ultimately follow. In America the case has
been directly the reverse. Instead of meeting with
favor they have been passed by in silence, questioned,
doubted, denied, and even in some cases treated almost
with scorn and ridicule. Against such an extreme as
this also, reaction seems inevitable.

The chief value of Weismann’s work, regardless of
how correct or incorrect it may ultimately prove to be,
is the fact that he has attacked the problem of heredity
from an entirely new point of view, and has set the sci-
entific world to thinking. It will be advisable to con-
sider his speculations in some detail, as bearing directly
upon the subject under discussion.

Weisinann commences his discussion with an inquiry
into the nature of death. He asks why mortality should
be a necessary consequence of life. In unicellular organ-
isms reproduction takes place by fission. The life of one
amieba cones to an end by the division of the parent into
two equal halves, cach of which forms a new individual.
“But,” as Weismann says, ‘this process cannot be truly
called death. Where is the dead body’—what is it that
dics? Nothing dies; the body of the animal only divides
into two similar parts, possessing the same constitution.
Each of these parts is exactly like its parent, lives in the

EVOLUTION OF THE COLORS OF BIRDS. 9

same manner, and finally also divides into two halves.
As far as these organisms are concerned, death can only
be spoken of in the most figurative sense.”* From this
view of Weismann’s in regard to the immortality of pro-
tozoa there has been but little dissent. The most seri-
ous objection that has been raised to it is that of Maupas,
who has shown that even among amceba conjunction is
occasionally necessary to effect rejuvenescence, and who
holds that, in consequence of this, even unicellular or-
ganisms are mortal. Geddes and Thomson have well
stated,} however, that Maupas’ experiments, instead of
being contradictory to Weismann’s view, should be in-
serted as a saving clause, for in a state of nature this
rejuvenescence by coalescence does take place when
needed and neither the organism as a whole nor any
part of it dies.

Having established the immortality of unicellular or-
ganisms, Weismann attempts to account for the intro-
duction of death into the economy of nature on the
principle of the advantage to the race as a whole, of the
sacrifice of the old and decrepit to the young and vig-
orous. The weaknesses and fallacies of this part of his
theory have been pointed out with especial force by Lloyd
Morgan.?

The fact seems to be tolerably well established that in
protozoa death never normally ends the career of the
organism, although of course violent death by accident
is perfectly possible, while in metazoa death is the nat-
ural outcome. Or, as Weismann explains it, there is no
limit to the number of times an amceban cell can divide
itself, but the cells of a more complex organism are lim-

* Essays upon Heredity, 1891, I, p. 26.
tEvolution of Sex, Humboldt Library, p. 240.
tAnimal Life and Intelligence, pp. 184 and 193.

10 CALIFORNIA ACADEMY OF SCIENCES.

ited in their powers of reproduction and ultimately wear
out. To this latter rule Weismann makes one important
exception, viz: the germ cells. In the case of some in-
sects and other invertebrates it has been demonstated
that almost at the commencement of embryonic devel-
opment the reproductive cells are setapart. In the case
of these organisms it is evident that there is an immor-
tal chain of reproductive cells from one generation to
another. Observation has proved, however, that it is
only in rarely exceptional cases that the reproductive
cells are thus set apart, and that in ordinary metazoa
they appear after the embryo is well advanced toward
maturity. Weismann argues very reasonably that it is
at variance with the laws of development to assume, as
Neegeli does, the existence of a nucleoplasm which first
develops into the more complex body cells and then be-
comes simplified into reproductive cells; and he accord-
ingly substitutes an hypothesis of hisown. He assumes
the existence of germ-plasm intermingled with the body
plasm and capable of producing the latter, although
body plasm cannot be converted into germ-plasm.
There is, then, in the higher metazoa, not an immortal
chain of reproductive cells, but an immortal chain of
germ-plasms.

From this standpoint the heredity of acquired char-
acters is obviously impossible. The germ-plasm cannot
be influenced by the body plasm. The hypothesis, from
its very nature places an effectual barrier against the in-
heritance of acquired characters, and in case it could be
demonstrated as true, it would be necessary to explain
all such supposed cases of transmission in some other
way. This is what Weismann and his followers have
done, but before considering the evidence for and against
such transmission of acquired characters, it will be ad-
visable to consider how well founded this theory is.

EVOLUTION OF THE COLORS OF BIRDS. 11

Prof. S. H. Vines has criticised it in an article enti-
tled ‘‘ An Examination of Some Points in Prof. Weis-
mann’s Theory of Heredity,” which appeared in Nature,
October 24, 1889 (pp. 621-626). Prof. Vines admits the
immortality of protozoa, but questions the explanation
of Prof. Weismann as to how the immortal protozoa
evolved into the mortal metazoa. He objects to Weis-
mann’s suggestion of unequal fission as being no ex-
planation, and asserts that if unequal fission were the
cause it would be necessary to assume that a potential
mortality already existed in protozoa. ‘‘It is impossible
to conceive,” he says, ‘‘that unequal fission can take
place in a cell consisting throughout of essentially the
same kind of substance.’? Furthermore, as Prof. Vines
points out, Weismann claims that the germ-plasm is
located chiefly in the nucleus of the germ-cell, but does
not explain of what the remaining portion of the germ-
cell consists. Obviously it must be somatoplasm, which
is mortal, despite the fact that Weismann has asserted
that the entire germ-cell is immortal. Prof. Vines
suggests as an explanation of the paradox, ‘‘ the assump-
tion that the substance of the nucleus determines the
nature and character of the cell.” Admitting the above
explanation that the protozoon contains both somato-
plasm and germ-plasm, it is easy to understand how un-
equal fission might separate the one from the other, thus
originating two forms of cells, mortal and immortal; but
this Weismann is not likely to admit, asserting as he
does that the germ-plasm becomes changed into somato-
plasm.

‘SIt is not a little remarkable,” says Prof. Vines,
‘that Prof. Weismann should not have offered any sug-
gestion as to the conception which he has formed of the
mode in which the conversion of germ-plasm into so-
matoplasm can take place, considering that this assump-

2

12 CALIFORNIA ACADEMY OF SCIENCES.

tion is the key to his whole position. He has been at
considerable pains to controvert the view that somato-
plasm may be conyerted into germ-plasm; but in mak-
ing the attack he has overlooked the necessity for de-
fense.”’ Prof. Vines then gives quotations from Prof.
Weismann illustrative of his theory of heredity, and of
his assertion that germ-plasm must be a substance of
great stability in order to be able to transmit all of the
complex modifications which it acquires. He then con-
tinues his objections as follows: A part of the germ-
plasm, Weismann claims, goes to the formation of the
somatoplasm of the developing embryo, while what re-
mains goes to the formation of the nucleus of the germ-
cells of the embryo. But the germ-plasm of the ovum,
Prof. Vines claims, cannot influence the somatoplasm of
the embryo, even from Prof. Weismann’s standpoint.
‘“‘This function caunot be discharged,” he says, ‘“‘ by
that portion of the germ-plasm of the ovum which has
become converted into the somatoplasm of the embryo,
for the simple reason that it has ceased to be germ-plasm
und must therefore have lost the properties characteris-
tic of that substance. Neither can it be discharged by
that portion of the germ-plasm of the ovum which is
aggregated in the germ-cells of the embryo, for under
these circumstances it is withdrawn from all direct rela-
tion with the developing somatic cells. The question
remains without an answer.” So much for the criticism
from Prof. Weismann’s own standpoint. From Prof.
Vine’s position it is open to a still more vital attack.
Claiming as he does that the possibility of germ-plasm
being converted into somatoplasm is an unwarrantable
assumption on the part of Prof. Weismann, Prof. Vines
cannot but assert that the entire theory of germ-plasm
which is built upon this assumption, must collapse.
Furthermore, inasmuch as the embryo is not formed

EVOLUTION OF THE COLORS OF BIRDS. 13

solely from that part of the nucleus which is purported
to be the chief bearer of the germ-plasm, but from the
whole nucleus plus a portion of the cytoplasm of the
ovum, it is evident that the somatoplasm must have some
constructive powers as well as the germ-plasm; but this
is the very thing denied by Prof. Wiseman, and if true,
what would be the necessity of introducing the germ-
plasm at all. The somatoplasm alone would be able to.
be the bearer of hereditary characters, and we would
then have a continuity of somatoplasm instead of a con-
tinuity of germ-plasm.

Prof. Weismann’s reply to the criticism of Prof. Vines*
is of especial interest for it contains an epitome of his
theories brought up to date. From it, it is possible to
comprehend which of his views he still holds and which
he rejects, a task of some difficulty in depending upon
the series of his collected essays. In reply to the objec-
tion of Prof. Vines that an immortal cell could not have
changed into a mortal cell by fission unless there already
existed within it a latent principle of mortality, Prof.
Weismann appeals to the division of labor, saying:
‘From the one cell which performed all functions comes
a group of several cells which distribute themselves over
the work. In my opinion, the first such differentiation
produced two sets of cells, the one the mortal cells of
the body proper, the other the immortal germ-cells.”
Surely Prof. Weismann does not consider immortality a
function of amceban cells, or, if he does, cannot hold
that mortality is another function possessed by the same
cells! His explanation of the distinction between im-
mortality and eternity is opportune and may throw some
light on the subject. By biological immortality Weis-

*Prof. Weismann’s Theory of Heredity; Nature, February 6, 1890, pp.
317 -323.

14 CALIFORNIA ACADEMY OF SCIENCES.

mann simply means that an organism has the potential
power of renewing the cycle of its existence, as long as
the environment conditioning its existence remains
suitable. While it can be destroyed, it does not bear
within it the seeds of decay. ‘‘If, then,” says Prof.
Weismann, ‘‘this true immortality is but cyclical, and
is conditioned by the physical constitution of the pro-
toplasm, why is it inconceivable that this constitution
should be under certain circumstances and to a certain
extent, so modified that the metabolic activity no longer
follows its own orbit, but after more or fewer revolutions
comes to a standstill and results in death? All living
matter is variable; why should not variations in the
protoplasm have occurred which, while they fulfilled cer-
tain functions of the individual economy better, caused
a metabolism which did not exactly repeat itself, 2. e.,
sooner or later came to a condition of rest?”

This explanation, although rather vague, does indeed
seem to throw some light upon the way in which mortal-
ity might have originated, but his appeal to panmixia
to aid him seems wholly unwarrantable. He says: ‘‘I
believe that organs no longer in use become rudiment-
ary, and must finally disappear solely by ‘ panmixie’;
not through the direct action of disuse, but because
natural selection no longer maintains their standard of
structure. What is true of an organ is true also of its
function, since the latter is but the expression of the
qualities of material parts, whether we can directly per-
ceive their relations or not. If, then, as we saw, the
immortality of monoplastids depends on the fact that
incessant metabolism of their bodies is ever returning
exactly to its starting point, and produces no such modi-
fications as would gradually obstruct the repetition of
the cycle, why should that quality of its living matter
which causes immortality —nay, how could it be re-

EVOLUTION OF THE COLORS OF BIRDS. 15

tained—when no longer necessary?” The above pas-
sage is open to two objections. First, he assumes pan-
mixia as proved. In discussing this subject later on it
will be shown that, on the contrary, panmixia appears
to be largely untenable. Secondly, panmixia means a
cessation of natural selection. If we assume with Weis-
mann, as there seems every reason to assume, that the
original unicellular organisms, and their living repre-
sentatives to-day, possess a potential immortality or
possibility of indefinite existence, we certainly cannot
assume that some are more immortal than others. But
if potential immortality be a natural attribute of life,
why should natural selection be necessary to preserve
this attribute, or from what could it make its selection?
If natural selection is not requisite to maintain this
standard of immortality, panmixia, assuming its po-
tency in other instances, could have no influence in
causing mortality, being merely the negative of natural
selection.

Prof. Weismann then replies to Prof. Vine’s criticism
of his theory of embryogenesis and the continuity of
germ-plasm. He asserts that Prof. Vine’s criticism is
due toamisconception, that he does not claim that germ-
plasm is ever converted into somatoplasm. In his second
essay he had indeed contrasted the somatoplasm or the
entire substance of the body with the germ-plasm or
entire substance of the germ-cells, not having arrived at
the time at the conclusions of Strasburger and O. Hert-
wig, that hereditary transmission was effected solely by
the chromatin of the nuclear loops. This view he had
adopted when the fourth essay was written, and his theory
was accordingly somewhat modified. He made use of Na-
geli’s term, idioplasm, in an essentially different manner,
applying it to the chromatin not only of the ovum-
nucleus, but also of every cell in the body. This idio-

16 CALIFORNIA ACADEMY OF SCIENCES.

plasma he asserted to be the vital formative principle of
every cell, what remained being merely nutritive. The
general term somatoplasm was then of course abandoned.
There are, then, according to this theory, two series
throughout the body, one formative, the other reproduc-
tive. The formative, which is the chromatin of the nu-
cleus, passes under the general term of idioplasma.
When present in the germ-cells it is called germ-plasm;
in the body cells,‘‘ somatic idioplasm.”’ The nutritive sub-
stance is what was previously known as somatoplasm.
To it also a new term was given, ‘‘cytoplasma.” Hav-
ing made these distinctions, Weismann explains his idea
of embryogenesis. This takes place, according to his
theory, by the successive halvings of the nuclear loops
or germ-plasm. ‘‘ Each fresh cell-division,” he says,
‘sorts out tendencies which were mixed in the nucleus
of the mother-cell, until the complex mass of embryonic
cells is formed, each with a nuclear idioplasma which
stamps its specific histological character on the cell.” A
minute part of the idioplasm he assumes to remain un-
changed when the first transformation occurs, in order
to preserve the continuity of the germ-plasm. This
fragment of germ-plasm migrates in an inactive condi-
tion from cell to cell, until it comes to the spot where it
develops into the germinal cells of the next generation.

How, then, does the controversy between Prof. Vines
and Prof. Weismann rest? Briefly, Prof. Vines has
shown that Weismann has not accounted for the intro-
duction of mortality in the order of life, although it is
upon the assumption of such a differentiation of cells
into mortal and immortal that his entire theory rests.
Vines has not shown, however, that such a differentia-
tion is impossible. Weismann, on the other hand, has.
refuted the charge of the inconsistency of assuming that
germ-plasm can be converted into somatoplasm, although

EVOLUTION OF THE COLORS OF BIRDS. 17

the reverse is impossible. From the destructive, it may
now be well to turn to the constructive form of criticism.

There have been two theories recently advanced in
opposition to Weismann’s—one by J. A. Ryder and the
other by Lloyd Morgan. Ryder has presented his
theory in an article entitled ‘A Physiological Hypothe-
sis of Heredity and Variations,” in the ‘‘ American
Naturalist.* He considers Weismann’s views to be
fanciful and visionary and admits of no possible com-
promise. He agrees with Morgan that the introduction
of a hypothetical germ-plasm, instead of making the
question of heredity more simple, in reality complicates
matters. He believes with H. Milne Edwards and Hux-
ley that in the division of labor of the various cells of
the body, all have been specialized beyond the point
where further embryonic development is possible, with
the exception of the reproductive cells which remain un-
specialized, and hence capable of development. Spencer
confirms this view of the simplicity of the reproductive
cells. He says: t‘‘ The marvellous phenomena initiated
by the meeting of the sperm-cell and germ-cell, natur-
ally suggest the conception of some quite special and
peculiar properties possessed by these cells. It seems
obvious that this mysterious power which they display,
of originating a new and complex organism, dis-
tinguishes them in the broadest way from portions of
organic substance in general. Nevertheless, the more
we study the evidence the more is this assumption
shaken—the more are we led towards the conclusion
that these cells have not been made by some unusual
elaboration, fundamentally different from all other cells,
* * * the organs for preparing sperm-cells and germ-

*Vol. 24, p. 85.
tPrinciples of Biology, I, pp. 219-220.
2

18 CALIFORNIA ACADEMY OF SCIENCES.

cells have none of the specialty of structures which
might be looked for, did sperm-cells and germ-cells need
endowing with properties essentially unlike those of all
other organic agents. On the contrary, these reproduc-
tive centers proceed from tissues that are characterized
by their low organization.”

Besides the accusation of producing unnecessary con-
fusion by the introduction of the mystical germ-plasm,
Ryder has a much more serious charge to prefer against
Weismann. He claims that the isolation of the germ-
plasma in the germ-cell is in conflict not alone with the
principles of metabolism, upon which modern physiology
stands, but also with the law of the conservation of en-
ergy. ‘‘ Modern physiology,” he says, ‘‘as well as the
doctrine of the conservation of energy, positively forbids
us to interpose any barrier between the plasma of the
parent-body and that of the germ-cells, as is done by the
promulgators of the hypothesis of the continuity and
isolation of the germ-plasma.” What, then, is Ryder’s
theory? Briefly this: All cells of the body haye some
reproductive power, as shown by the healing of a wound
among the most specialized organisms, by the restora-
tion of a lost lim} among lower forms, or of a lost organ,
as the eye, for example, by still lower, and by the power
of the lowest metazou and some plants of forming anew
individual from a fragment of the parent. The lower in
the scale of life we penctrate, the more generally diffused
and potent do we find this regenerative power. The
logical inference from this is that reproductive force is
most powerful where the specialization is least. *The
reproductive cells would accordingly be the least special-
ized cells of the body. Moreover, they are the only cells
which are normally passive and functionless. The spe-
cific molecular character of the reproductive cells, then,
according to Ryder, together with the molecular tenden-

EVOLUTION OF THE COLORS OF BIRDS. 19

cies of all the cells of the body form the efficient force for
the production of a new individual. This theory would
allow, nay, even necessitate, the inheritance of acquired
character. Prof. Ryder states this as follows: ‘‘ Molecu-
lar impressions experienced in the course of variations
in the modes of manifestation of, or of disturbance of the
balance of the metabolism of the parent-body, are sup-
posed upon this view to be transmitted as molecular
tendencies to the idle or passive plasma of the germ-
cells. Variations in the molecular constitution and
tendencies of the germinal matter are supposed to thus
arise at different times in the same parent, and that,
consequently, successive germs may be thus differently
impressed.”

The above view does not seem to be fundamentally
different from Haeckel’s plastidule theory, although Ry-
der classes the latter with those from which his own is a
departure. The chief objection to it is its vagueness,
and it is to be hoped that Prof. Ryder may elaborate it
at some time. There appears to be nothing expressed in
Lloyd Morgan’s views contradictory to the theory above
stated. Morgan lays the greatest stress upon cellular
continuity. He regards ‘‘the sharp distinction between
body-plasm and germ-plasm as an interesting biological
myth.’”? He expresses his views on cellular continuity
as follows:*

‘The nucleus is the essence of the cell. And the doc-
trine of cellular continuity emphasizes the fact that the
nuclei of all the cells of the body are derived by a pro-
cess of divisional growth from the first segmentation
nucleus which results’ from the union of the nuclei of
the ovum and the sperm. In this sense, then, however
late the germinal cells appear as such, they are in direct

* Animal Life and Intelligence, p. 142.

20 CALIFORNIA ACADEMY OF SCIENCES.

continuity with the germinal cell from which they, in
common with all the cells of the organism, derive their
origin. In this sense there is a true continuity of germ-
cells.”

Morgan, like Ryder, believes the reproductive cells
have been set apart in the division of labor, and in
this he is in accord with Weismann. The agreement
is not carried out in the details, however. Morgan says:*
‘« Cell-reproduction is, however, in the metazoa of two
kinds. There is the direct reproduction of differentiated
cells, by which muscle-cells, nerve-cells, or others re-
produce their kind in the growth of tissues or organs;
and there is the developmental reproduction, by which
the germinal cells under appropriate conditions repro-
duce an organism similar to the parent. The former is
in the direct line of descent from the simple reproduc-
tion of amceba. The latter is something peculiarly
metazoan, and is, if one may be allowed the expression,
specialized in its generality.”

Prof. Eimert expresses himself in similar terms with
regard to the continuity of body cells. He says: ‘If
the body of the multicellular organism is thus, even
according to Weismann’s ideas, of secondary importance
in comparison with the germ-plasm, if the latter corres-
ponds to the unicellular organism, it follows that the
multicellular is just as immortal or mortal as the uni-
cellular. And thus it is impossible to see why, between
the germ-plasm of the multicellular on the one hand,
and that of the unicellular on the other, there should
exist this profound difference, that the latter acquire
characters during life and transmit them by heredity,
the former not, how the former any more than the

“le, p. 143.

tOrganic Evolution; English Translation, p. 71.

EVOLUTION OF THE COLORS OF BIRDS. 21

latter can nourish itself and grow without being influ-
enced in its nature by its nurture.”

The above discussion of the views of Weismann and
his opponents is merely intended as a statement of
the case in an unprejudiced light so that a provis-
ional conclusion may be reached, at least on some of the
points in dispute. A brief summary of the details in
which the two factions agree and differ may now be
given: They agree:

(1.) In the immortality of protozoa.

(2.) That mortal metazoa have been evolved from
lunmortal protozoa.

(3.) That the reproductive cells have been set
apart by the principle of the division of labor.

Weismann claims:

(1.) That there are two forms of plasma, germ-
plasma which has a formative and cytoplasma
which has a nutritive function.

(2.) That the germ-plasma has the immortality of
protozoa, while the cytoplasma is mortal.

(3.) That the two plasmas are mutually isolated.

Hence:

(4.) That whatever may affect the cytoplasma of
the body-cells can have no influence on the
germ-plasma of the ovary. Or, in other words,
that acquired characters cannot be inherited.

His opponents claim:

(1.) That there is only one form of plasma which
may be called either somatoplasm or idioplasm.

(2.) That, inasmuch as every individual is formed
by repeated cell divisions of the germ-cell,
there is an immortality of somatoplasm.

(8.) That the inheritance of acquired characters
is not in opposition to any known biological
law.

22 CALIFORNIA ACADEMY OF SCIENCES.

According to Ryder’s hypothesis acquired characters
must be inherited, but this hypothesis not yet having
been demonstrated does not afford a proof that such
characters are inherited. Morgan, on the contrary,
points out the difficulty of framing an intelligible theory
which will satisfactorily explain such inheritance, al-
though at the same time claiming that there is no theo-
retical evidence against it. This being the state of
opinion in regard to the theories, it will be well to look
to the so-called proofs of the inheritance of acquired
characters; but before so doing one other theory of
Weismann’s bearing upon the subject must be examined,
viz: panmixia, or ” according to the cor-
rected version.

Panmixia, according to Weismann, means the ‘ sus-
pension of the preserving influence of natural selec-
tion,” over an organ, part or function no longer
necessary for the welfare of the species. So far all very

ce

pammixis,

ce

well. We cannot quarrel with Weismann for giving a
name to this cessation, but we can disagree with him as
to the result which will be brought about; and this
Lloyd Morgan las done so ably that I cannot do better
than refer the reader to the passage.* Weismann claims
that by this failure on the part of natural selection to
maintain the standard of excellence of an organ, it will
degenerate and ultimately disappear. Lloyd Morgan
shows that pammixis can only produce a reduction from
the survival mean to the birth mean. Selection, by
chminating such individuals as possess inferior parts,
makes the standard of excellence of the survival mean
considerably above the standard of birth mean. If
then, selection cease to operate, the birth mean stand-
ard will be again restored. Ory, to express it in figures,

“Animal Life and Intelligence, p. 1S9.

EVOLUTION OF THE COLORS OF BIRDS. 23

if the birth mean standard of an organ equal 3, and the
survival mean 5 when selection is operative, the standard
cannot possibly fall below 3 when selection ceases.

Mr. J. T. Cunningham criticises pammixis as follows:*
“The fallacy of this argument is so obvious that it is
surprising it should be for a moment accepted. For
what is stated of the maxima variations is equally true
of the minima. In the absence of all selection the
minima variations will be combined in sexual union
with variations superior to themselves, and therefore in
each successive generation the minimum will be raised.
Thus the only possible result of pammixis, on Weis-
mann’s theory of variation, will be the production of
uniformity in a disused or useless organ, and the de-
generation or disappearance of such an organ will be
absolutely impossible.”

In an article on ‘‘ The Factors of Organic Evolution,”’}
Prof. Geo. J. Romanes calls attention to the fact that he
had enunciated the principle of pammixis under the
name ‘‘ Cessation of Selection,” as early as 1873. He
had not claimed, however, that this cessation of selec-
tion could of itself produce the total disappearance of an
organ or part. As an instance of this, he supposes a
structure to have been raised from 0 to an average of
100, and then to have become wholly useless, so that
natural selection would be no longer operative in main-
taining the standard. Reversal of selection would then
set in, due to economy of growth, and variations 101,
102, 108, etc., would be eliminated, while variations 99,
98, 97, etc:, would be favored. To continue the explana-
tion in the writer’s own words: ‘‘For the sake of
definition, we shall neglect the influence of economy

* The New Darwinism, Westminster Review, July, 1891, p. 23.
+ Nature, XXXVI, Aug. 25, 1887, pp. 401-407.

24 CALIFORNIA ACADEMY OF SCIENCES.

acting below 100, and so isolate the effects due to the
mere withdrawal of selection. By the conditions of our
assumption, all variations above 100 are eliminated,
while below 100 indiscriminate variation is permitted.
Thus, the selective premium upon variation 99 being no
greater than upon 98, 98 would have as good a chance
of leaving offspring which would inherit and transmit
this variation as would 99; similarly, 97 would have as
good a chance as #8, and so on.” He then shows how
there would be a constant tendency toward reduction in
the part, but that the greater the reduction the less pos-
sibility of future reduction would remain. ‘‘ Thus,” he
says, ‘theoretically the average would continue to
diminish at a slower and slower rate, until it comes to
50, where the chances in favor of increase and diminu-
tion being equal, it would remain stationary.”

Prof. Romanes then gives examples of parts which he
thinks have degenerated through cessation of selection.
He considers the cases where the phylogenic stages are
omitted in the developing embryo to be instances in
point, and argues that such omissions cannot be ex-
plained by economy of growth, for in allied forms where
economy would be equally operative the structure per-
sists. Neither can the absence of such parts be due to
disuse, Prof. Romanes contends, for they were not gen-
erally produced by use. The case of hard coverings,
which are developed by natural selection as a protection
to certain unimals, and afterwards lost when their period
of usefulness is past, is also cited as an example of de-
ecneration without disuse

Pammnixis, then, is not new as a theory, nor is it
the only ayailable explanation of degeneration. Ro-
mancs himself suggested three alternatives. Besides
reversal of selection, he has stated two other factors, as
follows: “(The first of them is inheritance at carlier

EVOLUTION OF THE COLORS OF BIRDS. 25

periods of life, which progressively pushes back the de-
velopment of a rudiment to a more and more remote
embryonic stage of growth; and the second is the event-
ual failure of the principle of inheritance itself. For,
‘whether or not we believe in Pangenesis, we cannot but
deem it in the highest degree improbable that the influ-
ence of heredity is of unlimited duration.’”” The former
of these two factors is the same as Prof. Cope’s principle
of retardation, while the latter has also been independ-
ently stated by Cope, as will be seen in a subsequent
quotation. There are, then, five theories besides pam-
mixis by which degeneration of parts may be explained,
viz: (1) reversal of selection, (2) economy of energy, (3)
disuse, (4) retardation, and (5) the failure of heredity.
In some cases the loss of an organ or member is an ad-
vantage, and then natural selection may aid in elimina-
ting it. An instance of this is to be found in insects
inhabiting islands, which would be blown to sea and
perish if they possessed the power of flight, and in which
the wings are aborted or entirely absent. There are
many cases, however, in which there is no such direct
advantage to be discovered in the reduction of a part.
To some of these cases the principle of economy enun-
ciated by Darwin might well apply. This principle
assumes that the organism has a given amount of force
to expend, and that if one part be useless the growth
force which has been expended in maintaining it will be
diverted to some other channel. Thus birds, like the
ostrich, in which the wings are aborted, have legs pro-
portionately powerful. The third principle is disuse.
It is universally admitted that whenever a part is not
used during the lifetime of an individual it degenerates
to a certain extent. The principle of disuse merely
assumes that such acquired degeneration can be inher-
ited. Cope enunciated the fourth and fifth principles,

26 CALIVORNIA ACADEMY OF SCIENCES.

independently of Romanes.* When any part or the
whole of an individual is retarded in its growth, so that
the animal begins to breed before reaching maturity,
Cope holds that the descendents will be deficient in such
parts as were not fully developed in the parent. The
statement of the action of this law in producing degene-
ration of parts is as follows: ‘‘ ‘ Retardation’ continued
terminates in extinction. Examples of this result are

common; among the best known are those of the atrophy
of the organs of sight in animals inhabiting caves. *

I would suggest that the process of reduction illustrates
the law of ‘retardation’ accompanied by another phe-
nomenon. Where characters which appear latest in em-
bryonic history are lost, we have simple retardation—
that is, the animal in successive generations fails to
grow up to the highest point of completion, falling
farther and farther back, thus presenting an increasingly
slower growth in the special direction in question.
Where, as in the presence of eyes, we have a character
early assumed in embryonic life, retardation! presents a
somewhat different phase. Each successive generation,
it is true, fails to come up to the completeness of its pre-
decessor at maturity, and thus exhibits ‘retardation;’
but this process of reduction of rate of growth is followed
by its termination in the part long before growth has
ceased in other organs. This is an exaggeration of
retardation, and means the carly termination of the
process of force-conversion, which has been previously
diminishing steadily in activity.”

The subject of use and disuse need not be considered
in further detail. From the above it is evident that there
are many explanations of the phenomena of degenera-
tion and that pamminxis cannot be at best more than one
of several factors.

* Origin of the Fittest, p. 13.

EVOLUTION OF THE COLORS OF BIRDS. 27

Having disposed of the theoretical aspect of the ques-
tion of use and disuse and the inheritance of acquired
characters, a few words on the practical side of the case
may now be in order. In the first place it will be advis-
able to enquire what an acceptance of the views of
Weismann involves. Some of the consequences are
well shown by Mr. J. T. Cunningham, in his introduction
to the English translation of Eimer’s Organic Evolution.
He calls attention to the abnormally lengthened tongue
of the woodpecker, which can be greatly protruded and
thrust into holes to extract insects. The lengthened
tongue, the Neo-Darwinians claim, has been produced
solely by the selection of those individuals in which it
was longest. They cannot but admit with the Lamarck-
ians, however, that constant exercise of the tongue in
the individual, especially the constant stretching to
which it would be subjected in the effort to reach far-
ther, would increase its length; but in admitting this
they have involved themselves in the paradox of assum-
ing that the tongue has become lengthened during the
course of ages, and that it has also been lengthened in
the individual by the Lamarckian factor of use and dis-
use, but that the lengthening which has occurred in the
race is in no wise related to the lengthening that has
taken place in the individual. ‘‘ Which is very like the
argument,” says Cunningham, ‘‘ that the /liad and the
Odyssey were not written by Homer, but by another man
of the same name who lived at the same time.”

Another difficulty in the way of the Neo-Darwinian
argument to which Cunningham, among others, has
called attention, is its inability to account for the origin
of totally new characters. Even though it may be able
to account for the lengthened neck of the giraffe by
selection, it is impossible, Cunningham argues, to ex-
plain the origin of horns by this principle. From what

28 CALIFORNIA ACADEMY OF SCIENCES.

is the selection to be made? ‘‘ No other mammals,” he
says, have ever been stated to possess two little symmet-
rical excrescences on their frontal bones as an occasional
variation; what then caused such excrescences to appear
in the ancestors of horned ruminants? Butting with
the forehead would produce them, and no other cause
can be suggested which would.”

An invonsisteney which has been pointed out by
Romances, Osborn and Le Conte, ix the fact that the Neo-
Darwinians admit the Lamarckian factors among proto-
zoa. Romanes has pointed this out with especial clear-
ness in his article entitled ‘ Weismann’s Theory of
Heredity ”’* He calls attention to the fact that inas-
much as natural selection is unavailing without varia-
tion, aud that variation, according to Weismann’s view
is due to the sexual admixture of different traits, there
can be no individual variation among unicellular and
parthenogenetic organisms, and hence natural selec-
tion cunnot be a factor in producing new forms. Weis-
mann, indeed, sees this to be the case and admits that
modifications in such animals must be due solely to the
direct action of the environment. Two objections have
here been interposed. Prof. Osborn asks why, if the
direct action of the environment was once a factor of
evolution, as Weismann adinits, it should ever have
ceased to be such if its period of usefulness did not ter-
minate.t Now it is apparent that the period of useful-
ness of the Lamarckian factors does not terminate with
the protozoa, and couscquently natural selection itself
would have tended to preserve them. Prof. Romanes’
suggestion was not stated in the form of an objection,
although such is clearly imphed. It is in brief as fol-

Contemporary Review, May, 1890, pp. 686-699.

tAmevican Naturalist, xxiii, p.

EVOLUTION OF THE COLORS OF BIRDS. 29

lows: The germ-plasm of Prof. Weismann is a highly
stable substance unaffected from within or without.
Variations occur by sexual admixture, but as there is an
immortal chain of germ-plasm all variations may be
ultimately referred back to the unicellular organism, and
hence all variations must have heen due originally to
the direct actions of the environment in producing
changes in the protozoa.

In considering the observed cases of transmission of
acquired characters, the subject of the supposed trans-
mission of mutilations may be first discussed. As is well
known, many cases of such transmission are on record,
but Weismann has shown that a large number of these
are untrustworthy. Undue significance however, has
been attached to the validity of such cases. In his in-
troduction to the English translation of Eimer’s Organic
Evolution, Mr. J. T. Cunningham has made a sugges-
tion, the importance of which has been generally over-
looked. He says: ‘‘The fact that artificial malforma-
tions are not usually inherited is no argument against
the inheritance of acquired characters. In all animals,
from the lowest up to reptiles, recrescence of lost parts
takes place, and the reappearance of lost parts in the
next generation in mammals and birds seems to me to
be simply recrescence slightly postponed.”

Lloyd Morgan, speaking of the evidence of the inherit-
ance of acquired characters, says:* ‘‘Attempts have been
made to furnish such evidence by showing that certain
mutilations have been inherited. I question whether
many of these cases will withstand rigid criticism. Nor
do I think that mutilations are likely to afford the right
sort of evidence one way or the other. We must look to
less abnormal influences. What we require is evidence

*Animal Life and Intelligence, p. 163.

30 CALIFORNIA ACADEMY OF SCIENCES.

in favor of or against the supposition that mudéfcautions
of the body-cells are transmitted to the germ-cells.
Now these modifications must clearly be of such a nature
as to be receivable by the cells without in any way de-
stroying their integrity. The destruction or removal of
cells is something very different from this. If it were
proved that mutilations are inherited, this would not
necessarily show that normal cell-modifications are trans-
missible. And if the evidence in favor of inherited
mutilations breaks down, as I believe it does, this does
not show that more normal modifications such as those
with which we are familiar, as occurring in the course
of individual life, are not capable of transmission.”

Weismann has devoted some attention to the reported
cases of the inheritence of mutilations, and has reached
the conclusion that all the published instances are either
untrustworthy or of such a character that they do not
conclusively prove that a case of mutilation has ever
been inherited.

In commenting upon his explanation of reputed cases
of the transmission of rudimentary tails, he says:* ‘We
have seen that the rudimentary tails of cats and dogs,
as far as they can be submitted to scientific investiga-
tion, do not depend upon the transmission of artificial
mutilation, but upon the spontaneous appearance of de-
generation in the vertebral volumn of the tail. The
opinion may, however, be still held that the customary
artificial mutilation of the tail, in many races of dogs
and cats, has at least produced a number of rudimentary
tails, although, perhaps, not all of them. It might be
maintained that the fact of the spontaneous appearance
of rudimentary tails docs not disprove the supposition
that the character may also depend upon the transmis-
sion of artificial mutilation.

* Essays on Here lity, 1891, I, p. 443.

EVOLUTION OF THE COLORS OF BIRDS. 31

‘‘Obviously, such a question can only be decided by
experiments; not, of course, experiments upon dogs and
cats, as Bonnet rightly remarks, but experiments upon
animals the tails of which are not already in a process
of reduction. Bonnet proposes that the question should
be investigated in white rats or mice, in which the
length of the tail is very uniform, and the occurence of
rudimentary tails is unknown.”

Weismann accordingly performed these experiments,
and there can be no doubt that they have been done in
a thorough and scientific manner. The result has been
purely negative, mice of the fifth generation bearing
young with the tails of normal length. Weismann, in-
deed, admits that these experiments do not constitute a
complete disproof of a possibility of transmitting mutila-
tions, but justly claims that, in comparison with the
cases of such supposed transmission occurring com-
pletely in a single generation, and where but one parent
was affected, the possibilities of transmissions were in-
finitely greater in his experiment. If, however, the
suggestion of Cunningham, that the non-inheritance of
mutilation in higher animals is comparable with regen-
eration of lost parts among lower forms, but delayed to
the following generation, we can easily understand that
while mutilations might be inherited, the contrary
would be the rule, and such inheritance would be due
to an abnormal condition of the organism. Cunning-
ham, furthermore, not unjustly, insinuates that even
the cautious Prof. Weismann may have been betrayed
into asserting a little more than he knew in order to
prove his point. For example, he says:* ‘‘ Prof. Weis-
mann mentions the feet of Chinese ladies, which he
says are still, when uncompressed, as large as if the

* Organic Evolution, Translator’s Preface, p. x.

32 CALIFORNIA ACADEMY OF SCIENCES.

practice of artificially compressing them had not been
practiced for centuries. But he does not tell us whether
he ever saw a Chinese young lady, or if he has made
any observations on the feet of Chinese young women.”
Eimer has mentioned a number of cases of supposed
mutilation which I have not seen refuted. In arguing
for the inheritance of mutilations, he says:* ‘‘ That
injuries, when continued for an extremely long time,
may be inherited is proved, to my mind, by atrophied
(rudimentary) organs. The degeneration of these organs
depends incontestably on disuse; in consequence of dis-
use the blood-supply is diminished, in consequence of
the decrease of nutrition degeneration takes place. If
we consider the course of gradual degeneration, e. g. of
the tail as it must have taken place in the higher mam-
mals, to have proceeded in this purely physiological
manner from the tip toward the root, the process is
much the same as if the tip of the tail had been in
many successive generations amputated, and the short-
ening had been inherited and then the shorter tail thus
acquired had been farther shortened artificially, and so
on. In any case, in the degeneration of the tail an
acquired character has been inherited by the offspring,
a character which, in the causes of its origin, is closely
similar to a perpetually repeated mutilation. Great
periods of time, however, have been necessary in this
case for the accomplishment of a final result.”

This example of Prof. Eimer’s seems, however, to be
an assumption of the validity of the point in dispute.
His opponents could, of course, claim that the reduction
in the length of the tail was due solely to the selection
of the shortest. His examples of observed cases of in-
heritance of mutilations, however, do not appear to be

* Organic Evolution, Eng. translation, p. 176.

EVOLUTION OF THE COLORS OF BIRDS. 33

susceptible of the criticism of a misinterpretation of the
facts. Two only need here be given.* ‘A. Decandolle
describes one such case with the assurance that it is per-
fectly true. In the year 1797 a girl twenty-one years
old was thrown from a carriage, and in consequence had

_a scar about five centimeters wide over the left ear and
temple which remained without hair. Married in 1799,
she bore a son in 1800, in whom the hair was absent
from the same area and remained so. The son of the
man, born in 1836, had no such defect, but it was pres-
ent in his grandson born in 1866, and in 1884 in this
last individual when he was eighteen years old the pecu-
liarity was disappearing.

‘* Dr. Meissen of Falkenberg, records in the number
of Humboldt for June, 1887, the following case of in-
heritance of an injury in his own family: ‘When I was
seven or eight years old I had the chicken-pox, and I
recollect with complete distinctness that I scratched one
of the pustules on the right temple in consequence of
which I had a small white scar at this spot. Exactly
the same scar, which I had of course ceased to think of,
on exactly the same spot, was present on my little son,
now fifteen months old, when he came into the world.
The resemblance is so perfect that it surprises everyone
who sees the little mark.’ ”’

It is unnecessary to multiply examples of reported
cases of the inheritance of mutilations. A single trust-
worthy instance is sufficient to offset an indefinite
amount of evidence to the contrary; more especially
when the Neo-Lamarckians themselves assert that
‘such inheritance, if it occurs at all, is abnormal.
The explanation given by Weismann of the inheritance
of epilepsy in guinea pigs as described in the experi-

* Ti Cis ps HPF:
3

34 CALIFORNIA ACADEMY OF SCIENCES.

ments of Brown-Séquard and Obersteiner appears forced
and inexcusably far-fetched, but it will be unnecessary
to dwell longer upon such cases. Suffice it to say, that
epilepsy superinduced by severing certain peripheral
nerves has been transmitted to the following generation,
but that Weismann evades the experiment, first by sug-
gesting that it was merely the predisposition to the dis-
ease—due to a general derangement of the nervous sys-
tem that was inherited, and secondly suggesting that
the disease was transmitted by microbes penetrating the
reproductive cells.

An almost endless number of instances have been re-
ported of the transmission of acquired characters, while
the Neo-Darwinians have replied to many of them, ex-
plaining the facts after their own fashion. It will be
obviously impossible to give all of the cases which have
been brought to light, but a few representative ones may
be detailed. The evidence divides itself into two classes,
viz: observed instances of transformation which appear
to be due to the direct action of the environment; and,
second, structures, functions, or traits which could only
have resulted from the inheritance of acquired charac-
ters.

In the first category may be mentioned the case of
the Japanese goldfish of Dr. Wahl, which Prof. Ryder
records,* which, by close confinement and abundant
food, were greatly modified, ‘‘ enormous and abnormally
lengthened pectoral, ventral, dorsal, double and caudal
fins’ being developed. ‘‘Some of the races of these fishes
have obviously been affected in appearance by abundant
feeding,” he states, ‘‘as is attested by their short, almost
globular bodies, protuberent abdomens and greedy hab-
its,’ etc. Lloyd Morgan gives some interesting instances

“Am, Nat., vol. 24, page 84.

EVOLUTION OF THE COLORS OF BIRDS. 35

of observed cases of the direct action of the environment
in affecting a race. One of the finest is the transforma-
tion of the brine-shrimp, Artemia, reported by Schman-
kewitsch: ‘‘One species of this crustacean, Artemia sa-
lina,” says Lloyd Morgan, ‘‘ lives in brackish water,
while A. milhausenii inhabits water which is much salter.
They have always been regarded as distinct species, dif-
fering in the form of the tail lobes and the character of
the spines they bear. And yet, by gradually altering
the saltness of the water, either of them was transformed
into the other in the course of a few generations. So
long as the altered conditions remained the same the
change of form was maintained.”

What is to be said of such cases? Do they constitute
a proof positive of the inheritance of acquired characters?
At first sight they certainly do appear to be conclusive
and final, but a candid examination of them compels us
to admit of the possibility of a different interpretation.
The two examples mentioned are fairly representative
of a large number of similar instances. The whole diffi-
culty with them lies in the fact that when the organism
is restored to its former conditions the change does not
persist. Does the son of a blacksmith inherit the strong
arms of his father? Probably he does, but this is not
equivalent to asserting that he inherits the strength
acquired by his father in following his vocation. It is
not improbable that his father became a blacksmith be-
cause he was naturally strong or was predisposed to
develop unusual strength. We must first in every case
assure ourselves that the predisposition does not exist in
the animal and only requires a favorable environment
to be developed. Secondly, it must be demonstrated
that there is a progressive change from one generation
to the next. This there apparently was in the case of
the goldfish, for a period of six years was required for

36 CALIFORNIA ACADEMY OF SCIENCES.

the production of some of the forms. But even in cases
where the change lasts only us long as the altered envi-
ronment is operative, although the permanent transmis-
sion of acquired characters is not demonstrated, it is
rendered highly probable. Cunningham gives an excel-
lent analogy illustrative of this. He says:* ‘Ifa plant
with a vertical stem is placed in a horizontal position,
the ight coming from above, the end of the stem will
bend up toward the light, partly by growth, partly by
flexure. Sucha plant wasso placed, and, after a certain
time, when the upward flexure was established, it was
turned round so that the tip pointed downwards. Of
course the flexure was gradually reversed until the tip
pointed upwards again. After the same interval the
plant was reversed once more. This was continued for
some days, the plant being reversed at regular intervals.
At last, when the time came for turning the plant round,
the operation was not performed, it was left undisturbed.
But then the plant began to reverse its flexure of its own
accord, and actually turned its tip downwards, away
from the light. By the regularly repeated reversal of
position a rhythm had been set up in the life of the
plant, and even when the cause which excited this
rhythm ceased, the rhythm continued.”

Weismann himself gives some instances of these
‘after-effects 7 in plants, such as the case of the sun-
flower, which is as follows: “ If vigorous plants of the
sunflower, grown in the open air, be cut off close to the
ground and transferred to complete darkness, the exam-
ination of a tube fixed to the cut surface of the stem
will show that the escape of sap does not take place uni-
formly, but undergoes periodical fluctuation, being
stronyest inthe afternoon and weakest in the early

“The New Darwinism, Westminster Review, July, 185], p. 416.
t Essays upon Heredity, 189], I. p. 416.

EVOLUTION OF THE COLORS OF BIRDs. 37

morning. Now the cause of this daily periodicity in the
flow of sap depends upon the periodical changes due to
the light to which the plant was exposed when it was
growing under normal conditions. When plants which
have been grown in darkness from the first are similarly
treated, the flow of sap does not exhibit any such peri-
odicity.” In commenting on these instances, Weis-
mann says: ‘‘ All this is certainly very interesting, and
it proves that periodical stimuli produce periodical pro-
cesses in the plant, which are not immediately arrested
when the stimulus is withdrawn,and only become uniform
gradually andafter the lapse of a considerable time. But
I certainly claim the right to ask what connection there is
between these facts and the transmission of acquired
characters? All these peculiarities produced by exter-
nal influences remain restricted to the individual in
which they arose; most of them disappear comparatively
soon, and long before the death of the individual.”

From the theoretical standpoint of Prof. Weismann
this is doubtless correct. But it has been shown, I think,
that Prof. Weismann’s theory of the continuity of germ-
plasm is not only unproven but highly improbable.
Moreover, it has been shown that the continuity of the
body plasm is highly probable. But if this be true,
then there is, to say the least, a strong analogy between a
rhythm which can be established in the somatoplasm
and repeated during the life of the individual and a
rhythm which can be established in the history of a
race.

Cunningham has well pointed out, however, that the
Neo-Lamarckians do not claim that a change due to the
environment can be established in a single generation.
All that is claimed is, that when a particular environ-
mental influence is continued from generation to gen-
eration, the offspring successively inherit a progressive

38 CALIFORNIA ACADEMY OF SCIENCES.

predisposition to be modified, until, in the course of
time, the modification takes place, when the cause is no
longer operative, as in the case of the after effects just
considered.

Mr. D. G. Elliot, in a recent address on the inheritance
of acquired characters delivered before the American
Ornithologists’ Union,* relates an instance of the ob-
served inheritance of an acquired habit in birds as fol-
lows: ‘‘Currituck Sound, in North Carolina, where wild
fowl are accustomed to pass the greater portion of the
winter, is a great resort of sportsmen, who pursue the
birds in every way to accomplish their destruction.
This, at length, was carried to such a degree that the
fowl] had no place left for them to rest during the day.
Some years ago the gunners were surprised to find that
whenever the weather permitted, as soon as a gun was
fired in the early morning the birds would rise and _ be-
take themselves to the ocean, and remain congregated
on the water just beyond the line of the breakers, and
would not return until night closed in. This custom
was acquired by birds of succeeding years, until the
habit has become apparently established. Now it may
be said that this is not an aeguéred habit, but the result
of example, the old birds leading the young to the sea.
But this would be to assume that the majority of the
birds which commenced this habit had survived to re-
turn to this locality every winter. And even if the
young, without at first comprehending the reason for so
strange a proceeding, merely followed the old birds, is it
reasonable to suppose they would remain in such an
unusual locality throughout the day, deprived of their
food, which could be obtained in profusion on the other
side of the narrow beach? It must have been something

* Auk, ix, Jan. 1892, pp. 77-104.

EVOLUTION OF THE COLORS OF BIRDS. 39

more powerful than the mere example of the flight of
the old birds to the ocean, witnessed by the young for
the first time, which compelled them to remain. Can
we not more reasonably presume that it was the knowl-
edge acquired by the parents that this was a secure
method to escape from a threatened danger, and trans-
mitted to the young, who assumed the habit as a part cf
their nature?”

Mr. Elliot then mentions numerous cases of the
change of nesting habits in birds. He alludes to the
instance reported by Coues in Birds of the Northwest,
of the geese of the Yellowstone, which build in trees in-
stead of on the ground, which is the usual habit of these
birds. He furthermore calls attention to the case noted
by Audubon of the change in nesting habits of the her-
ring gulls on White Head Island, in the Bay of Fundy,
which during the lifetime of a single man had
deserted the ground in favor of trees owing to the perse-
cution to which they had been subjected. ‘‘A remark-
able effect of this transmission of an acquired charac-
ter,” says Mr. Elliot, ‘‘ is that the young hatched in the
trees do not leave the nest until they are able to fly,
while those hatched in nests on the ground run about
in less than a week and conceal themselves at the sight
of man among the moss and plants.”

But cannot these instances of Mr. Elliot’s be otherwise
interpreted? JI think they can. It is impossible to
prove the inheritance of acquired habits by citing ex-
amples which could best be thus explained, but which
might be otherwise accounted for. In the first instance
mentioned, the majority of the birds which commenced
the habit mght have survived. Or, a smaller number
might have been sufficient if we admit that birds have
sufficient language to give warning of danger. More-
over, Mr. Elliot’s argument that the inducement of ob-

40 CALIFORNIA ACADEMY OF SCIENCES.

taining food would be so strong as to compel the young
birds to ignore the example of their elders really reflects
upon his own assumption, for it seems difficult to con-
ceive that a new adjustment of such force as this instinct
is supposed to be, could be inherited in the course of a
few generations. The argument derived from the change
in nesting habits appears to have still less force. Mr.
Elliot assumes that nest building is an instinct, but this
assumption is unproven and has been emphatically
doubted by Wallace. In his Philosophy of Birds’ Nests,*
he says: ‘‘ At all events, till the crucial experiment is
made, and a pair of wild birds, raised from the egg with-
out ever seeing a nest, are shown to be capable of mak-
ing one exactly of the parental tvpe, I do not think we
are justified in calling in the aid of an unknown and
mysterious faculty to do that which is so strictly analog-
ous to the house-building of savage men.”

Darwin has given many instances of the inheritance
of acquired habits which, however, have been skillfully
combated by William Platt Ball.f He cites, for ex-
ample, the inheritance by a colt of the paces of her
mother, but suggests that “selection of the constitu-
tional tendency to these paces, and imitation of the
mother by the colt, may have been the real causes.”’ He
calls attention to the fact that the songs of birds are not
inherited, but are learned from their parents, and says:
“Tf use-inheritance las not fixed the song of birds, why
should we suppose that in a single generation it has
transmitted a newly-taught method of walking or trot-
ting.” He speaks of the supposed inheritance by dogs
of the intelligence acquired by contact with man, which
he explains thus: ‘‘ But selection and imitation are so

“Natural Selection, pp. 108-109.
+ Ave the Effects of Use and Disuse Inherited?) Humboldt Library, pp

31-33.

EVOLUTION OF THE COLORS OF BIRDS. 41

potent, that the additional hypothesis of use-inheritance
seems perfectly superfluous. Where intelligence is not
highly valued and carefully promoted by selection, the
intelligence derivable from association with man does
not appear to be inherited. Lap-dogs, for instance, are
often remarkably stupid.” It seems to me that Ball
does not establish his point in this instance. To be
sure, it might be claimed that thoroughbred dogs had
attained their intelligence through selection alone
{although this I should be inclined to question), but
such dogs are generally, if not universally, bred with
one especial end in view, either speed, hunting qual-
ities, fighting qualities, beauty or eccentricty; but how
often are they bred for intelligence? Moreover the
most intelligent dogs are not infrequently curs. A large
number of the most remarkable stories of canine sagac-
ity are told of animals without a pedigree. But these
dogs have not been selected at all, for the most part.
What is the fate of a large litter of puppies of a cur? A
part of them are generally destroyed in early infancy,
and this in a manner practically impartial so far as in-
telligence is concerned. The rest are generally given
away, but what evidence have we that the less intelli-
gent of them are killed by their new masters, while the
more intelligent survive to perpetuate the race?
Romanes has called attention* to the inheritance of
an instinct in dogs which he considers especially invul-
nerable in support of the inheritance of acquired habit.
For an instinct to have been established solely by nat-
ural selection, it must be of sufficient importance to be
essential to the life of the race, so that those individ-
uals possessing it may alone survive. This is not the

*The Factors of Organic Evolution. Nature. August 25, 1887-XXXYVI,
p. 406.

42 CALIFORNIA ACADEMY OF SCIENCES.

case with the instinct which dogs possess of turning
around several times before lying down, in order to
trample down a bed, a relic of primitive comfort which
surely could not have become established by the survival
of the fittest. ‘ Or,” says Romanes, ‘‘if this instance
be held doubtful, what shall we say to the courting in-
stincts in general, and to the play-instincts of the bower-
bird in particular, which are surely quite without mean-
ing from any utilitarian point of view? And these in-
stincts naturally lead to the wsthetic faculties of mankind,
few of which can be possibly ascribed to natural selec-
tion, as Mr. Spencer very conclusively shows.”
Weismann and Ball have both combated this, as, in-
deed, they are bound to do to be consistent with their
theory. Thus Ball says: ‘‘ The emotional susceptibility
to music and the delicate perceptions needed for the
higher branches of art, were apparently the work of
natural and sexual selections in the long past. Civiliza-
tion, with its leisure and wealth and accumulated knowl-
edge, perfects human faculties by artificial cultivation,
develops and combines means of enjoyment and dis-
covers unsuspected sources of interest and pleasure.
a * But modern wsthetic advance seems to he
almost entirely due to the culture of latent abilities, the
formation of complex associations, the selection and
encouragement of talent, and the wide diffusion and
imitation of the accumulated products of the well-culti-
vated genius of favorably varying individuals. The
fact that uneducated persons do not enjoy the higher
tastes, and the rapidity with which such tastes are ac-
quired or professed ought to be sufficient proof that
modern culture is brought about by far swifter and more
potent influences than use-inheritance.”” What has Mr.
Ball shown in the above paragraph? If he has proved
anything it certainly is that natural selection has not

EVOLUTION OF THE COLORS OF BIRDS. 43

originated, even if it has been instrumental in the de-
velopment of the wsthetic faculties. In speaking of the
culture of latent abilities he has yielded a most import-
ant point. There was obviously a time in the evolution
of organic life when piano playing, or, in more general
terms, appreciation of harmony, was not even a latent
faculty. It would be taxing our credulity to assume, for
example, that amceba possessed it. Then it must, at
some time, have come into existence as a latent faculty,
and later on been developed by use, or culture, as Ball
calls it; and this developed faculty has been inherited.

Spencer published in the Vineteenth Century for April
and May, 1886, two essays, which have since appeared
in a separate form, entitled ‘‘The Factors of Organic
Evolution.” ‘‘ Among the most important criticisms of
this work is the one by Romanes in an article bearing
the same title, which appeared in Vutwre August 25,
1887; and Ball’s criticism in his pamphlet, ‘‘ Are the
Effects of Use and Disuse Inherited?” Spencer gives
three forms of evidence in proof of the inheritance of
acquired characters—(1) the crowding of teeth in dogs
and reduced size of the jaw in civilized man; (2) the
correlation of different parts of the organisin, and (3)
the apparent direct influence of the environment in
altering the surface of an organism.

Romanes and Ball agree that Spencer has failed to
prove his first point. Romanes says, in regard to this:
‘Be it observed, I am not disputing that disuse may in
both these cases have co-operated with the cessation of
selection in bringing about the observed result. In-
deed, I am rather disposed to allow that the large amount
of reduction described in the case of the dogs as having
taken place in so comparatively short a time, is strongly
suggestive of disuse having co-operated with the cessa-
tion of selection. But at present I am merely pointing

44 CALIFORNIA ACADEMY OF SCIENCES.

out that Mr. Spencer’s investigations have here failed to
exhibit the crucial proof of disuse as a reducing cause,
which he assigns to them; it is not true that in this case
disuse ‘remains as the only conceivable cause.’ ’’*

The third argument, as Romanes asserts, is too theo-
retical to be considered as a proof, but the second, in
regard to the correlation of parts of the organism is of
great importance, and, according to the opinion of
Romanes, ‘‘ virtually proves the truth of the Lamarckian
assuinption.”

So important is this particular case that it is worthy
of a somewhat extended consideration. Spencer takes
the giraffe as illustrative of his point. He calls atten-
tion to a statement of Darwin’s that ‘the prolonged
use of all the parts together with inheritance will have
aided in an important manner in their co-ordination.”
“A remark,” observes Spencer, ‘‘ probably having
reference chiefly to the increased massiveness of the
lower part of the neck; the increased size and strength
of the thorax required to bear the additional burden,
and the increased strength of the fore legs required to
carry the greater weight of both. But now I think that
further consideration suggests the belief that the en-
tailed modifications are much more numerous and re-
mote than at first appears; and that the greater part of
these are such us cannot be ascribed in any degree to the
selection of favorable variations but must be ascribed
exclusively to the inherited effects of changed func-
tious.” Mr. Spencer then describes the mechanism of
locomotion in the giraffe, the short hind limbs which
must keep pace with the long fore limbs, and the con-
sequent complex series of changes of bones, muscles and
nerves which must have taken place in order to bring

loc. p. 405.

EVOLUTION OF THE COLORS OF BIRDS. 45

about this result. This might very easily be explained,
indeed, by the effects of inherited use and disuse. ‘‘ If
the effects of use and disuse of parts are inheritable,”
says Mr. Spencer, ‘‘then any change in the fore parts
of the giraffe which affects the action of the hind limbs
and back will simultaneously cause, by the greater or
less exercise of it, a remoulding of each component in
the hind limbs and back in a way adapted to the new
demands; and generation after generation the entire
structure of the hind quarters will be progressively fitted
to the changed structure of the fore quarters, all the ap-
pliances for nutrition and innervation being at the same
time progressively fitted to both.” But the factors of
use and disuse aside, we must assume that all of these
complex changes occurred simultaneously. It might be
contended that slight variations in one direction which
were advantageous might take place in one generation,
and the correlative changes in other parts at some future
time. In reply to this Mr. Spencer says: ‘‘ Besides the
fact that until this secondary variation occurred the
primary variation would be a disadvantage, often fatal,
and besides the fact thai before such an appropriate
secondary variation might be expected in the course of
generations to occur, the primary variations would have
died out; there is the fact that the appropriate variation
of one bone or muscle in the hind quarters would be
useless without appropriate variations in all the rest—
some in this way and some in that—a number of appro-
priate variations which it is impossible to suppose.”
Mr. Ball either cannot or will not see the force of
Spencer’s objections. He says: ‘‘ All that is needed is
that natural selection should preserve the tallest giraffe
through times of famine by their being able to reach
otherwise inaccessible stores of foliage. The continual
variability of all parts of the higher animals gives scope

46 CALIFORNIA ACADEMY OF SCIENCES.

for innumerable changes and nature is not in a hurry.
Mr. Spencer, however, says that the chances against any
adequate readjustments fortuitously arising must be in-
finity to one. But he has also shown that altered de-
gree of use does not cause the needed concomitant
variation of co-operative parts. So the chances against
a beneficial change in an animal must be, at a liberal
estimate, infinity to two. Mr. Spencer, if he has proved
anything, has proved that it is practically impossible
that the giraffe can have acquired a long neck, or the
elk its huge horns, or that any species has ever acquired
any important modification.”

Mr. Ball then draws attention to the facts which Wal-
lace has adduced in his recent work, ‘‘ Darwinism,”
proving that constant and independent variation is the
rule among all animals and plants. He then says:
‘The lengthened wing might be gained in one genera-
tion, and the strengthened muscle at a subsequent pe-
riod; the bird in the meanwhile drawing upon its surplus
energy, aided (as I would suggest) by the strengthening
effect of increased use in the individual.” This expla-
nation is open to two objections: First, the one already
raised by Mr.Spencer, that before the second correlative
variation appeared the first would be lost; and, second,
the suggestion of Cunningham in regard to assuming
that use could develop the character required, but that
the individual thus favored could not transmit the vari-
ation, but that posterity must wait for the same vari-
ation to arise spontaneously. This hypothesis is so forced,
illogical and absurd, that so long as a better one can be
found it should be adopted.

But one more class of evidence need be discussed, that
furnished by paleontology. Prof. Henry F. Osborn has
called special attention to this subject on two different

lad

EVOLUTION OF THE COLORS OF BIRDS. 47

occasions.” He says: ‘‘ The evidence is of a direct and
indirect character. The direct evidence is that by actual
observation in complete paleontological series, the origin
of adaptive structures is found to conform strictly to the
lines of use and disuse. The indirect proof is that the
natural selection of chance variations is unsupported by
observation and is inadequate to explain the various
phenomena of the second class.” Special attention is
drawn to the evolution of teeth, in which every grada-
tion may be traced from the simple conical reptilian
tooth to the highly complex molars of some mammals.
Osborn has enunciated the two following laws of cusp
growth:

“‘(1) The primary cusps first appear as cuspules, or
minute cones, at the first points of contact between the
upper and lower molars in the vertical motions of the
jaw. :

(2) The modeling of cusps into new forms, and the
acquisition of secondary position, is a concomitant of
interference in the horizontal motions of the jaws.”

From the above laws it is evident that the variations
in the race are the same as the variations in the indi-
viduals, caused by the use and disuse of parts, and a
causal connection between the two is inferred. This
proof of the inheritance of the characters of use and dis-
use has been criticised by Poulton. In a foot-note to
Weismann’s Essays upon Heredity,t he says: ‘‘ One of
the most remarkable forms of this revival of Lamarck-
ism is the establishment in America of a ‘ Neo-Lamarck-
ian School,’ which includes among its members many
of the most distinguished American biologists. One of
the arguments upon which the founders of the school

t Page 437.

48 CALIFORNIA ACADEMY OF SCIENCES.

have chiefly relied is derived from the comparative mor-
phology of mammalian teeth. The evolution of the
various types are believed to be due to modifications in
shape, produced by the action of mechanical forces
(pressure and friction) during the life of the individual.
The accumulation of such modifications by means of
heredity explains the forms of existing teeth.

“Tt may be reasonably objected that the most element-
ary facts concerning the development of teeth prove that
their shapes cannot be altered during the lifetime of the
individual, except by being worn away. The shape is
predetermined before the tooth hascutthe gum. Hence
the Neo-Lamarckian School assumes, not the transmis-
sion of acquired characters, but the transmission of
characters which the parent is unable to acquire!”

In replying to the criticism of Mr. Poulton, Osborn
says: ‘To the objection that the teeth are entirely
formed before piercing the gum, and that use produces
an actual loss of tissue as contrasted with the growth of
bone, it may be said that by our theory, it 1s not the
growth itself but the reactions which produce this growth
in the living tissue, which we suppose to be transmitted.”
Osborn also criticises Weismann’s theory as follows:
‘‘In Weismann’s variation theory the preponderating
influence must be conservative; however it may explain
progressive modification, or even correlation of old char-
acters, it does not admit that the genesis of new char-
acters should follow definite lines of adaptation which
are not pre-existent in the germ-plasma. We find that
new characters of the second class do follow such pur-
posive or directive lines, arising simultaneously in all
parts of the organism, and first appearing in such min-
ute form that we have no reason to suppose that they
can be acted upon by selection. The old view of nature’s
choice between two single characters, one adaptive, the

EVOLUTION OF THE COLORS OF BIRDS. 49

other not adaptive, must be abandoned, since the latter
does not exist in the second class.”

I have attempted to present a fair statement of both
sides of the case in this controversy in regard to the in-
heritance of acquired characters, and to all of the import-
ant arguments which have been adduced both pro and
con, illustrated by typicalexamples. It will be remembered
that the scepticism in regard to the possibility of ac-
quired characters being transmitted arose from the
theory of heredity eflunciated by Prof. Weismann. A
study of the different theories of heredity disclosed the
fact that while Darwin’s hypothesis of pangenesis, or
any subsequent modification of it, was a merely formal
and provisional scheme for explaining the supposed facts
of heredity, Weismann’s theory of germ-plasm appears
to be a speculative deduction from real facts which the
facts themselves do not warrant. That consequently,
although this theory may be true, it appears rather vis-
ionary and certainly should not be unconditionally
accepted as true or even as a working hypothesis, unless
the facts dependent upon it can be thus best explained.
The practice of ‘stretching facts to fit into a theory is a
habit which cannot be too strongly condemned.

If Weismann’s theory of germ-plasm is unproven
there is no apparent reason why acquired characters
may not be inherited. An examination of the testimony
on this point seems to indicate that both sides have cited
cases which did not prove their point. Many so-called
proofs which have been brought forward by the Neo-
Lamarckians in reality prove nothing, but, on the other
hand, some of the interpretations of the Neo-Darwin-
ians appear forced and illogical.

It is obvious that a single established case of inherit-
ance of an acquired character would be sufficient to prove
the principle, however many other cases might be dif-

4

50 CALIFORNIA ACADEMY OF SCIENCES.

ferently explained. And it seems to me that some
fairly well established, if not absolutely convincing,
cases have been adduced. Furthermore, the Neo-Dar-
winians maintain the illogical assumption that the
changes which are observed in the individual have no
possible causal connection with the changes which take
place in the race, this assumption being contradictory
to the law of correspondence of the ontogenic and phylo-
genic series. Lastly, it is impossible to explain, from
the Neo-Darwinian point of view, simultaneous varia-
tions of an adaptive nature.

From all this we may come to a provisional conclu-
sion that acquired characters are trausmissible. We
are justified in using this assumption as a working
hypothesis, and in feeling confident that future investi-
gation will place it upon a footing where it is beyond
the possibility of refutation.

‘

VARIATION AND NATURAL SELECTION.

The end of science is the establishment of natural
law, which is merely the orderly relationship existing
between phenomena, and consequently cannot be con-
sidered as an ultimate cause. Thus gravitation is merely
a name for the observed relation between bodies, but
does not in any way tell us why these relations exist.
Evolution is a term expressive of change in form, and
modern biology is striving to determine the precise laws
conditioning such changes as occur in organic beings.
There are two sets of changes—ontogenic and phy-
logenic. It has become the fashion of late years
among certain scientists to attribute changes of the
second class solely to the action of natural selection,
and it thus becomes necessary in inquiring into the
laws of evolution to consider first this principle, and de-
termine what it can effect.

EVOLUTION OF THE COLORS OF BIRDS. 51

It will be necessary to understand clearly the scope
and meaning of the term natural selection, or Darwin-
ism, as it is frequently called in recognition of its enun-
ciator, before inquiring into what it can accomplish.
Darwin himself used the term in two different senses,
more narrowly as synonymous with Spencer’s term sur-
vival of the fittest and in the broader interpretation of
the cause of modifications including the facts of varia-
tion. In this latter application, however, it is inac-
curate and misleading. Indeed, the process, isasarule,
not one of selection, as Lloyd Morgan has pointed out,*
but rather of rejection, in which case natural elimina-
tion becomes a more correct term. But how can any
change be brought about in a species by this process?
Prof. J. G. Schurman in a chapter on tae Metaphysics
of Darwinismt writes as follows of natural selection:
‘‘There have been objections to the theory, especially
to the somewhat startling assumption that the results of
man’s purposive selection in breeding could be attained
and that, too, on a much larger scale—by the blind and
purposeless operations of nature; but granting all that
the hypothesis requires of us, we are still in presence of
the fact that natural selection, or survival of the fittest,
can accomplish nothing until it is supplied with mate-
rial for ‘ selection,’ until there has appeared upon the
field an antecedent ‘ fittest ’—a fittest organ, function,
habit, instinct, constitution, or entire organism. Natu-
ral selection produces nothing; it only culls from what is
already in existence. The survival of the fittest is an
eliminative, not an originative, process.”

So obvious is the above assertion that it needs no dis-
cussion. A variation must be originated before it can

* Animal Life and Intelligence, p. 79.
tThe Ethical Import of Darwinism, pp. 77-78.

52 CALIFORNIA ACADEMY OF SCIENCES.

be selected. We find then, that before variations can
occur in the phylogenic series they must take place in
the ontogenic series, or in other words, natural selection
is strictly conditioned upon individual variation. It
therefore becomes a matter of no small moment to en-
quire into this subject, from the standpoint of observa-
tion to determine what are the possibilities of variation,
and from the theoretical aspect to establish the condi-
tions of variation. Wallace, more than any other
scientist, probably, has given attention to the record-
ing of individual variations, and his most recent work,
Darwinism, furnishes us with a large number of facts of
this sort. An inspection of the instances there given*
and of cases adduced by other writers, as Semper in his
Animal Life, discloses the fact that they are all quanti-
tive and not qualitative. Variations may occur in size,
in shape, in position, in number and inecolor. Varia-
tions in size are most numerous and most marked, per-
haps, but such variations if in any special direction
entail variations in shape. Variation in the position
of parts is almost equally great, perhaps, although less
readily observed and consequently less frequently re-
corded. Variations in number may be trivial and
unnoticeable, or may become monstrous as with six-
fingered men and two-headed calves. Besides the
above mentioned deviations I know of none, except
the occasional suppression of parts, either internal or
external.

The above variations might at first sight appear suf-
ficient to produce changes in structures already in ex-
istence, but the old difficulty still remains of accounting
for the origination of new parts. Wallace’s answer to
this is certainly apologetic.t Have we aright to assume

* Darwinism, pp. 41-82.
tl.c., pp. 128-131.

EVOLUTION OF THE COLORS OF BIRDS. 53

that variations occurred in past geological ages different
in nature from those which occur to-day? If we do not
do so, from which class of the variations above enumer-
ated were feathers evolved, or horns, for example?
Singularly enough, Wallace, an avowed opponent of
Lamarckianism, triumphantly appeals to the principles
of use in the one difficult variation which he does really
explain, the shifting of the eye of the flatfish.* If in-
dividual variation does not normally originate new parts,
such as horns, for example, and I know of no reason
either theoretical or practical for assuming that it does,
then there must be some other factor occasionally called
into play; but if some factor exists which must be opera-
tive at rare intervals it is but reasonable to assume that
it is constantly operative to some extent rather than to
suppose that it is lying idle until natural selection is
obliged to call upon it to lend a helping hand.

The above considerations lead to another frequently
urged difficulty that even supposing the necessary varia-
tions did occur, we are obliged to assume that they were
advantageous. That incipient modifications of an organ-
ism are always advantageous is considerable of an assump-
tion. It seems incredible, indeed, that some very pro-
nounced distinctions can be of utility. Thus it seems
difficult to explain, by utility or adaptation, the fact that
in California a species of magpie is found identical with
the one found east of the Sierra Nevada Mountains, with
the exception of being smaller and having the bill bright
yellow instead of black. The birds are well isolated geo-
graphically throughout the greater part or the whole of
their range, so the difference could hardly be accounted
for by the theory of recognition markings. This is but
one of a great number of instances of a similar nature

*1iu., pp. 129-130.

54 CALIFORNIA ACADEMY OF SCIENCES.

which might be adduced. Examples of insular species
are especially noteworthy in this regard.

Mr. Romanes has considered these difficulties at some
length in a book which has but just appeared.* The
Duke of Argyll in advocating design in nature, made
use of the above objections to natural selection, and it
is to him that Mr. Romanes addresses his reply. It is
hardly necessary to say that the difficulties above stated
were not mentioned with a view to show the necessity
for design in nature, or some direct supervising power
to control the origination of variations, but merely to
call attention to the proposition that natural selection,
with the help of purely fortuitous variation, does not
seem sufficient to account for all structural and specific
details. Mr. Romanes, in his reply to the Duke of
Argyll, shows how careful it is necessary to be in any
particular instance in saying that a character is of no
use. Thus the eye must have been of use in its most
incipient stages as a black pigment spot which indicated
vaguely the presence of light to the nerves in its vicin-
ity. The wing in its incipient stages must also have
been of use, as Mr. Romanes points out. Even if a
variation were of no use at first 1b might be indircctly
preserved by natural selection through correlation of
growth. ‘‘Mr. Darwin, who has paid more attention to
this matter than any other writer, has shown, in consid-
erable detail, that all the parts of any given organism are
so intimately bound together, or so mutually dependent
upon each other, that when one part is caused to change
by means of natural selection, some other parts are very
likely to undergo modification as a consequence.”

The theories which have been put forth to explain
variation are, forthe most part, very incomplete. Darwin

* Darwin and After Darwin. I—The Darwinian Theory, pp. 350-373.

EVOLUTION OF THE COLORS OF BIRDS. 55

freely confesses his ignorance on this point, saying:
“Our ignorance of the laws of variation is profound.
Not in one case out of a hundred can we pretend to as-
sign any reason why this or that part has varied.” *
Spencer suggests a number of causes for variation.t He
considers the influences of environment in altering
functions to be one factor in determining variations,
asserting * * * ‘that organisms produced by the
same parents at the same time, must be more or less
differentiated both by insensible initial differences, and
by slight differences in the conditions to which they are
subject during their evolution.” He then appeals to first
principles to show that no two parts of a homogeneous
substance can be exactly alike, and that consequently
there must alavays be a difference at least in the number
of physiological units composing a reproductive cell.
Thus we have a clue to the differences existing in the
young of a single litter.

Weismann’s theory of variation is probably the most
carefully worked out in its details, and is perfectly con-
sistent with his theory of the isolation of the germ plasm.
It appears impossible, however, that any progressive
modification could take place according to this view, for
it depends exclusively upon the number of ahnenplas-
mas of each sex which enter into combination, this
number being largely determined by the amount of sur-
plus germ plasm which is disposed of in the extrusion
of the two, polar cells. But variation according to this
theory, it would seem, must be strictly conservative, for
all modifications must be within the extremes of an-
cestral modification. Thus, let us suppose that every
living individual of some species has been measured,
and that 500 represents the number of units in the

* Origin of Species, p. 73, Humboldt.
tPrinciples of Biology, I, pp. 257-272.

56 CALIFORNIA ACADEMY OF SCIENCES.

smallest individual of the species, and 550 in the largest.
There could be, then, no variation due to sexual admix-
ture which fell below 500 or rose above 550, and hence
no progress would be possible.

Another difficulty in Weismann’s theory of variation
has been recently communicated to Nuture, by Prof.
Marcus Hartog.* He argues from the assumption that
Weismann’s theory is proved, and the result is, as he
says, indeed a startling one. He first presents five theses
expressing the main assumptions of Weismann’s theory,
which Poulton has since admitted to be an impartial and
correct statement of the case. ‘lhey are as follows:

‘‘T. Each primitive germ-cell of either sex, contains
a number of ancestral germ-units, the ahnenplasmas;
and this number is constant, for the species at least.

II. These ancestral germ-units are far more con-
stant and unchangeable in character than the species
itself.

III. They lie associated together in the germ-cell
without loss or alteration of their individual peculiar-
ities.

IV. The number contained in the mature ovum
and spermatzoon is reduced by one-half, and in the
fertilized ovum or oosperm the number is restored to the
normal by the summation of the ahnenplasmas of the
two fusing cells. This process is comparable to the
shuffling of two packs of cards by taking half from each
and joining the talons or remainders to form a new
pack.

V. The possible combinations under this process
are so numerous as to explain the variations among the
offspring of sexual union.

Accepting these statements, we next inquire, How

*Nature, October 29, 1891, p. 613.

EVOLUTION OF THE COLORS OF BIRDS. 57

are we to conceive of these ancestral units, the ahnen-
plasmas? Two hypotheses may be given in answer to
this question:

A. Each ahnenplasma unit corresponds to an in-
dividual of the species itself; and if put under proper
tropic conditions, would, singly, reproduce such an in-
dividual. :

B. The ahnenplasmas correspond to the primitive
Protozoan ancestors, which, according to theory, could
alone reproduce modifications due to external causes
(acquired modifications).”’

Prof. Hartog then shows that if hypothesis A be ac-
cepted the ahnenplasmas must have varied with the
race, but this would make the shuftling process super-
fluous as an explanation of variation, and would also be
contradictory to thesis IT.

‘‘According to hypothesis B,”’ he continues, ‘‘ the
ahnenplasmas of all Metazoa being similar and Proto-
zoan, if the numbers are equal and the shuffling fair, any
two parents may beget any offspring whatever; on the
plane of thesis V, a lioness might be expected to bring
forth a lobster or a starfish or any other animal, which
as we know, does not take place in nature. The only
escape from this result is to assume the postulates—(1)
that the number of ahnenplasmas varies from species to
species; (2) that the number in the combination and
not the character of the ahnenplasmas determines the
species. And as there is nota particle of evidence for
the latter postulate, we may say that on hypothesis B,
the theory breaks down by its non-conformity with the
facts.

We have then the dilemma, from which I see no
escape, that the theory is inconsistent, on A with itself,
on B with the facts.”

It is too soon to attempt to pass judgment upon this

58 CALIFORNIA ACADEMY OF SCIENCES.

objection, as the subject is still under discussion. Cer-
tain it is, however, that the replies which have been
made by Poulton and Trow are incorrect and Hartog
demonstrates their inconsistency with the views of Weis-
mann by a letter from that gentleman.

Lloyd Morgan suggests as an hypothesis of variation
a modification of Spencer’s theory of physiological units,
which has much to commend it, viz., the organic com-
bination of the elements of the two sexes into a specific-
ally new compound. Morgan explains the need of this
hypothesis as follows:* ‘* * * if, in sexual union,
there is a mere mixture, a mere commingling of heredi-
tary characters, it is quite impossible that new characters
should result, or any intensification of existing charac-
ters be produced beyond the mean of those of ovum and
sperm. * * * Let us suppose, for the sake of illus-
tration, that a pair of organisms have each an available
store of forty units of growth-force, and that these are
distributed among five sets of organs, « to e, as in the
first two columns. Then the offspring will show the
organs as arranged in the third column.

——— Parents — Offspring.
a. rime lO a PLO: . 10
Ong ike 8. 10. 9
Cesraretatels v, 5 7
d 7 9. 8
é 6 6. 6
40 40 40

‘«There is no increase in the set of organs a, which
are strongly developed in both parents; and no decrease
in the set of organs e, which are weakly developed in
both parents. By sexual admixture alone there can be
no increase or decrease beyond the mean of the two

“ Animal Life and Intelligence, pp. 150, 151.

EVOLUTION OF THE COLORS OF BIRDS. 59

parental forms. If then the union of sperm and ovum
be the source of new or more favorable variations other
than, or stronger than those of either parent, this must
be due to the fact that the hereditary tendencies not
merely commingle, but under favorable conditions com-
bine, in some way different indeed from, but perhaps
analagous to, that exemplified in chemical combina-
tion.”

According to this theory it is of course possible that
new variations could be originated by sexual admixture.
It would also help to explain the presence of useless
specific characters, which would, according to the old
view, be obliterated by the swamping effects of inter-
crossing. ‘‘If, however,” says Morgan,* ‘‘on the
hypothesis of combination, we have definite organic
compounds, instead of, or as well as, mere hereditary
mixtures, if, in other words, variations take definite lines
determined by the laws of organic combination (as the
nature and properties of chemical compounds are deter-
mined by the laws of chemical combination), then this
difficulty disappears.” .It would account also for the
introduction of new parts or‘organs, and for their pres-
ervation before they had reached a point of usefulness.
The chief objection to this view, aside from its purely
hypothetical nature, which the author freely concedes,
is the fact that it appears to prove too much. There
are three general hypotheses concerning modifications:
(1.) The variations may be ‘‘spontaneous,” or due to
some unknown cause, and occurring in all directions
indiscriminately, as Wallace and other Neo-Darwinians
claim, so there would be ample material for natural
selection to work upon. According to this view external
influences have nothing to do with variations. (2.)

Ds Cay De 152s

60 CALIFORNIA ACADEMY OF SCIENCES.

Variations may be caused chiefly by the response of the
organism, or of its parts, to influences external either to
the organism as a whole or to the parts thus affected.
Or (8) variations may, as in the first instance, be wholly
independent of external influences, but due, not to the
selection of such modifications as happen to be suitable,
but to inherent properties of the organism itself, or to
predetermined properties of the combinations of organ-
isms. It is Nageli who advanced the first part of this
third hypothesis, while Morgan’s organic combination
theory would, if pushed to its logical conclusion, it seems
to me, lead to the second part. If certain combinations
must inevitably lead to éertain fixed and definite results,
regardless of environment, not only the incipient stages
but also the more advanced condition of an organism
should be due exclusively to the operation of this force.

In reality there are but two alternatives in regard to
the origin of variations. They are due either to some
force or tendency or property resident within the organ-
ism itself, whether it be Weismann’s sexual combination
of different germ-plasmas, Nageli’s idioplasm with the in-
herent tendency to vary in the right direction, or Mor-
gan’s,organic compound of sexually different elements
which must on uniting produce a new given result; or to
the workings of forces outside of the whole organism or
of the parts affected. The possibility, of course, exists
of both alternatives being valid. There is, however, a
very fundamental objection to the first class of forces as
being sufficient to originate variations, viz.: that it is at
variance with the law of the conservation of energy, as
suggested by Ryder, that an organism can create any-
thing new within itself without the aid of any force ex-
ternal to it—a feat no less difficult than for a man to
lift himself by the straps of his boots. The delusion of
supposing that it is only a very little which is created
new each time only seryes to confuse the mind.

EVOLUTION OF THE COLORS OF BIRDS. 61

It is still not inconceivable that both external and in-
ternal forces may influence variation. There is no pos-
sible doubt that sexual combination is productive of
variations, but I think it has been shown that these are
not progressive. It is doubtless this factor which pro-
duces individual variation in species which have re-
mained unmodified through long geological ages. This
class might well be termed conservative variations.

It is a fact well known to gardeners and breeders that
when a species is placed under entirely new environ-
mental conditions it is apt to produce variations which
are well marked, known as sports. Itis such variations
as these, due to the influence of environment, which, it
would seem, are originative of new characters and might
be called progressive variations. According to this view
not only certain unusual modifications are due to en-
vironment, but all variations which are new have been
thus produced. It must not be supposed that the en-
vironment acts definitely upon the organism, compelling
it to vary in a single direction. Such may be the case
in some instances, but not universally. Change in nu-
triment may produce very different results in different
individuals, even of the same species, and would thus
furnish natural selection with a very diverse, although
by no means unlimited, assortment to pick from. Climate
would often act in a more direct manner in modifying
organisms as will be seen later on in considering the
varieties of North American birds. Use and disuse of
parts would also be productive of definite variations.
According to the principles above stated, variations may
be classified as follows:

I. Conservative. Occurring in all directions within
the limits of variation of the
species. Produced by sexual com-
bination of unlike individuals.

62 CALIFORNIA ACADEMY OF SCIENCES.

II. Progressive. Produced by the action of enyiron-
ment upon the whole organism
or by the interaction of parts.

1. Definite. Direct action of environment in
one direction, including use and
disuse.

2. Indefinite. General action of environment
in producing variations which
can be accumulated by natural
selection.

Dr. W. K. Brooks in his book on heredity (p. 213)
quotes the following secondary laws of variation which
should be borne in mind:

(1) ‘‘ Specific characters are more variable than gen-
eric characters.” (Darwin, Origin of Species, p. 122.)

(2) ‘Species of the larger genera in each country
vary more frequently than species of the smaller gen-
era.” (1. ¢., p. 44.)

(3) <‘‘A part developed in any species in an extra-
ordinary degree or manner, in comparison with the
same part in allied species, tends to be highly vari-
able.” (1. ¢., p. 119.)

(4) ‘If any given character is very variable in one
species of a group, it will tend to be variable in allied
species, and if any given character is perfectly constant
in one species of a group it will tend to be constant in
allied species.”” (Walsh, Proc. Ent. Soc. Phila., Oct.
1863, p. 213.)

The above discussion has answered the question, is
natural selection creative? in the negative. An attempt
has been made to show that it is not even admissible to
assume unlimited variation as affording material for
natural selection. Prof. Schurman goes even a step
farther than this with regard to the origin of varia-

EVOLUTION OF THE COLORS OF BIRDS. 63

tions. He says:* ‘‘They originate, we know not how,
in the nature of the organism. Nor would the state of
the case be essentially altered if it were domonstrated,
in opposition to Darwin, that every organic modifica-
tion was occasioned by some external stimulus. For the
change thus set up in the organism in response to the
foreign excitation would obviously derive its character
from the constitution of the organism, just as, to use
Darwin’s own example, the peculiarity of a flame is due
to the constitution of the combustible materials, and not
to the igniting spark.” Prof. Schurman is arguing for
a theistic conception of the origin of variations, and
hence, of the creation of species, but his conclusion
here does not effect the scientific, but only the met-
aphysical aspect of the case. As stated in the in-
troduction to this chapter, natural law does not explain
first cause, with which, indeed, science is not concerned.
The ultimate reason for the enlargement of a muscle
when exercised, for example, cannot be ascertained, but,
nevertheless, if it could be shown why such exercise pro-
duced in successive generations variations in the direction
of increased dimension, we would have as good an expla-
nation of the cause of those particular variations as we
have of the cause of the earth’s attraction by the sun
through gravitation. That variations are conditioned
by the reaction of the organism on the influences of the
environment, is undeniable, but this does not detract
from the validity of an explanation of the origin of
variations, so far as science is concerned. An analogy
may illustrate this better, perhaps. Certain qualities of
the mind are developed by education. It is possible to
formulate the exact process by which these mental
traits were brought about and it is justifiable to state

* The Ethical Import of Darwinism, p. 81.

64 CALIFORNIA ACADEMY OF SCIENCES.

that the origin of these qualities is known, and the
cause of their presence in the mind, despite the fact
that had not the mind the potential possibility of de-
veloping them they could never have been introduced.

LAWS CONDITIONING EVOLUTION.

If the above considerations upon individual variation
contain within them even a flimsy core of truth; if
natural selection even in its widest interpretation be not
creative; then, indeed, it is necessary to discard the
dogma of chance so much preached by the scientists of
the day, and admit that justas the formation of the
crystal is due to the working of natural law, so, too,
is the evolution of man and of all the diversified life
of the globe due to workings of the same natural law.
Prof. Cope, Prof. Hyatt and Haeckel have placed
especial stress upon the importance of the laws of
biology, and it will be necessary to consider some of
the laws which they have enunciated. The laws goy-
erning the evolution of life may be stated under three
heads: (1) laws of development, (2) laws of struc-
ture, and (3) laws of heredity. They may be diagram-
atically classified as follows:

I. Laws of development,

1. Bathmism or growth force.
2. Phylogenic extent and density.
3. Metabolism.
a. Anabolism
b. Katabolism.
4. Sexual intensification.
5. Acceleration and retardation.
6. Law of concentration.
II. Laws of structure.
1. Homology.
2. Successional relation.

EVOLUTION OF THE COLORS OF BIRDS. 65

Parallelism.
Adaptation.
Geratology.

Bilateral symmetry.
Correlation of growth.

ITI. Tis of heredity.

1. Uninterrupted or continuous transmission.
2. Interrupted or latent transmission.

3. Sexual transmission.

4. Mutual or amphigonous transmission.

Of the laws of development, growth force or bath-
mism, as Prof. Cope has termed it, is the most funda-
mental. Protoplasm is a great store house of energy
from without. It, alone, is capable of converting into
its own substance foreign matter—inorganic among
plants and organic among animals. By this conversion
of foreign substance into the body of the organism,
growth is induced, but this growth requires an expendi-
ture of energy. The Century Dictionary refers to the
following passage of Cope’s for a definition of bath-
mism:* ‘It is here left open whether there be any
form of force which may be especially designated as
‘ vital.” Many of the animal functions are known to
be physical and chemical, and if there be any one which
appears to be less explicable by reference to these forces
than others, it is that of nutrition. Probably in this
instance force has been so metamorphosed through the
influence of the originative or conscious force in evolu-
tion, that it is a distinct species in the category of forces.
Assuming it to be such, I have given it the name of
Bathmism.”

Bathmism, then, is the vital force inducing growth.

Prof. Cope has stated as a fundamental law of bath-

MS TUR go

* Method of Creation, p. 26.
5

66 CALIFORNIA ACADEMY OF SCIENCES.

mism* * * * ‘that growth-force, uninfluenced hy
inherited peculiarity, or any stronger influence locating
a nutritive fluid, must develop extent in the direction of
least resistance, and density on the side of greatest resist-
wnce, when not too great. The illustration of this state-
ment would be that of a globular mass of cells brought
to the point of junction of two media, as water and
earth or air and earth, elongates in the direction of the
medium presenting the least resistance, i.¢., air.” This
law may be further illustrated by the growth of the
foliage of a tree. When a tree is located in the midst
of a forest, it is crowded on all sides and accordingly
expends its energy in growing upward. If, on the
other hand, a barrier be placed to its upward growth,
the foliage will become dense and matted at the point of
obstruction. This daw of extent and density, as enun-
ciated by Prof. Cope, is apparently intended to account
for the distribution of force in individuals or species
only, but it seems to me possible that a broader inter-
pretation may be put upon it than Prof. Cope had in-
tended to imply. As an analogy representing the dif-
ferentiation of life the structure of a tree has been com-
monly used, the spreading branches of which illustrate
the divergent course of life through past geologic ages,
while the leaves and terminal buds constitute the life
of to-day. But the analogy of the tree may serve an-
other purpose. It has been stated that the growth of a
tree is conditioned by Cope’s law of extent and density.
This is alaw of ontogony. But just as the perfect tree
represents the direction of organic progress, so the law
of growth force which conditions the growth of the tree,
conditions also the growth of organic beings as a whole.
In other words, life as a whole has made progress or

* Origin of the Fittest, p. 30.

EVOLUTION OF THE COLORS OF BIRDS. 67

developed in extent along the lines of least resistance
and multiplied in forms on the same plane, or has de-
veloped in density along the lines of greatest resistance.
This may be called the Law of Phylogenic Extent and
Density. According to this view, living matter has
within it the potential possibility of indefinite growth
and modification. The growth is limited as Spencer has
shown by the mass outrunning the surface, while modi-
fication is limited (1) by the possibilities of environ-
ment to create variations, and (2) by the ability of the
organism to adapt itself to its environment. If it be
asked how an organism can adapt itself to an environ-
ment which is itself the cause of the variations, the re-
ply is by the selection of such environmental variations
as are adaptive, or conversely by the elimination of such
as are unadaptive. Living matter, then, like a gigantic
tree or bush, spreads out in all directions where the en-
vironment offers least resistance, and man was not an
avcident or happy coincidence in the aimless wander-
ings of blind forces, but rather the inevitable outcome of
natural law.

Metabolism, according to the Century Dictionary, is
‘‘the sum of the chemical changes within the body, or
within any single cell of the body, by which protoplasm
is either renewed or changed to perform special func-
tions, or else disorganized and prepared for excretion.”
Geddes and Thomson in their work on the Evolution
of Sex, have especially emphasized the importance of
the laws of metabolism in the development of species.
Metabolism should be clearly distinguished from bathm-
ism or growth force. The latter is a force analogous to
electricity, for instance, which conditions the growth of
an organism; while the former is an expression of the
changes continually taking place in protoplasm. There
are two phases of metabolism. When the protoplasm is

68 CALIFORNIA ACADEMY OF SCIENCES.

in a passive growing state and storing away energy, it
is spoken of as anabolic, but when it is in a disruptive
condition, giving out energy and breaking up into
simpler compounds it is said to be katabolic. The
metabolism of an organism is constantly varying be-
tween the anabolic and katabolic state, although of course
not in rhythms of definite duration.

Messrs. Geddes and Thomson have devoted a consider-
able space in their work on the Evolution of Sex to estab-
lishing the principle that the male nature is naturally
katabolic, the female naturally anabolic. They have en-
forced this principle by many examples and arguments,
and it is justifiable, it seems to me, to use it as a work-
ing hypothesis. The law of sexual intensification pro-
posed hy me ina recent paper on the Colors of West
Coast Mammals* is based upon this principle. Among
west coast mammals are certain species which have as-
sumed a black color in harmony with their surround-
ings, which are dark in hue, owing to the volcanic rock
of which the soil is formed. A particular species was
taken as an example, the black-headed ground squirrel
(Spermophilus granomurus vtvicapillus), and an attempt
was made to account for the black color. For three
reasons it was assumed that the ancestors of this species
were paler in color, probably gray or brown: (1) Be-
cause black is never a primitive color but rather a mark
of specialization. (2) Because the races and species
which are not blackjoccupy a much greater geographical
area, the black form being very local; and (3), be-
cause the young are paler than the adult. If the pre-
ceding discussion has established the impossibility, or
even the improbability of natural selection exercising a
creative power, it becomes proper to ask how the black

* Zoe, li, pp. 203-216.

EVOLUTION OF THE COLORS OF BIRDS. 69

color of this race was produced from brown or gray an-
-cestors. It is found that the male averages considerably
darker than the female, although females may be found
as dark as the lightest males. This is in accordance with
the principle enunciated by Geddes and Thomson, that
the tendency of the male is katabolic, of the female
anabolic. The surplus of energy of the male would be
expended in pigment, making the color more intense
than in the female.

It will now be necessary to anticipate two of the laws
of heredity as quoted in Zoe from Haeckel.* ‘‘A third
law of conservative transmission may be called the law
of sexual transmission, according to which each sex
transmits to the descendants of the same sex peculiari-
ties which are not inherited by the descendants of the
other sex. * * * A fourth law of transmission,
which has here to be mentioned, in a certain sense con-
tradicts the last, and limits it, viz.: the law of mixed or
mutual (amphigonous) transmission. This law tells us
that every organic individual produced in a sexual way
receives qualities from both parents, from the father as
well as from the mother.”

Now, according to this law of mutual transmission,
there would be a constant tendency for the characters of
the male and female to combine more or less in the off-
spring. This tendency, if acting alone, would result in
an average color for both sexes. The females would be-
come darker and the males lighter. The katabolism of
the males, however, would tend to keep them at an
average distance in advance of the anabolic females,
and a tolerably constant ratio would be established. If,
then, there were nothing to hinder the female from ap-
proaching the characters of the male, the latter would

* History of Creation, 1, pp. 209-210.

70 CALIFORNIA ACADEMY OF SCIENCES.

be constantly pushed ahead by katabolism and drag his
mate after him. Thus, in the case in point, suppose
the following proportions of color existed in a pair of
these ground squirrels:

MALE. FEMALE.
Brown : .10 20
Black .... Subs are .60 40
White.. eee be 380 40

In accordance with the law of mutual transmission
the two sexes are approaching the average: brown 15,
black 50 and white 85. As the first ratio had been ad-
justed by the difference in metabolism which is sup-
posed to be constant, the real goal toward which the
male of the next generation is advancing is shown in
the following proportion:

IsT GENERATION. 2D GENERATION.

MALE. FEMALE, MALE. FEMALE.
Brown .. .10 : 20 § x : 15
Black... .60 : 40 = x ; 50
White... .80 : 40 a x F 35

The male of the second generation would then have
the proportion of colors as follows: brown 7.5, black 75,
and white 26.25, the black having increased consider-
ably. In nature no such rapid increase as this is sup-
posed to occur. The conservative law of sexual trans-
mission and innumerable limiting and modifying cir-
curostances would retard it.

After attaining a certain goal it would be impossible
for any further intensification to take place in the male,
as, for example, when it had become perfectly black. The
color of the male then becoming stationary, it would be
possible for the female to become more and more like
the male in accordance with the law of mixed transmis-
sion, the characters of the one sex being transferred to
the other. This, apparently, has actually taken place in

EVOLUTION OF THE COLORS OF BIRDS. 71

another black form, the jack-rabbit of Espiritu Santo
Island, Lower California, Lepus insularis, in which both
male and female are equally black. This law, as stated
in Zoe, referred only to transformations of color, but
would apply equally to any form of modification. Varia-
tions, according to the principles stated in this chapter,
are due to influences of the outer world upon the organ-
ism. One of the most potent environmental influences
is nutriment. If there be an abundance of nourishment
during a period of time, the tendency will at first be
anabolic, or constructive. There is, at such times, a
storing away of energy for future use. Geddes and
Thomson have shown that a preponderance of females
is born in times of plenty, of males in times of want.
After a time of bountiful nourishment suppose a period
of scanty food supply to follow. There will be a ten-
dency toward katabolism, which means a breaking up of
the energy which has been accumulating, a tendency
toward variation and a preponderance of males. The
males being more katabolic than the females, will tend
to vary in advance of the latter in every direction in
which a surplus of energy might be expended. In
noting the different kinds of variation which have been
observed it was found that they might consist of differ-
ences in size, shape, position, number, color or the
addition of parts. It is obvious that any increase of
size of the whole organism or of any particular part is
due to an increase in the number of constituent cells,
while variations in shape are due simply to a different
distribution of the cells or to an increase or decrease in
the number in any particular region. Variations such
as ordinarily occur in organisms then, are apparently
either variations in number or position. Number and
position depend on the amount and location of growth-
force, which in turn is conditioned by the metabolism

72 CALIFORNIA ACADEMY OF SCIENCES.

of the organism, which last is determined by nutrition.
The law of sexual intensification would thus seem to be
applicable to any form of modification, which may be
encouraged by natural selection, in which the male takes
the lead, but would in no wise serve to originate any new
part or structure.

At first sight this law might appear to be a mere re-
statement of the view of Dr. W. k. Brooks. According
to his view, the male cells are the bearers of gemmules
inducing variation, and in hard times the variable males
being in preponderance, would induce progressive varia-
tion. Rev. J. T. Gulick has criticised this view as fol-
lows:* ‘* There can be no doubt that in many species
the males are more variable than the females, and that
in some of the same species the proportion of males in-
creases with the degree of adversity; but this does not
seem to be sufhcient ground for maintaining that the
increase in the proportion of males will increase the
variability of the offspring. Increase in the number or
amount of the variable element does not necessarily in-
volve increase in the variability of either element, or in
the offspring of both. I find need of additional factors
in order to bring these facts into any relation to the in-
crease of variability. Granting that the sperm-cell is
the source of variation and the germ-cell the source of
fixity, and that increased tendency to variation in the
offspring will be secured by an increased range of varia-
tion in the sperm-cells, it does not follow that increase
in the relative number of males will increase the range
of variation in the sperm-cells, and therefore in the off-
spring.”

This difficulty, which implies that, however great the
majority of the variable sex may be, unless each one

* Journ. Linn. Soc., xxiii, p. 317.

EVOLUTION OF THE COLORS OF BIRDS. 73

founda mate the variability of the species would not
increase, does not, of course, apply to the law of sexual
intensification, which has, it seems, been placed upon a
firm basis through the researches of Messrs. Geddes and
Thomson.

Cope first enunciated the law of acceleration and re-
tardation. Two passages may be cited from his writings
explanatory of this law: ‘‘I believe that this is the
simplest mode of stating and explaining the law of varia-
tion; that some forms. acquire something which their
parents did not possess; and that those which acquire
something additional have to pass through more numer-
ous stages than their ancestors; and those which lose
something pass through fewer stages than their ances-
tors; and these processes are expressed by the terms ‘ac-
celeration’ and ‘retardation.’”* On another occasion
he had expressed the law thus: ‘‘It was also shown that,
if the embryonic form were the parent, the advanced de-
scendant was produced by an increased rate of growth,
which phenomenon was called «acceleration; but that if
the embryonic type were the offspring, then its failure
to attain to the condition of the parent is due to the
supervention of a slower rate of growth; to this phenom-
enon the term retardation was applied.” +

With regard to the cause of acceleration Prof. Cope
first made the following suggestion:{ ‘‘ The successively
higher degree of oxidization of the blood in the organs
designed for that function, whether performing it in
water or air, would certainly accelerate the performance
of all the vital functions, and among others that of
growth. Thus it may be that acceleration can be ac-
counted for, and the process of the development of the

* Origin of the Fittest, p. 297.
fl.c., p. 125.
tlie, p. 143-144.

74 CALIFORNIA ACADEMY OF SCIENCES.

orders and sundry lesser groups of the vertebrate king-
dom indicated; for, as already pointed out, the defini-
tions of such are radically placed in the different struc-
tures of the organs which aérate the blood and distri-
bute it to its various destinations.

‘‘ But the great question: What determined the direc-
tion of this acceleration? remains unanswered. One
cannot understand why more highly oxidized blood
should hasten the growth of partition of the ventricle
of the heart in the serpent, the more perfectly to separate
the aérated from the impure fluid; nor can we see why
a more perfectly constructed circulatory system, sending
purer blood to the brain, should direct accelerated growth
to the cerebellum or cerebra] hemispheres in the croco-
dile.”

The above statement of .the cause of acceleration is
in reality no explanation, for after all the real question
is what determined the direction of the acceleration, for
which Prof. Cope does not here attempt to account. In
the paper on the Method of Creation of Organic Forms *
he offers another explanation, which appears to be final.
He asks, ‘‘What are the influences locating growth-
force?” and answers, ‘‘ The only efficient ones with which
we are acquainted are: first, physical and chemical
causes; second, use; and! would add a third, viz.: ef-
fort.” To this third influence Prof. Cope ascribes a
most important function—that of originating new parts;
to it, indeed, may be attributed, according to Prof. Cope,
the origin of the fittest. He says: ‘“ You cannot rub
the sclerotica of the eye without producing an expan-
sion of the capillary arteries and corresponding increase
in the amount of nutritive fluid. But the case may be
different in the muscles and other organs (as the pig-

*1ow., p. 195.

EVOLUTION OF THE COLORS OF BIRDS. 75

ment cells of reptiles and fishes), which are under the
volition of an animal. Here, and in many other in-
stances which might be cited, it cannot be asserted that
the nutrition of use is not under the direct control of
the will through the mediation of nerve force. There-
fore I am disposed to believe that growth-force may be,
through the motive force of the animal, as readily de-
termined to a locality where an executive organ does not
exist, as to the first segment or cell of such an organ
already commenced, and that effort is, in the order of
time, the first factor in acceleration.”

All that can be said to the above is that it may be
true, but that it has not yet been demonstrated. There .
is, indeed, a vast difference between the assumption
that use can modify a part which already exists, and the
assumption that desire or effort can originate something
which does not exist. Moreover, even if effort be a
valid factor in creation, it cannot, it seems to me, have
the general application ascribed to it by Prof. Cope.
For example, it could apparently have no influence upon
the origination of new colors. Does the bird desire to.
be protectively colored? If so, it must decide what
colors would be most in harmony with its surroundings
and then make an effort of will to have these colors
developed; all of which is, on the face of it, inconceiv-
able. Or by what imaginable sort of effort could feathers
be originated? Effort, then, if it can be shown to have
any creative power, must be relegated to a very special
field, and cannot be considered as the sole or even
principal originator of the fittest.

What then can be considered the originator of the
fittest? This I have attempted to indicate in the discussion
of variation. It is an emphasis of the Hilairian rather
than the Lamarckian factor. It is rather morein accord
with the views of Eimer than Cope, although both are

76 CALIFORNIA ACADEMY OF SCIENCES.

considered of importance. It is the recognition of the
importance of physical and chemical causes, as the
originators of new parts. The environment produces
changes of which natural elimination destroys the least
advantageous, leaving only the fittest to survive. There
are metabolic rhythms in life. When the anabolic ten-
dency is in the ascendent life is non-progressive, or
there may even be retardation; but when the katabolic
tendency is in the ascendent, life progresses along lines
determined: first, by the nature of the variations, and
second, by the operation of natural selection or elimi-
nation.

It may be of interest to note the bearing of the law of
phylogenic extent and desiiny. In the growing tree
whenever its progress is checked in any direction, if the
resistance be not too great the foliage becomes dense and
matted. So with life as a whole. When a new plan
of organization was originated, nature ran riot for a
time in the wealth and multiplicity of forms she dis-
played. Take for example the age of reptiles. In their
palmy days they were without competitors; climate and
vegetation conspired for their well being; the soil had
not been cultivated and was capable of bearing an im-
mense crop. This was a period of acceleration. There
was no resistance to the upward progress of growth-force
and it advanced rapidly along certain lines. Having
originated this great variety and multiplicity of forms
quite suddenly, comparativcly speaking, a barrier was
presented to their future progress. The climate may
indeed have remained as propitious and the food as
abundant, yet an obstacle to their advance was intro-
duced. And this obstacle was apparently simply the
fact that they had reached the goal of their specializa-
tion. The growth-force was still there to be expended
and was used in increasing density or mass to speak

EVOLUTION OF THE COLORS OF BIRDS. OK

figuratively, or in other words, in the great multiplica-
tion of such species as already existed. In course of
time the conditions of life became less favorable to
them, more powerful antagonists arose, and they began
to dwindle. Retardation began, and to-day there are
left a few poor fragments of that mighty host that lived
in the age of reptiles.

Prof. Alpheus Hyatt’s law of concentration is very
closely related to the preceding, being in fact a form of
acceleration. Prof. Hyatt thus alludes to it:* ‘‘ The
law of concentration in development seems to express
an invariable mode of action of heredity, in the earlier
reproduction of hereditary characteristics of all kinds
and under all conditions. In progressive series it acts
upon healthy characteristics and appears to be an
adaptation to favorable surroundings, and in retrogress-
ive series upon pathological characteristics, and is prob-
ably an adaptation to unfavorable surroundings usually
leading to the extinction of the series or type.”’

This ends the discussion of the laws of development.
Of the laws of structure and heredity little need be said.
For the sake of completeness, however, and especially
as it will be necessary to refer to them in the second part
of the work, it will be advisable to include a brief state-
ment of them. Prof. Cope, in the Chapter on Evolution
and its Consequences,f states the following four laws of
structure:

‘1. Homology. This means that animals are com-
posed of corresponding parts; that the variations of an
original and fixed number of elements constitute their
only difference. * * *

‘¢2. Successional Relation. This expresses the fact

* Fossil Cephalopoda in the Museum of Comparative Zoology. Proc.
A. A. A. S., 1883, xxxii, p. 360.
tl.c., pp. 6-7.

78 CALIFORNIA ACADEMY OF SCIENCES.

that species naturally arrange themselves into series in
consequence of a mathematical order of excess or defi-
ciency in some feature or features. Thus species with
three toes naturally intervene between those with one
and four toes. * * *

“3, Parallelism. This states that while all animals in
their embryonic and later growth pass through a num-
ber of stages and conditions, some traverse more and
others traverse fewer stages; and that, as the stages are
nearly the same for both, those which accomplish less
resemble or are parallel with the young of those which
accomplish more. * * *

‘4, Teleology. This is the law of adaptation so much
dwelt upon by the old writers, and admired in its ex-
hibitions by men generally. It includes the many cases
of fitness of a structure for its special use, and expresses
broadly the general adaptations of ananimal to its home
and habits.”

5. Geratology. Prof. Hyatt has enunciated this
principle of distorted or pathological types.* It is the
law that there ‘‘ is an exact correspondence between the
life of an individual and the group to which it belongs:
namely, the young and adolescent stages having direct
correspondence and repéating the past history of its
own group to a greater or less extent, the adult corre-
sponding to the present with all the peculiarities and
differences of its group, and the metamorphoses of old
age to the pathological modifications and changes found
in the types which arose in the unfavorable localities, or
which were found as a rule to terminate the history of
the group in time.”

To this list may be added the following special laws
of structure, both of which may be more or less closely
related to homology:

* Proc. A. A. A. S., xxxii, p. 349.

EVOLUTION OF THE COLORS OF BIRDS. 79

6. Bilateral symmetry. It is not clearly apparent why
bilateral symmetry should be so prevalent in organic
structures, whether it be due to some fundamental law
of growth, or persistence of type, or to some principle
of utility. Wallace has advocated the last view with re-
gard to the bilateral symmetry in the colors of animals,
arguing from the fact that where protective or recogni-
tion markings are no longer of utility, as with domesti-
cated animals, bilateral symmetry is lost.

7. Correlation of growth. This law has been most
clearly enunciated and established by Darwin. Owing
to the close interdependence of parts in organisms, any-
thing which affects one structure in the body may cause
a similar or corresponding modification to appear in
some other structure which is not directly influenced.

The laws of heredity may be condensed after Haeckel,
thus:*

1. Uninterrupted or continuous transmission. Chil-
dren are in general like their parents. Thisis expressed
by the phrase ‘‘like produces like,” or more accurately
‘similar things produce similar things.”

2. Interrupted or latent transmission. Among certain
lower forms of life there is an alternation of generations,
the children being like the grandparents, the parents a
different organism. ‘‘If we express this general law
and the succession of generations by the letters of the
alphabet, then A= C = E,whilst B= D=F, and so on.”
8, Sexual transmission. Each sex transmits to its
offspring certain peculiarities not possessed by the other
sex, as, for example, the antlers of a deer.

4. Mixed or mutual (amphigonous) transmission.
‘This law tells us that every organic individual pro-
duced in a sexual way receives qualities from both
parents, from the father as well as from the mother.”

* History of Creation, pp. 205-213.

80 CALIFORNIA ACADEMY OF SCIENCES.

5. Abridged or simplified transmission. In embryonic
development the organism does not pass through all
ancestral stages, but omits a portion of them in order
more rapidly to reach a state of maturity.

Having clearly defined my attitude in regard to the
non-creativeness of natural selection and the necessity
of qualifying it by the laws of development, structure
and heredity, it will in the future frequently be found
convenient to speak figuratively of selection as the orig-
inator of this or that character. Whenever a use is
found for any structure or peculiarity of a species it is
justifiable to assume that such a character has been
encouraged by selection or elimination, and this is a
most important, although not the ultimate step, in deter-
mining how the feature under consideration came about.
It is indeed the only step which can be explained in most
cases, the causes of the origination of anything new
being so intimately dependent upon the nature of the
organism, that comparatively little is known of them.

Two forms of selection still remain for consideration,
sexual and physiological. Both have a direct and vital
bearing upon the subject of the colors of birds; for if
sexual selection be a valid factor in evolution it is
undoubtedly an agent in the production of the bright
plumage and gay ornamentation of male birds; while if
the hypothesis of physiological selection be correct much
light is thrown upon specific color marks which would
otherwise appear useless. It thus becomes important to
consider both of these theories with considerable care.

SEXUAL SELECTION.

Darwin originated the theory of sexual selection and
Wallace has been his most determined opponent. Both
should be heard from before a decision is reached.
Darwin says;* ‘‘ This form of selection depends, not on

* Origin of Species. I, p. 108.

EVOLUTION OF THE COLORS OF BIRDS. 81

a struggle for existence in relation to other organic
beings or to external conditions, but on a struggle be-
tween the individuals of one sex, generally the males,
for the possession of the other sex. The result is not
death to the unsuccessful competitor, but few or no
offspring.” There are two forms of sexual selection, the
law of battle which is universally admitted, and ‘ pref-
erential mating ’’ as Lloyd Morgan has termed it,* which
is doubted by many. It is known that the males of many
animals, especially among mammals and birds, fight
among themselves for a female, the victor carrying off
the prize. There can be little doubt that in such cases the
more powerful males become the parents of the race, and
that every advantage which they may possess in the way
of strength, agility, or special weapons, such as antlers
or spurs, will be selected in accordance with the law of
battle. Preferential mating is thus described by Darwin:t
‘‘ Amongst birds, the contest is often of a more peaceful
character. All those who have attended to the subject,
believe that there is the severest rivalry between the
males of many species to attract, by singing, the females.
The rock-thrush of Guiana, birds of paradise, and some
others, congregate; and successive males display with
the most elaborate care, and show off in the best mauner
their gorgeous plumage; they likewise perform strange
antics before the females, which, standing by as specta-
tors, at last choose the most attractive partner. Those
who have closely attended to birds in confinement well
know that they often take individual preferences and
dislikes. Thus Sir R. Heron has described how a pied
peacock was eminently attractive to all his hen birds. I
cannot here enter on the necessary details, but if man
can in a short time give beauty and an elegant carriage

* Animal Life and Intelligence, p. 198.
tl.c., p. 109.

6

82 CALIFORNIA ACADEMY OF SCIENCES.

to his bantams, according to his standard of beauty, I
can see no good 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.”

Darwin’s theory of sexual selection with regard to the
colors of birds is simply this, that both sexes were orig-
inally dull colored and alike, but the females, in se-
lecting those males for mates which were more brightly
colored than their fellows, have gradually brought about
the brilliant and diversified plumage of the male birds
of to-day. Wallace takes a directly opposite view, as
expressed in the following passage: * ‘I have long
held this portion of Mr. Darwin’s theory to be erroneous,
and have argued that the primary cause of sexual diver-
sity of color was the need of protection, repressing in
the female those bright colors which were normally
produced in both sexes by general laws; and I have
attempted to explain many of the more difficult cases on
this principle (‘A Theory of Birds’ Nests,’ chap. VI.
ante).”” :

Mr. Wallace then proceeds to elaborate his views,
explaining how the colors of male birds have become
more brilliant without reference to sexual selection. He
makes the valuable suggestion that brilliant colors are
concomitants of a healthy organization, dull hues of a
diseased system. The vitality of the male he considers
to be greater thau of the female, especially during the
breeding season, and the brilliant colors and long plumes
which are donned at that time are due to the necessity
for this vital energy to find some outlet. ‘‘ The greater
intensity of coloration in the male” he says, + ‘‘ which
may be termed the normal sexual difference, would be

* Natural Selection 1891, pp. 364-365.
tl. c., p. 366.

EVOLUTION OF THE COLORS OF BIRDS. 83

further developed by the combats of the males for the
possession of the females.- The most vigorous and ener-
getic usually being able to rear the most offspring inten-
sity of colour, if dependent on, or correlated with vigor,
would tend to increase. But as differences of colour
depend upon minute chemical or structual differences in
the organism, increasing vigor acting unequally on
different portions of the integument, and often produc-
ing at the same time abnormal developments of hair,
horns, scales, feathers, etc., would almost necessarily
lead also to variable distribution of colour and thus to
the production of new tints and markings.” In follow-
ing out this line of argument Mr. Wallace suggests that
color is proportionate to integumentary development,
reaching its maximum among butterflies and birds
where there is the greatest expanse and variation of
wings, but it has been asked why, if this hypothesis
be a true.one, bats are generally so dull colored, or
beetles so brightly. ‘‘ The endless processes of growth
and change during the development of feathers, and
the enormous extent of this delicately organized sur-
face,” he says,* ‘‘must have been highly favorable to
the production of varied colour effects, which, when not
injurious, have been merely fixed for purposes of specific
identification, but have often been modified or sup-
pressed whenever different tints were needed for purposes
of protection.” This appeal to the extent and structure
of the feather can have but little weight, for, in the
majority of cases, it is the exposed edge of the feather
alone which is colored; the basal half being as a rule
white or some dull shade of buff or gray.

Whether the appeal of Mr. Wallace to the general
laws of growth is or is not justifiable as an explanation

*1.o., p. 369.

84 CALIFORNIA ACADEMY OF SCIENCES.

of the markings of birds, the question with which we
are at present concerned is whether the female exerts
any choice of a mate dependent upon color, and whether,
if she does so, it is possible that the markings cfa
species might in course of time be changed by this
means. Wallace asserts that there is no evidence that
the female is in any way influenced by color in choosing
amate. Among butterflies it does indeed seem incredi-
ble that any such selection can take place, and Mr.
Wallace presents the argument against this in the
strongest possible light. His argument is controverted
by Poulton, however, with considerable force.* Prof.
Geo. W. and Elizabeth G. Peckham have published a
paper entitled Observations on Sexual Selection in
Spiders of the Family Attide,f in which they advocate
the factor of sexual selection in the production of the
colors of spiders. They show that in this group the
female is fierce and pugnacious, which trait, according
to Wallace, should be accompanied with a surplus of
vitality and accordingly with brilliant colors, as he
claimed to be the case with the humming-birds. But in
this group the males are nearly always brilliantly colored,
while the females are inconspicuous. On the other hand,
the dull colors of the female could not have been originated
for protection, for ‘‘all the species of this family have
covered nests.”” The most important part of the paper
is devoted to a minute description of the courting habits
of different species of Attidee. In conclusion the authors.
sum up the results of their observations as follows:
‘¢The fact that in the Attidw the males vie with each
other in making an elaborate display, not only of their
grace and agility but also of their beauty, before the
females, and that the females, after attentively watching

*The Colors of Animals, 291-297.
t Natural History Society of Wisconsin.

EVOLUTION OF THE COLORS OF BIRDS. 85

the dances and tournaments which have been executed
for their gratification, select for their mates the males
that they find most pleasing, points strongly to the con-
clusion. that the great differences in color and in orna-
ment between the males and females of these spiders are
the result of sexual selection.” Indeed, anyone who
will attentively follow the account of these elaborate and
painstaking investigations of the courting habits of this
family of spiders, must admit that in this instance at
least, there is considerable reason to believe that sexual
selection has played an important part in the modifica-
tion of color.

Among birds it is difficult if not impossible to secure
such definite results from observation. It is known,
however, that many male birds display their markings
before the female in a very elaborate way. Darwin
pointed this fact out as being especially confirmatory of
his theory of sexual selection. It is thus explained by
Wallace:* ‘‘At pairing-time the male is in a state of
excitement, and full of exuberant energy. Even un-
ornamented birds flutter their wings or spread them out,
erect their tails or crests and thus give vent to the
nervous excitability with which they are overcharged.
It is not improbable that crests and other erectile
feathers may be primarily of use in frightening away
enemies, since they are generally erected when angry
or during combat. * * * But if those portions of
the plumage which were originally erected under the
influence of anger or fear became largely developed and
brightly colored, the actual display under the influence
of jealousy or sexual excitement becomes quite intelligi-
ble.”” Mr. Wallace, in the above passage, has introduced
a new theory explaining the brilliant colors of male

*L.¢., p. 377.

86 CALIFORNIA ACADEMY OF SCIENCES.

birds, which it seems to me would need far greater proof
to establish than the theory of sexual selection which
itis intended to supplant. According to this new hypoth-
esis the brilliant crests, etc., of birds have been largely
developed in order to frighten away enemies, but it
would require a great amount of observational evidence
to demonstrate this. On the contrary, in the majority
of cases the colors of the male bird are not in the least
calculated to inspire an enemy with fear. Let us take a
familiar example and compare the two views as explana-
tions. The house-finch (Carpodacus mexicanus frontalis),
is now at the height of the courting season and certainly
displays a great amount of energy in the prosecution of
his love-making. Three or four males may frequently
be seen following a single female from fence to fence, or
tree to tree. The wings are slightly lowered and the
birds either face the object of their devotions displaying
the bright crimson of the head, throat and breast, or
hop directly away exposing the rump patch of the same
color. Their vivacious song is also a feature of the
courtship. The dull colored female eventually flies away
with one of the competitors for her favor and the re-
maining males start in quest of other mates. It would
of course be utterly impossible to assert that the female
had chosen the most brilliantly attired of her suitors,
but it is indeed a significant fact that several males, or
at least two, have presented themselves to her, display-
ing their charins of song and dress, and that one of
them is successful without any fighting with his com-
petitors. Another fact of some importance,—I have
seen a single male house-finch paying assiduous atten-
tions to a female, and utterly repulsed and deserted by
the object of his devotions. From the above it is
evident that it is not mere perseverance which deter-
mines which male shall be successful, as Mr. Wallace

EVOLUTION OF THE COLORS OF BIRDS. 87

has argued, but the choice is really made by the female.

Mr. Wallace has, however, interposed another objec-
tion of great importance. He says: *‘‘ Again, evidence
collected by Mr. Darwin himself, proves that each bird
finds a mate under any circumstances. He gives a num-
ber of cases of one of a pair of birds being shot, and the
survivor being always found paired again almost im-
mediately. This is sufficiently explained on the assump-
tion that the destruction of birds by various causes is
continually leaving widows and widowers in nearly equal
proportions, and thus each one finds a fresh wate; and
it leads to the conclusion that permanently unpaired
birds are very scarce, so that, speaking broadly, every
bird finds a mate and breeds. But this would almost or
quite neutralize any effect of sexual selection of color
or ornament, since the less highly-colored birds would
be at little or no disadvantage as regards leaving healthy
offspring.” If it were indeed true that every male bird
found a mate there would be a serious difficulty in the
way of sexual selection, but Mr. Wallace has not estab-
lished this in the passage above quoted. It seems quite
as natural to suppose that there are always a number of
birds who have not found mates, and that consequently
there are constantly a supply of bachelors and old maids
on hand to mate with any unfortunate widow or widower.
Collectors of birds have often commented on the fact
that several males are nearly always shot to one female.
To be sure the duller colors and more retiring habits of
female birds would insure their protection, but even
where special search is made for them they are found to
be less common than the other sex. This is noted,
moreover, among species in which the sexes are alike.
If there be such a preponderance of males as seems

“le., p. 370.

88 CALIFORNIA ACADEMY OF SCIENCES.

highly probable, then, of course, every bird does not
find a mate, and Mr. Wallace’s objection is invalidated.
Prof. and Mrs. Peckham have shown that among the
Atti every male does not geta mate. They say: *‘In
spiders, as the females gradually become adults, they
have a choice from among a number of males, as these
mature several days earlier. The males will pair as often
as they have the opportunity, and as the mating season
lasts for two or three weeks, the more brilliant males may
easily be selected again and again.”’

Of considerable significance as showing the unsettled
state of scientific opinion upon this difficult question, is
the fact that two eminent English naturalists have each
just published a work in which the subject is discussed,
and have arrived at directly opposite conclusions. Prof.
Frank E. Beddard, in his book entitled Animal Color-
ation, concludes that sexual selection, if operative at all,
is a very insignificant factor, while Prof. George J.
Romanes, in the first volume of the series on Darwin and
After Darwin, writes to the contrary in the following
emphatic sentence: ¢ ‘‘And, as regards the particular
case now before us, I think I have shown, as far as space
will permit, that in the phenomena of decorative colour-
ing (as distinguished from merely brilliant colouring),
of melodious song (as distinguished from merely tune-
less cries), of enormous arboresceut antlers (as distin-
guished from merely offensive weapons), and so forth—
I say that in all these phenomena we have phenomena
which cannot possibly be explained by the theory of
natural selection; and, further, that if they are to be ex-
plained at all, this can only be done, so far as we can at
present see, by Mr. Darwin’s supplementary theory of
sexual selection.” Inasmuch as these two scientists have

"els u., Pp. 60.
tp. 400.

EVOLUTION OF THE COLORS OF BIRDS. 89

arrived at these opposite conclusions from consulting in
the main the same works that have been referred to in
the preceding discussion, it may be profitable to notice
the grounds for their decisions.

Prof. Beddard, in his chapter on Sexual Coloration,*
first enumerates instances of sexual dimorphism and
dichromatism among forms where sexual selection is ob-
viously precluded by the nature of the case, as, for ex-
ample, among some species of Echinoderms. Te states
that even among birds when the sexes are different in
color the female is sometimes as beautifully marked as
the male, as in certain species of curassows, parrots,
and the upland goose. He contends with Wallace that
these brilliant colors are most common and striking
in butterflies and birds where the nature of the expanded
surface would facilitate their development. He points
to the very slight exhibition of sexual dichromatism in
mamumals, and cites instances showing the dependence
of sexual dichromatism in birds upon the sexual organs.

What is to be said of those objections to sexual
selection? His first objection is, indeed, a valid one so
far as it goes, viz., in showing that there can be sexual
diversity in color and form without any selection, but
this by no means disproves sexual selection in forms
where it might be possible. As regards certain isolated
instances of birds in which the female, although differ-
ently colored from the male is equally beautiful, there is
much room for argument. In the first place it would
be necessary to know in each specific instance in what
the degree of difference consisted. Certain colors in
birds have complementary colors which are more prim-
ative but not necessarily less beautiful. Thus many
scarlet male birds when kept in captivity become yellow,
which color is often characteristic of the female of the

* pp. 253-282.

90 CALIFORNIA ACADEMY OF SCIENCES.

same species. A bird might accordingly develop a
brilliant scarlet color in its plumage through the instru-
ientality of sexual selection, while the female, through
partial inheritance, would be colored yellow upon cor-
responding parts of the body. In the same manner
various colors in one sex might be imperfectly inherited,
producing a female bird but little inferior to her mate,
although quite differently marked. The instance of the
upland goose (Bernicla mugellanica) in which ‘ the
female is a rich brown diversified by white marks, while
the male is black and white,” would be a case in point.
To the suggestion that the structure of the feather offers
greater facilities mechanically, for the display of color,
the objection has already been raised that only the tip
of the feather is colored. In another connection I have
suggested * that the absence of brilliant colors in mam-
mals, may be due to the fact that they are characteristi-
cally nocturnal in their habits, and the faculty of color
perception would very naturally be less exercised and
less highly developed than among birds. That the
color of birds may be influenced by the removal of the
sexual organs is an undoubted fact, but it does not
throw any light upon the origination of the color.
Even though a color had been developed by sexual selec-
tion, it would eventually become an attribute of one sex
only, andmight be expected to chanye if the bird were
unsexed.

Prof. Beddard also calls attention to the difficulty of
believing ina highly developed esthetic sense, which has
been urged by Wallace with such force. He alludes to the
excitability during the breeding season of animals among
which there is no pairing, but this does not appear to be
as significant as he imagines. Granting it to be a fact
that low forms of life do show signs of excitability

* Zoe, ii, p. 209.

EVOLUTION OF THE COLORS OF BIRDS. 91

during the breeding season, which is due to the state of
their nervous organization, and granting, even, that the
excitement shown by birds at this time may be due to
the same cause, to a large extent, this does not account
for the fact that the male birds display their ornaments
in a painstaking and elaborate way. Prof. Beddard
states Mr. Stolzmann’s view as set forth in the Proceed-
ings of the Zoological Society.* This theory is indeed an
ingenious one and may be used as supplementary to the
theory of sexual selection. It is based upon the assump-
tion that male birds are much more numerous than the
females, an assumption which Mr. Wallace rather too
hastily discarded. Mr. Stolzmann suggests that this
preponderance of male birds would be a disadvantage to
the species, for the unpaired males would be apt to
annoy the setting females and prevent the raising of the
brood. Accordingly those species in which the males
greatly predominated would stand less chance of per-
petuating their kind than species in which the sexes
were more evenly divided, and any device by which the
surplus males could be killed off would enable such as
were left to rear their offspring with greater success.
Such a device is to be found, according to Mr. Stolz-
mann’s view in the long tail feathers and brilliant
colors of many male birds. If Mr. Stolzmann’s view
be correct then natural selection, it would appear,
must be causing individuals to assume instruments for
their own destruction in order that the species may live.
This conclusion is arrived at from a merely superficial
consideration of the case, however. Natural selection
is in reality only preserving the lives of the greatest
number of individuals. In any given generation the
fittest individuals to survive would be those males which
were least ornamented, but these would be the least fit to

* 1885, p. 615.

92 CALIFORNIA ACADEMY OF SCIENCES.

leave offspring, because from their very advantages over
the females in point of numbers and strength, they
would prevent them from rearing their young. Conse-
quently these unornamented and inconspicuous males
would leave fewer offspring than the conspicuous males
which were fewer in numbers. The survival of the fittest,
then, should be regarded in the long run as the survival
of the individuals best fitted to leave offspring.

And now it may be well to enquire how this theory
might be made to support sexual selection instead of
opposing it, as Prof. Beddard intended it should. A
very weighty objection to sexual selection has been
raised by Mr. Wallace, viz., that even admitting the fact
of selection on the part of the female bird its influence
would be entirely neutralized by natural selection, for if
these ornaments were developed solely for the sake of
beauty, the individuals possessing such charms would
be at a disadvantage in the struggle for existence, and
perish. If the paradox can be demonstrated, on the
contrary, that these disadvantageous appendages which
have been acquired for the purpose of adornment, are
in reality an advantage, this difficulty is entirely done
away with.

Prof. Romanes does not consider Mr. Wallace’s objec-
tions in great detail, but takes issue with him on the
leading points in question. He says:* ‘‘ There is no
conceivable reason why mere brélliwncy of color, as an
accidental concomitant of general vigor, should have
run into so extraordinary, so elaborate, and so beautiful a
design of colors. Morcover, this design is only unfolded
when the tail is erected, and the tail is not erected in
battle (as Mr. Wallace’s theory of the erectile function
in feathers would require), but in courtship; obviously,
therefore, the purpose of the pattern, so to speak, is cor-

*lou, pp. 394-395.

EVOLUTION OF THE COLORS OF BIRDS. 93

related with the act of courtship—it being only then, in
fact, that the general purpose of the whole structure, as
well as the more special purpose of the pattern, becomes
revealed.” Mr. Romanes also calls attention to the fact
that in many cases, such as the appendage of the bell-
bird, a very elaborate structure has been evolved which
is used only in courtship. Such a tube as the bell-bird’s
inflatable tube, which is present only in the male, cer-
tainly could not have been developed by any excess of
vitality, or in accordance with any general laws of
growth.

Mr. Romanes is inclined to throw aside the difficulty
of constancy in esthetic taste in birds a little too
lightly. Hesays: * ‘‘Although we know very little about
the psychology of the lower animals, we do observe in
many cases that small details of mental organization are
often wonderfully constant and uniform throughout all
members of a species, even where it is impossible to
suggest any utility as a cause.”

In commenting on the display of ornaments by male
birds, Mr. Wallace writes:+ ‘‘ But it by no means fol-
lows that slight differences in the shape, pattern, or
colors of the ornamental plumes are what lead a female
to give the preference to one male over another; still less
that all the females of a species, or the great majority of
them, over a wide area of country, and for many suc-
cessive generations, prefer exactly the same modifica-
tion of the color or ornament.”’ Or to put the difficulty
in the words of Lloyd Morgan, { ‘‘ Sexual selection of
preferential mating involves a standard of taste; that
standard has advanced from what we consider a lower
to what we consider a higher esthetic level, not along

=p. 399.
+ Darwinism, p. 285.
+ Animal Life and Intelligence, p. 205.

94 CALIFORNIA ACADEMY OF SCIENCES.

one line, but along many lines. What has guided it
along these lines?” There are apparently but two views
to take in regard to the beautiful in life, either that
which appeals to the wsthetic taste of man in animal and
floral colors has been developed solely for the utility of
the individual possessing it, in accordance with the
general laws of growth (such as the direct action of the
environment, the structure of the integument, ete.), in
which event any beauty which it may possess for man is
purely incidental or a mere coincidence; or else it has
been produced by the selection of the most beautiful,
generally by the female.

Even Wallace admits that the beautiful colors of flow-
ering plants have been produced by the selective agency
of insects which aid in fertilization. Prof. Peckham has
shown that in all probability the brilliant colors of some
spiders have been produced by the selective agency of
the female, while the argument for sexual selection in
birds has a great deal in its favor. But it is not a little
remarkable that those things which appeal to man as
beautiful should be the same ones that affect animals as
low in the scale as insects. Mr. E. B. Poulton believes
this esthetic sense to be generally present in the ani-
mals as well as in man. He says: *‘‘ If an artist, en-
tirely ignorant of natural history, were asked to arrange
all the brightly colored butterflies and moths in Eng-
land in two divisions, the one containing all the beau-
tiful paiterns and combinations of color, the other
including the staring, strongly contrasted colors, and
crude patterns, we should find that the latter would con-
tain, with hardly an exception, the species in which in-
dependent evidence has shown, or is likely to show, the
existence of some unpleasant quality. The former
division would contain the colors displayed in courtship

“Loo, p. 316

EVOLUTION OF THE COLORS OF BIRDS. 95

and when the insect is on the alert, concealed at other
times.

‘The immense difference between the two divisions,
the one most pleasing, the other highly repugant to our
esthetic susceptibilities, seems to me to be entirely un-
explained if we assume that the colors of both are
intended for the purposes of recognition. But these
great differences are to be expected if we accept Mr.
Darwin’s views; for the colors and patterns of the latter
division appeal to a vertebrate enemy’s sense of what
is conspicuous, while those of the former appeal to an
insect’s sense of what is beautiful. It is, of course,
highly remarkable that our own esthetic sense should
so closely correspond with that of an insect. I believe,
however, that it is possible to account for this wonder-
ful unanimity in taste.”

Mr. Poulton accounts for it by supposing that ‘‘ our
standards of beauty are largely derived from the con-
templation of the numerous examples around us, which,
strange as it may seem, have been created by the esthe-
tic preferences of the insect world.” But this does uot
explain why insects should have the same standards of
beauty as man. Morgan indeed takes a stand decidedly
opposed to Poulton. Thus he says:* ‘To sum up,
then, concerning this difficult subject, the following are
the propositions on which I wouldlay stress: (1) What
we term an esthetic sense of beauty involves a number
of complex perceptual, conceptual, and emotional ele-
ments. (2) The fact that a natural object excites in
us this pleasurable emotion does not carry with it the
implication that the object was evolved for the sake of
its beauty. (3) Even if we grant, as we fairly may,
that brightly colored flowers, in association with nectar,
have been objects of appetence to insects; and that

* Animal Life and Intelligence, p. 413.

96 CALIFORNIA ACADEMY OF SCIENCES.

brilliant plumage, in association with sexual vigour, has
been a factor in the preferential mating of birds;—this
is a very different thing from saying that, either in the
selection of flowers by insects, or in the selection of
their mates by birds, a consciously wsthetic motive has
been a determining cause. (4) In fine, though ani-
mals may be incidentally attracted by beautiful objects,
they have no wsthetic sense of beauty <A sense of
heauty isan abstract emotion. Esthetics involve ideals;
and to ideals, if what has been urged in these pages be
valid, no brute can aspire.’’

There is certainly much truth in these propositions of
Mi. Morgan. His second statement, however, seems
to need some qualification. As previously stated, the
beauty in nature must be a pure accident or else have
heen evolved, because it appealed to the sense of beauti-
ful in the loweranimals. A landscape or beautiful sun-
set gencrally appeals to our sense of the sublime rather
than the beautiful. But there are many objects in
inorganic nature that certainly do appeal to our sense of
the beautiful. Crystals may be beautiful both in form
and color, but when such is the case the beauty cannot
be looked upon as anything buta coincidence. Objects
in the organic just as in the inorganie world may or
may not be beautiful, but in this case the beauty cannot
be always considered incidental. Ifa feather is beauti-
ful it is doubtless a mere accident, inasmuch as it was
evolved for utility, and the same may be said of the
leaf. But if flowers, insects and birds, display beautiful
effects which hear an observed relation to the inter per-
ceptual and emotional faculties of these organisms, it is
but reasonable to ussume a causal connection between
them, and to suppose that these beautiful effects have
been evolved by insects and birds, because they gave
them pleasure. Mr. Weismann, in speaking of the tail-

EVOLUTION OF THE COLORS OF BIRDS. 97

less cats of Japan, suggests that novelty may be the

motive for choice in sexual selection. He says:* ‘‘We
thus see how a slight but striking variation may at once

cause an energetic process of artificial selection, which
helps this variation to predominance: a hint for us to
be careful in passing judgment upon sexual selection,
for the latter also works upon such functionally indiffer-
ent but striking variations. In the case of the cats,
man has favored a particular variation, because the
novelty rather than the beauty of the character sur-
prised and attracted him. * * * I see no reason
why the same process should not take place in animals
by the operation of sexual selection.”

If the colars of birds and insects had been produced
for the sake of novelty, as Weismann suggests, we might
indeed expect to find occasional or even frequent exam-
ples of beauty among them, but the rule would be that
glaring contrasts would prevail. There would be no
harmony in the arrangement or contrast of colors, and
the majority would possess the characteristics of those
insects that are branded with warning colors.

Mr. Romanes, in his recent work, devotes more atten-
tion to the beautiful in the lower forms of life than has
been previously done, and affords an excellent explana-
tion of its occurrence. He says:} ‘‘ Turning, then, to
the animal kingdom below the level of insects, here we
are bound to confess that the beauty which so often
meets us cannot reasonably be ascribed either to natural
or to sexual selection. Not to sexual selection for the
reasons already given; the animals in question are
neither sufficiently intelligent to possess any esthetic
taste, nor, as a matter of fact, do we observe that they
exercise any choice in pairing. Not to natural selection,

* Essays upon Heredity, I, 1891, p. 443.
tl.c., p. 408.
7

et

98 CALIFORNIA ACADEMY OF SCIENCES.

because we cannot here, as in the case of vegetables,
point to any benefit as generally arising from bright
colors and beautiful forms. On the principles of natural-
ism, therefore, we are driven to conclude that the beauty
here is purely adventitious, or accidental. Nor need we
be afraid to make this admission, if only we take a suffi-
ciently wide view of the facts. For, when we do take
such a view, we find that beauty here is by no means of
invariable, or even of general, occurrence. There is no
loveliness about an oyster or alob-worm; parasites, as a
rule, are positively ugly, and they constitute a good half
of all animal species. The truth seems to be, when we
look attentively at the matter, that in all cases where
beauty does occur in these lower forms of animal life,
its presence is owing to one of two things—either to the
radiate form, or to the bright tints.””, Mr. Romanes then
shows that neither of these circumstances have primarily
any reference to beauty, and we must consequently
conclude that the beauty of such structures is a purely
accidental feature of their organization.

But having shown this to be the case, might it not be
justifiable to extend the conclusion, as Mr. Wallace has
done, and decide that wherever beauty is found in ani-
mate nature it is accidental and due to the mechanical
or organic necessities of the case? Clearly the general-
ization should not be made without looking at the ques-
tion from other aspects. On looking at color in the
broadest possible manner, it is found to be of two sorts:
colors incidental to the nature or properties of a sub-
stance or organism, as the red of blood, green of leaves,
brown of earth and blue of sky; and colors which do
not appear in any way mechanically essential to the
organisms possessing them, but to have been produced
for an effect upon some percipient being. It is hardly
necessary to call attention to the fact that all color is

ted

EVOLUTION OF THE COLORS OF BIRDS. 99

color only in so far as it is seen as such, being, from a
mechanical point of view, nothing but molecular vibra-
tions. In spite of this fact, some colors appear to have
been brought about from some relation existing between
the being possessing them and some other percipient
being or beings, while other colors are purely adventi-
tious. It is indeed difficult in many cases to distinguish
between the two kinds, although there are some instances
-in which opinion is practically unanimous. For exam-
ple, no one can deny that certain animals are protect-
ively colored; but in asserting that an animal is pro-
tectively colored, the implication is necessarily made
that there are or have been other animals in the same
environment which would see it if it were colored other-
wise. Accordingly, in these protectively colored species,
the colors are not adventitious but have been produced
with the view of not being seen. Warning colors and
recognition marks, on the other hand, have been pro-
duced for the purpose of being seen. If, then, these
three great classes of colors in animals—protective,
warning, and recognition—have been produced in order
that the animal possessing them may be seen or not
seen, as the case may be, is it logical to assume that
the colors which appeal to us as beautiful, although it is
impossible, in the majority of cases, to draw any sharp
line between them and such as may be more or less of
use for protection or recognition—is it logical to assume
that these beautiful colors have been produced without
any reference to some other individual perceiving them?
Why, in this one instance of the colors which above all
others attract the human eye, and, apparently, the eye
of all the higher animals, should we assume that they
have been produced without any reference to their fellow
beings which are continually seeing them?
Having stated at some length the leading arguments™

L.ef C.

100 CALIFORNIA ACADEMY OF SCIENCES.

for and against sexual selection, it will be appropriate
to end the discussion with some provisional conclusions
concerning it. In the first place, it will be necessary to
distinguish between the origination and guidance of
sexual colors. Critics of the theory of sexual selection
frequently appear to imagine that its acceptance requires
the assumption that a female bird has simply to get fixed
in her mind in some unaccountable manner a particular
style of dress for her lord and master, and then by a
patient process of trying on one garment after another,
and with perfect scope to embellish the dress at her
pleasure, she will at last get one to suit. The more
rational view is, that the female simply passes judgment
upon the attire of the male, accepting that which is most
pleasing to her sense of sight.

Grant Allen, in his suggestive little volume on Physi-
ological Aisthetics, treats of the physical basis of color
perception, and of the harmony and discord of color.
He says:* ‘““We haveseen * * * that certain masses
of colour are in themselves, apart from any effects of com-
bination, pleasurable stimulants of the optic nerve.
They may thus be regarded as the analogues of musical
tones, which we saw to be similarly gratifying in isola-
tion, because they aroused normal amounts of action in
fully-nurtured and under-worked nervous structures.
But most pleasures of colour are not so simple in their
nature as these, nor do those we have already considered
rank very high in esthetic value. Savages are pleased
by yards of red or blue cloth, and even cultured eyes are
often attracted by acolour of unusual purity and richness,
unrelieved by contrast or harmony; but the greater num-
ber of artistic cffects depend upon combinational con-
siderations.” He then shows why certain combinations
of colors are more pleasing than others. A particular

"p 161.

EVOLUTION OF THE COLORS OF BIRDS. 101

color fatigues only such nerves of the eye as respond to
itasastimulant. Just as in music when the beats are
too frequent a dissonance is produced, so in looking
at a combination of colors if the recurrent interferences
are too frequent a discord is produced and the colors do
not please.

The eye of the bird is a more perfect mechanical con-
trivance than the human eye, and is doubtless fully as
sensitive to the stimulus of various shades of color. To
be sure, it has been ascertained that birds see colors
entirely differently from mammals. Blue, for example,
is apparently excluded from their list.* Lloyd Morgan
says:t ‘If these facts be so, it is not too much to say
that the colour-vision of birds must be so utterly different
from that of human beings, that, being human beings,
we are and must remain unable to conceive its nature.
The factors being different, and the blending of the
factors by overlap being, by specially developed struct-
ures, lessened or excluded, the whole set of resulting
phenomena must be different from ours.” This differ-
ence in the colors perceived by birds, however, does not
in the least interfere with the mechanical basis of color
harmony as laid down by Grant Allen. A female bird
would naturally prefer for a mate an individual in which
the feathers were not soiled and bedraggled, and also one
in which the colors did not produce any mechanical jar
(roughly speaking) upon the delicate optical mechanism.
Accordingly, given certain base colors, these might be
separated, refined, purified, intensified and even arranged
in certain patterns in order to produce a harmonious and
agreeable combination and contrast.

To state briefly the factors operating in the production

*Color-Vision and Color-Blinduess. R. Brudenell Carter: Nature, xlii,
p. 56. “
+ Animal Life and Intelligence, p. 285.

102 CALIFORNIA ACADEMY OF SCIENCES.

of the brilliant colors and plumes of male birds they are
as follows: Katabolism of the male supplies a surplus
of energy to be expended (Geddes and Thomson); some
color must be present in birds as in all objects, and its
distribution is limited (but not determined) by the
nature of the integumentand the general laws of growth
(Wallace); birds and some insects (?) have a visual
apparatus which is sensitive to contrasts of color and is
more easily fatigued by certain shades and patterns than
by others, and consequently certain combinations are
more pleasing than others; the female bird (generally)
chooses a mate from amoung a number of suitors, con-
forming to what is more or less pleasing or disagreeable
in the plumage of the males (Darwin); owing to the
surplus of males, the conspicuous plumes and colors
which are thus developed by the choice of the female
bird, are a positive advantage to the species, reducing
the numbers of the predominant sex, and their preser-
yation is thus insured by natural selection (Stolazmann);
in many cases the female bird inherits the bright colors
of the male, but these colors are more frequently sup-
pressed by the action of natural selection, from the
greater necessity for protection on the part of the female
during the process of incubation (Wallace).

According to the view thus summed up, each one of
the investigators who have contributed to the subject
have presented only a portion of the truth and have
attempted to make it stand for the whole. This synthe-
sis seems to satisfy all the various objections which have
been raised against sexual selection, and to supply a tol-
erably complete explanation of secondary sexual char-
acters in birds.

EVOLUTION OF THE COLORS OF BIRDS. 103

THE NATURE OF SPECIES.

Throughout all the foregoing consideration of the
various theories and speculations regarding the factors
of organic evolution, it will be noticed that little has
been said about the nature and origin of species. This
has been left for the conclusion, as the final problem for
evolution to solve. In the first place, a few words on
the term species will be necessary. In its broadest or
logical interpretation, any set of individuals having
some characteristic or group of characteristics in com-
mon, constitutes a species. Species may be determined
upon an artificial or upon a natural basis. For example,
the human race might be divided into species depend-
ing upon the occupation pursued, when we would have
a species of hunters, a species of mechanics, one of
doctors, another of lawyers, and so on, ad infinitum.
This would, indeed, be, to a certain extent, a natural
classification, from one point of view. Or we might
divide mankind more arbitrarily into species according
to height, weight, disposition, or attainments, which
would be a classification into artificial species. A natural
system differs from the foregoing in showing real rela-
tion through common ancestry. To be sure, in the
classification of the human race according to occupations,
a true relation through common ancestry is shown, for
the progenitors of the human race undoubtedly all had
much the same occupation, from which all subsequent
pursuits have been evolved; but a natural system of
division must do more than indicate a real connection
by evolutionary succession—it must show that partic-
ular connection or group of connections which is most
vital to the nature or being of the thing classified. In
fact that which is used as a basis for a natural classifica-
tion must be that which is most fundamental to the
being classified, and each stage in division must be based

104 CALIFORNIA ACADEMY OF SCIENCES.

upon that quality or property which most vitally con-
ditions it, and is common to all subsequent groups
which may be contained within it. To illustrate: A
natural classification of man must be based upon life,
which, from a scientific point of view at least, is the
most vital and fundamental property of the being under
consideration. The next stage in the classification
would be a division into the animal and vegetable life,
and the third the placing of man in the vertebrate series
of animals. It will be observed that this third process
of division is simply a particular form of the funda-
mental property with which we started. Continuing in
this same manner, we should ultimately get a classitica-
tion of men into the various races, Cuucasian, Ethio-
pian, Mongolian, ete., aud a subdivision of each of
these into the various tribes or nationalities, as Ger-
mans, French, Italians, Americans. It is possible to go
even astage farther, and divide Americans into Yan-
kees, New Yorkers, Southerners, etc., but a point is
ultimately reached where no further division is possible
because no characteristic can be found of any number
of individuals which is a more special expression of
some more universal and necessary property of their
being.

The point to be emphasized in this discussion is the
fact that logically any group, however extensive or re-
stricted, may be regarded as a species in relation to the
next more inclusive group which contains it, and con-
stitutes its genus. Thus, logically, the vertebrates form
one species of the genus animal. Furthermore, science
is more or less arbitrary in the making of species, unless
only the most ultimate and special division of a group
be considered as such. Such a distinction of specics
is never made, however, for what scientist would ven-
ture to wake the Yankee a distinet species, americanus,

EVOLUTION OF THE COLORS OF BIRDS. 105

of the genus Homo, even though every individual had a
nasal twang which unerringly indicated the section from
which he came? The Code of Nomenclature of the
American Ornithologists Union* contains the following
apt sentences on this subject: ‘‘There is no inherent
zoological difference between a ‘ generic’ and a ‘specific’
name,—the nomen genericwm and the nomen triviale of
earlier zoologists. Both alike designate a ‘group’ in
Zoology,—the one a group of greater, the other a group of
lesser classificatory value. Some necessary distinction,
which has been misconceived to exist between these two
names, is simply a fortuitous matter of the technique of
nomenclature, apparently arising from the circumstance
that the generic and the specific names form the con-
trasted though connected terms of a binomial designa-
tion. Recognition of the scientific fact, that a ‘species,’
so called, is not a fixed and special creation, as long
supposed, but simply a group of the same intrinsic
character as that called a ‘genus,’ though usually less
extensive, and always of a lower taxonomic rank, has
done more than any other single thing to advance the
science of Zoology; for the whole theory of evolution
turns, as it were, upon this point.”

From the above it is apparent that the decision of
whether any particular group of individuals constitutes
a species or some greater or lesser taxonomic division,
must be more or less arbitrarily settled. In general,
however, it may be said that a species in science is a
group of individuals morphologically isolated from every
other group by at least one peculiar character.t
Wherever a group is not completely isolated, but con-
nected by living intervening forms with some other

* pp. 26 -27.
+ The word morphological is here used in its broadest interpretation, as
distinguished from physiological, and of course includes color changes.

106 CALIFORNIA ACADEMY OF SCIENCES.

group, it is calleda variety. Species and varieties, then,
do not always represent real taxonomic values, for two
extreme varieties of one species may be morphologically
very much less alike than two closely allied species. It
all depends upon the accident of destruction of inter-
mediate forms.

Having now clearly defined the nature of a species
and the relativity of its value, it is time to consider the
relation of evolution to species. The end of evolution
is the establishment of successively higher types. Pro-
gress must be orderly, and species are merely crystal-
lized forms of orderliness. If each individual were to
start off on its own independent track there would be no
unity in nature. Organisms would not hold together,
and life as a whole would present no feature of ration-
ality. Real progress would be defeated, and evolution
would end in chaos. But in recognizing that species
are the indispensable instruments of orderly evolution,
is it necessarily imphed that species are the outcome of
natural selection? This implication certainly does not
inevitably follow, even using the term natural selection
in its widest sense, and admitting it as an important
factor in cvolution.

Before considering this fundamental question as to
whether or not natural selection, aided, of course, by all
the factors of heredity und variation, can originate
species, it will be well to enquire what it is that natural
selection preserves—the individual or the species. Mr.
Romanes, in the Darwinian Theory* says: ‘‘ Next, it
must be clearly understood that the life which it is the
object, so to speak, of natural selection to preserve, is
primarily the hfe of the species; not that of the indi-
vidual, Natural selection preserves the life of the
individual only in so far as this is conducive to that of

* Darwin and After Darwin, i, pp. 264-205.

EVOLUTION OF THE COLORS OF BIRDS. 107

the species. Wherever the life-interests of the individ-
ual clash with those of the species, that individual is
sacrificed in favour of others who happen better to sub-
serve the interests of the species. For example, in all
organisms a greater or less amount of vigour is wasted,
so far as individual interests are concerned, in the form-
ation and the nourishment of progeny. * * * * Again,
all unselfish instincts have been developed for the sake
of the species, and usually against the interests of the
individual. An ant which will allow her head to be
Slowly drawn from her body rather than relinquish her
hold upon a pupa, is clearly acting in response to an
instinct which has been developed for the benefit of the
hive, though fatal to the individual. And, in a lesser
degree, the parental ins$incts, wherever they occur, are
more or less detrimental to the interests of the individ-
ual, though correspondingly essential to those of the
race.”

Let us view these words of Mr. Romanes in the light
of what has just been said concerning the nature of a
species. Discarding any teleological conception as un-
worthy of scientific consideration, why should natural
selection preserve the species rather than any other more
or less inclusive taxonomic group? Natural selection is
the result of a struggle for existence. This struggle for
existence, as Darwin has shown, is most keen among
those individuals that are most alike; just as among
men, doctors come into closer competition with one
another than they do with lawyers, or a dry goods mer-
chant will have a harder contest with his fellow dry
goods merchants than he will with a hardware merchant.
The struggle for existence is then, first of all, a struggle
of individuals of one species among themselves, in
which battle only the fittest will survive. But how could
the self-sacrificing ant to which Mr. Romanes alludes,

108 CALIFORNIA ACADEMY OF SCIENCES.

pass muster in such a struggle for existence of individ-
ual with individual; or how could any altruistic trait be
evolved? Mr. Romanes, it seems, has not adequately
shown this. The parental instinct does not constitute a
strictly corresponding instance, for the survival of the
fittest must of course mean the survival of those indi-
viduals best fitted to leave offspring. In the case of the
dry goods merchant just referred to, he would only be
the fittest individual to survive in the long run, if he
were able not merely to win his place among his com-
petitors, but to maintain it through his heirs. Repro-
duction is merely growth beyond the individual, and
accordingly the longer the line of potential descendants
which an individual leaves, the longer will that indi-
vidual survive, speaking broadly. Consequently, no
sexual individual is really complete. It can only be-
come complete by perpetuating itself, which demands a
mate.

In this view of reproduction a clue may be found to
the introduction of altruistic traits. Every individual’s
bodily immortality is conditioned by the existence and
well being of some other individual. The survival of
the individual is thus clearly wrapped up with the sur-
vival of the family, and natural selection would there-
fore encourage any tendency which would promote the
family, even to the disadvantage of certain members of
it. Thus the habit of feigning lameness by some birds
to lure an enemy from the young, is a source of danger
to the parent but a protection to the young. If in any
case, however, the danger to the parent were greater
than the protection to the young, the habit could not
have been acquired, for whenever the parent were lost
through the exercise of this altruistic habit, the young
would in the great majority of cases die of neglect, and
the habit would not become established.

EVOLUTION OF THE COLORS OF BIRDS. 109

To admit that it is the family rather than the single
individual which natural selection preserves, is far dif-
ferent from granting that the species, or even the race
or tribe is what natural selection especially favors. Let
us pass on next to a consideration of this point. The
family, as we have seen, is absolutely indispensable to
the life of the individual, in the widest sense, but
this is not the case with the tribe. The individual can
have a potential bodily immortality without the tribe,
and in a large number of cases this actually occurs.
However, it sometimes happens that by associating in a
community with similar interests, a greater number of
individuals can enjoy an immunity from some common
danger. This would only occur when the struggle for ex-
istence was more severe with some outside enemy than
with each other, which would be a comparatively
exceptional state of affairs.

This has not brought us any nearer to Mr. Romanes’
main proposition, that it is the life of the species which
natural selection preserves, and not that of the indi-
vidual. In point of fact, natural selection does not
preserve species with anything like the persistence
with which she perpetuates genera, nor are genera as
lasting as orders. What natural selection really pre-
serves are those individuals which are capable of perpet-
uating something higher or more perfect. A species is
an aggregate of individuals in which some new feature
of excellence is universally present, and from the very
fact of their“possessing this new feature they will stand
a better chance of being preserved as a whole, than
another group of individuals which lack this higher or
advantageous characteristic. To return to the analogy
of the dry goods merchants, which, by the way, is a
true analogy because natural selection is actually in
operation in this instance: their struggle for existence,

110 CALIFORNIA ACADEMY OF SCIENCES.

as previously pointed out, is more intense among them-
selves than it is with the hardware or shoe merchants.

Now, if in some large city, a portion of the dry goods
merchants should introduce some new feature, such as
keeping open evenings and lighting their stores with
electricity, while the other dry goods merchants were
unable to afford this innovation, a new species of dry
goods store would arise, every individual of which
would be at an advantage over the old species. The
struggle for existence would then go on between the dif-
ferent individuais of the new species and those who
were able to afford the most brilliant illumination or to
keep open the longest would survive, while the others
would die. In this sense natural selection is protecting
the species regardless of the individual, but in a deeper
sense it is protecting the individual regardless of the
specics.

PHYSIOLOGICAL SELECTION.

Mr. Romanes has also raised the deeper question, has
natural selection, even in its widest sense, originated all
new species? His original paper on Physiological
Selection was read before the Linnean Society, May 6,
1886,* and shortly afterwards an abstract appeared in
the columns of Nature. This communication aroused a
great storm of opposition, Wallace in particular enlist-
ing himself with the enemy. Romanes asserts that
there are three cardinal difficulties in the way of natural
selection, considered as a theory of the origin of species.
These are: (1) the difference between species and vari-
eties in respect of mutual fertility; (2) the swamping
effect of free intercrossing upon an individual variation;
and (3) the inutility of a large number of specific char-
acters.

* Journ, Linn, Soc. xix, pp. 337-411.

EVOLUTION OF THE COLORS OF BIRDS. 111

To overcome these three difficulties he has proposed
a new factor, physiological selection or the segregation
of the fit. It is a known fact that isolation favors the
production of new species. Oceanic islands in partic-
ular attest to this. According to Mr. Romanes, how-
ever, the advantage of geographical isolation is not
always offered to incipient species, which often branch off
in the midst of the parent stock. It is a noteworthy cir-
cumstance, however, that of all parts of an organism
the reproductive organs are most susceptible to change,
and if a variation in these parts should occur among a
number of individuals making them sterile with the
rest of the species but fertile among themselves, they
would be physiologically isolated even in the midst of
their associates. In the words of the author, this view
enables us to regard many, if not most, natural species
as the records of variation in the reproductive systems
of ancestors.

In considering the three difficulties in the way of natu-
ral selection the author adduces considerable evidence to
show that the difficulties are real. He asserts that the
fundamental or primary difference between species is
sterility, this being a common distinguishing feature
in nearly every instance. Other differences consist in
innumerable distinctions of structure, color or markings,
which he calls secondary differences. Inasmuch as
these secondary differences are never exactly alike in two
species while the primary difference is always the
same, he demands an explanation of the constancy of
this distinction, asserting that all previous theories
have dealt only with the secondary differences. Even
where geographical isolation has originated species it is
not possible to account for the almost universal sterility
existing between them. ‘‘The consequence is,’ he
says, ‘‘that most evolutionists here fall back upon a

112 CALIFORNIA ACADEMY OF SCIENCES.

great assumption: they say that it must be the change
of organism which causes the sterility—it must be the
secondary distinctions which determine the primary.
But the contrary proposition is surely at least probable,
namely, that it is the sterility which, by preventing inter-
crossing with parent forms, has determined the sec-
ondary distinctions—or, rather, that it has been the
original condition to the operation of the modifying
causes in all cases where free intercrossing has not
been otherwise prevented.” The author then proceeds
to show that it is not logical to suppose that any one of
so infinite a number of changes could have the same
effect in modifying the reproductive system. Further-
more, these changes constantly occur among domesti-
cated animals without affecting the fertility of the
breed. Darwin has shown that among wild species the
ratio between structural affinity and the degree of ster-
ility is not always constant, some very distinct species
being found to hybridize with facility; but this is not in
accord with the view that the sterility is universally due
to structural differences. ‘‘ Mr. Darwin further shows
that, ‘independently of the question of fertility, in
all other respects there is the closest general resemblance
between hybrids and mongrels.’ Clearly, this fact
implies that natural selection and artificial selection run
perfectly parallel in all other respects, save in the one
respect of reacting on the reproductive system, where,
according to the views against which I am arguing,
they must be regarded as differing, not only constantly
but also profoundly. Lastly, Mr. Darwin concedes—or
rather insists—that ‘the primary cause of the sterility
of crossed species is confined to differences in their
sexual elements.’ A general fact which assuredly proves
that the primary specific distinction is one with which
the organism as a whole is not concerned: it is merely a

EVOLUTION OF THE COLORS OF BIRDS. 113

local variation which is concerned only with the sexual
system. Why, then, should we suppose that it differs
from a local variation taking place in any other part of
the organism? Why should we suppose that, unlike all
other such variations, it can never be independent, but
must always be superinduced as a secondary result of
changes taking place elsewhere?’’*

After stating at length his reasons for believing that
variations in the reproductive system may arise inde-
pendently of variations in the organism, Mr. Romanes
qualifies this by admitting that in some cases the varia-
tion in the sexual organs may be correlated with other
variations in the system, and may have been primarily
caused by natural selection; but in granting this he
shows that natural selection simply becomes one of the
causes determining physiological selection. ‘‘If,” he
says, ‘‘we thus regard sterility between species as the
result of what I have called a local variation, or a varia-
tion arising only in the reproductive system—whether
this be induced by changes taking place in other parts of
the organism, to changes in the conditions of life, or to
changes inherent in the reproductive system itself—we
can understand why such sterility rarely, though some-
times, occurs in our domesticated productions; why it so
generally occurs in some degree between species; and
why as between species it occurs in all degrees.”

As to the evidence of this infertility between indi-
viduals of the same species, which is a necessary
assumption of the theory, many cases of failure to
interbreed have been adduced by Darwin and others.
Or the time of flowering or mating may be accelerated
or retarded in certain individuals, thus isolating them
from the rest of the race.

* Nature xxxiv, pp. 338-339.
8

114 CALIFORNIA ACADEMY OF SCIENCES.

The second difficulty of the doctrine of natural selec-
tion as the originator of species, according to Mr. Ro-
manes, is the swamping effect of interbreeding.
Wherever any geographical barrier cuts off a portion of
a race the section so isolated is found to develop into a
new species. In fact, it is evident that if occasional
variations arise in individuals in the midst of others in
which they do not occur, they will soon be eradicated
by interbreeding with the dominant class, however ad-
vantageous they might be. Physiological selection
would of course prevent this.

Mr. Romanes’ third difficulty is a question of fact.
Are all specific characters useful? Natural selection
concerns itself solely with adaptations caring nothing
whatever for species as such. In case the specific char-
acter is also an adaptive character, it is reasonable to
suppose that it has been developed by natural selection;
but if, as Mr. Romanes contends, a large number of
specific characters are of no use, some other factor must
have developed them, which factor Mr. Romanes consid-
ers to be physiological selection. He asserts, moreover,
that without isolation evolution would be linear in
direction but never branching. The struggle for exist-
ence is most intense in the most populous districts, and
consequently the evolution of new species should be
most rapid in such regions, but the evolution of new
forms in a crowded district would be especially difficult,
in fact practically impossible without some form of iso-
lation. Physiological selection would afford this needed
vid.

That so revolutionary a theory as this appeared to be
should not be immediately accepted by scientists was
not to be wondered at. To be sure it had been sug-
gested previously in Mr. Belt’s Nicaragua and in
a letter to Nature by Mr. Catchpool, but Mr. Romanes

EVOLUTION OF THE COLORS OF BIRDS. 115

for the first time elaborated it and pointed out some of
its bearings upon the doctrine of natural selection. Mr.
R. Meldola was one of the first critics of the new theory.*
He asserted that the principal difficulty pointed out by
Mr. Romanes was “the sterility of natural species as
compared with the fertility of domesticated races.” In-
asmuch as the struggle for existence is most intense
between those individuals which are most alike, any-
thing which would cause them to vary would be an
advantage which natural selection would make use of.
The prevention of intercrossing by inter-sterility would
be such an advantage and natural selection would
accordingly favor it.

Mr. Francis Galton, soon after offered another sug-
gestion regarding physiological selection.t According
to his view certain individuals of a race are isolated
from the rest by having similar sexual likes and dislikes
which cause them to interbreed rather than to mingle
with the rest of the tribe. In replying to the above two
criticisms, Mr. Romanes attempts to show that neither
Galton nor Meldola have really opposed his views. Mr.
Galton had objected to physiological selection on the
ground that the intersterility which it presupposes would
not be externally apparent, and consequently many in-
fertile unions would result. There is nothing to show,
however, that infertile unions do not take place at times,
and Mr. Galton’s hypothesis of psychological selection
simply supplements among the higher animals the more
universal factor of physiological selection.

In replying to Mr. Meldola’s criticism, Mr. Romanes
shows that it is not his theory of physiological selection
which is called in question, but simply whether natural
selection may not in every case underlie and condition

* Nature xxxiv, pp. 384-395.
tle. pp. 395-396.

116 CALIFORNIA ACADEMY OF SCIENCES.

it. ‘But surely,” he says, ‘‘the burden of proof here
lies on the side of my critic. If he can show any suffi-
cient reason for going much farther than I have ventured
to go in out-Darwining Darwin—or for holding that
natural selection may not merely help in inducing ster-
tlity in some cases, but has been the sole cause of it in
all cases—then I should welcome his proof as showing
that the principles of physiological selection ultimately
and in all cases rest on those of naturalselection. But,
clearly, it is for him to prove his positive: not for me to
prove what I regard as an almost preposterous neg-
ative.’”’*

Mr. Meldola also noted the difficulty that physiologi-
cal selection must always be subservient to natural
selection, because if a race developed through isolation
did not possess some advantage over the main stock—
the struggle for existence being most severe among most
closely related forms—it would not be able to compete
with the dominant type of the species. In reply to this
Mr. Romanes states that if the character distinguishing
the new form be indifferent as regards utility the in-
dividuals possessing it will be on an exactly equal foot-
ing with those which do not possess it, more especially
if as at first, the variation be simply of the reproductive
organs. The fact that the individual possessing this
variation has reached the breeding age is in itself a
guarantee of its fitness to survive, and it was to empha-
size this fact that the alternative name of the ‘‘ segrega-
tion of the fit” was proposed.

Mr. Wallace has however presented a more sweeping
and serious line of criticism than any of the preceding,
and it is to this that attention must next be directed.
His objections were stated in an article entitled ‘‘Romanes
versus Darwin. An Episode in the History of the

*).e. p. 408.

EVOLUTION OF THE COLORS OF BIRDS. 117

Evolution Theory.” * Mr. Wallace objects most em-
phatically to the view that any specific characters are
without use. He points to the progress which has been
made of late years in discovering the use of structures
or characters which were formally thought to be of no
utility, and calls particular attention to the colors of the
higher vertebrates as illustrating his point. He men-
tions the fact that while the colors of wild animals are
so generally constant (white or pied varieties being
speedily obliterated) domestic breeds exhibit the greatest
inconstancy and diversity in this respect; proving, as
he thinks, that the colors of all wild animals must be
useful, even though we cannot always see how they are
so, and that their lack of uniformity under -domestica-
tion is due to the fact that they are no longer of utility.
He then gives illustrations of the various forms of
adaptive markings—protective and warning colors and
recognition markings. He says: ‘‘ This need of easy
recognition by each species of its own kind and of the
sexes by each other, will probably explain at once those
slight diversities of colour and marking, which, more
commonly than any other characters distinguish closely
allied species from each other, and also the constancy
and bilateral symmetry of the colouration of wild ani-
mals.”

With regard to the swamping effects of free intercross-
ing, Mr. Wallace says the difficulty is removed by con-
ceding ‘‘that the same variation occurs simultaneously
in a number of individuals inhabiting the same area.”
Mr. Romanes had denied this, but Mr. Wallace presents
an array of facts taken from Mr. J. A. Allen’s Winter
Birds of Florida, in support of his contention.

So far as the infertility of species with one another
is concerned, Mr. Wallace denies that the facts support

*Fortnightly Review, xlvi, pp. 300-316.

118 CALIFORNIA ACADEMY OF SCIENCES.

Mr. Romanes’ side of the argument. He points out
that experiments in hybridizing are generally made
with very distinct species, and that even these are some-
times fertile, while if two closely related forms hybridize
they are said to be one species. He quotes Dean Her-
bert’s experiments as proving that in several large
genera of plants not only are the hybrids often fertile
but sometimes more so than the parent stock, whence
he concludes ‘‘that the sterility or fertility of the
offspring does not depend upon original diversity of
stock; and that if two species are to be united in a
scientific arrangement on account of a fertile issue, the
botanist must give up his specific distinctions generally,
and intrench himself within genera.”

‘* Really close species,” says Mr. Wallace, ‘‘ which have
probably originated by one remove from a common an-
cestor have never yet been crossed in large numbers and
for several generations, under appropriately natural con-
ditions, so as to afford any reliable data. The mere fact
that not only animals of distinct genera, but even those
classed in distinct families—as the pheasant and the
black grouse—sometimes produce hybrid offspring in a
state of nature, is itself an argument against there being
any constant infertility between the most closely allied
species, since if that were the case we should expect the
infertility to increase steadily with remoteness of descent
till when we came to family distinctions absolute sterility
should be invariable.”

To these criticisms of Mr. Wallace, Mr. Romanes
replied in an article in the Nineteenth Century,* enti-
tled Physiological Selection. He freely grants that Mr.
Wallace has proved that some specific characters are
useful, but insists that this is not sufticient to prove that
all points of specific difference are so. The theory of

"xxi, pp. 59-80.

EVOLUTION OF THE COLORS OF BIRDS. 119

natural selection is based upon the assumption that cer-
tain characters are useful, but we are not justified in |
arguing from this that all characters have been devel-
oped by natural selection and are therefore useful.
Darwin himself freely conceded the inutility of many
characters, and, as Mr. Romanes says, ‘‘ there is positive
evidence to show that the slight changes of form and
colour which chiefly serve to distinguish allied species
are often due to what Mr. Darwin calls ‘the direct action
of external conditions,’ such as changes of food,
climate, etc., as well as to mere independent variation
on isolated areas, and in some of our domesticated pro-
ductions, etc.; and in none of these cases do the specific
changes which result present a meaning of any kind.”
In refuting Mr. Wallace’s argument on the utility of
color Mr Romanes, in a footnote, quotes Darwin to the
effect that ‘‘each of the endless variations which we see
in the plumage of our fowls must have had some
efficient cause; and if the same causes were to act
uniformly during the long series of generations on many
individuals, all probably would be modified in the same
manner,” and he adds: ‘‘The obvious truth of this
remark serves to dispose of Mr. Wallace’s argument in
the Fortnightly, that ‘the general constancy of colouration
we observe in each wild species,’ of itself furnishes
sufficient proof that the colouration must be ‘a useful
character.’ Moreover, when using this argument Mr.
Wallace forgets that uniformity of colouration (whether
useful or unuseful) is preserved in wild species by free
intercrossing. Where this is prevented—as by isola-
tion or migration—variations of colour very frequently
do take place, just as in the then analgous case of our do-
mesticated strains.”

Concerning the swamping effects of intercrossing, Mr.
Romanes shows that the variations cited from Mr.

120 CALIFORNIA ACADEMY OF SCIENCES.

Allen’s paper as quoted by Wallace would not be suffi-
cient to originate any new structure. He says: ‘‘It is
easy to see how natural selection could alter the general
size of the body, the relative sizes of parts, degrees of
colouration, etc., without encountering any great diffi-
culty from intercrossing. But if it were required to
produce, say, a fighting spur on a duck, clearly it could
not be done by natural selection alone, or when depend-
ing only on ‘accidental variations.’”” Mr. Romanes has
here granted far too much to his eritic; for, if physio-
logical selection is not a necessary factor in the modifi-
cation of size, shape (relative sizes of parts), and color,
all closely related species which depend upon such
features for their specific identity (and they constitute a
very large proportion of species), would be excluded from
the operation of the segregation of the fit. Nor would
this factor particularly assist in the production of spurs
-and horns, for example. Jt is thought by many that
such structures are due to the factor of use, in which
case physiological selection would indeed become re-
stricted in its function.

In replying to Wallace’s criticisms of his view of
sterility between species, Mr. Romanes says: ‘‘ Under
this head Mr. Wallace’s criticism amounts to nothing
more than a vague suggestion to the effect that all other
naturalists may have hitherto exaggerated the generality
of some degree of sterility between species. But he
allows that it is ‘a widespread phenomenon,’ and gives
no reasons for differing from Mr. Darwin’s careful esti-
mate of its frequency, he does not really furnish me
with any material to discuss.”

Mr. Seebohm in the introduction to his work entitled
The Geographical Distribution of Charadriidiwv, also
makes a sweeping criticism of physiological selection,
and as he and Mr. Wallace argue on the same lines upon

EVOLUTION OF THE COLORS OF BIRDS. 121

certain points, it may be well to pass on now to a brief
survey of his objections. The central idea of his argu-
ment is that variations are never spontaneous, but
always due to a definite cause, so that ‘‘ the inevitable
new species will be produced even in defiance of
increased fertility between the diverging forms.” He
lays great stress upon geographical isolation, however,
and doubts that any new species of bird has arisen with-
out the aid of this factor. He propounds a rather novel
explanation of the mutual infertility of natural species
as compared with the fertility of domesticated breeds.
Briefly stated it is this: Domestic animals are species
which from some unknown reason are capable of adapt-
ing themselves to a great variety of circumstances.
Just as the individual as a whole is capable of surviving
in various environments, so also, it may be supposed,
the germ cell of the male of such individuals would have
a greater vitality, and would be capable of surviving in
an unusual ovarian environment. A proportion is thus
established between the individual and its seed—as the
stable individual is to the plastic individual so is the
reproductive element stable or plastic. He calls atten-
tion to the fact that in a state of nature fertile hybrids
occur most frequently among the Phasianide and
Anatide, ‘the very families to which most of our do-
mestic birds belong.”’ This is certainly a very plausible
and ingenious theory, and 1 am not familiar with any
attempted refutation.

Unlike Mr. Wallace, Mr. Seebohm freely grants that
‘‘ specific differences are frequently, if not usually, with-
out utilitarian significance,” but he considers that this is
due to the fact that they, have been brought about by
definite variation witbout reference either to natural or
physiological selection. Upon one point, however, he
and Mr. Wallace agree, namely, that granting all that

122 CALIFORNIA ACADEMY OF SCLENCES.

physiological selection demands, the chance of all the
necessary contingencies arising contemporaneously
would be so slight as to make the theory unworkable.
Mr. Seebohm says: ‘‘To make it work we must pre-
suppose:—lIst, the special variation of the reproductive
organs must occur in two inviduals, otherwise the pos-
sible ancestor of the new species would have no
descendants; 2nd, it must occur at the same time in both;
3d, it must occur at the same place; 4th, the two indi-
viduals must be of opposite sexes; 5th, they must each of
them possess some other variation, or their progeny
would not differ from that of the rest of the species; and
6th, the variation must be the same in both, or appear
simultaneously in the majority of their children, other-
wise it would be swamped by interbreeding within the
physiologically isolated family.” Obviously Mr. See-
bohm does not think it possible for all these contingen-
cies to be realized, and that from their failure to
co-operate the theory of physiological selection must
break down.

Along the same lines as this Mr. Wallace also objects
to the segregation of the fit. In reply, Mr. Romanes
asserts that while it is true that the chances against
these physiologically isolated individuals mating may
indeed be thousands to one, so is it also true that the
number of fertile unions among animals com-
pared to the origination of every new species is in the
proportion of thousands to one, and he says: ‘‘I con-
fess it appears to me a somewhat feeble criticism to
represent that the conditions which my theory requires
for the origin of a new species are probably about
as rare in their occurrence as is the result which
they are supposed to produce.”” He furthermore states
that the variations in fertility of the sexual organs are
not due to chance, as his critics have assumed. They

EVOLUTION OF THE COLORS OF BIRDS. 123

may be due to the change in season of flowering or pair-
ing, or to difference in food or climate, or they may even
be originated by natural selection itself. Indeed, Mr.
Wallace has suggested as an ‘‘ alternative hypothesis”
to physiological selection, that specific sterility is due to
a relation existing between the external coloring and
sexual compatibility, so that whenever a change in color
is produced, the individuals so changed will not be fer-
tile with the parent stock. Mr. Romanes clearly shows
that instead of being an alternative hypothesis, if
tenable at all, it merely furnishes us with an additional
indirect cause of the infertility demanded by physio-
logical selection. Mr. Romanes gives a number of
forcible reasons for not granting this theory the promi-
nent function attributed to it by Mr. Wallace. Thus he
asserts that ‘‘many species which are mutually sterile
differ very little in color,” while ‘‘ most species which
are mutually fertile differ considerably ” in this respect.
Furthermore, ‘‘in the case of natural species, it often
happens that a great difference in respect of fertility
occurs, according to which has acted as the male and
which as the female, yet in both these crosses the colour
of each species is, of course, the same.”

The controversy which Mr. Romanes had with Mr.
W. Thiselton-Dyer, in the columns of Nature in 1889,
respecting phySiological selection, did not serve to eluci-
date many new points. Mr. Dyer’s objections to the
terms physiological selection and the segregation of
the fit, need not here detain us, for Mr. Romanes had
previously justified their use. The discussion centered
upon the relation between natural and physiological se-
lection. Mr. Romanes has so clearly expressed this in
an admirable figure that to quote it, will be a sufficient
epitome of the argument. He says: ‘‘In short, species
are like leaves, successive and transient crops of which

124 CALIFORNIA ACADEMY OF SCIENCES.

are necessary to the gradual building up of adaptations,
while these, like the woody and permanent branches,
grow continuously in importance through all the tree of
life. Now, it is the office of natural selection to see to
the growth of these permanent branches; physiological
selection has to do only with the deciduous leaves.”

Having now stated the principal arguments for and
against physiological selection, with as little bias as
possible, it only remains to pass judgment upon their
merits. This, however, does not appear to be an easy
task; but as usual in such cases, it will probably be
found that both sides are partly in the right and partly
wrong. Thus, with regard to sterility, natural species
are as a general thing sterile inter se, while domestic
races are usually fertile, but these conditions are by
no means uniformly constant. Physiological isolation
might explain this state of affairs, while Mr. Seebohm’s
hypothesis has quite an aspect of plausibility,—or both
views might be correct; whereas, various secondary
factors have been instrumental in bringing about this
condition. There seems to be hardly any evidence,
however, that sterility with the parent stock is generally
one of the first variations of a diverging race, but if this
be not so, then the chief aim of the theory—viz. to
account for the early stages of divergence—fails.

Isolation may be advantageous in preventing the
swamping effects of free intercrossing, although Mr.
Romanes has granted that the size, shape, or degree of
coloration, may be modified without its aid. Further-
more, geographical isolation is an ever-present factor,
while a slight change in habits on the part of certain
individuals would effect their isolation. psychological
isolation, as suggested by Galton, may also be a valid
factor, while the operation of physiological selection
itself need not be excluded.

EVOLUTION OF THE COLORS OF BIRDS. 125

Thirdly, some specific characters, at least, are ap-
parently of no conceivable use, but these may be due
to the direct action of the environment or to the use
and disuse of parts.

As regards the bearing of the above discussion upon
the evolution of the colors of birds, in the light of the
evidence adduced, it would hardly seem justifiable to use
physiological selection, unless the facts admitted of no
other rational explanation. It is always an easy loop-
hole for escape in accounting for any difficult character,
and for this very reason should be treated with especial
caution.

GULICK ON ISOLATION.

One of the most valuable contributions made of late
years to the literature of evolution is the series of
articles upon isolation by the Rey. J. T. Gulick. His
views coincide with those of Mr. Romanes upon many
important points, while their theories overlap to a cer-
tain extent. Without considering in detail the relation-
ship between their respective views, it may be said that
they both agree that many specific characters are use-
less, and that isolation is a necessary factor in their
origination, but Gulick does not emphasize physiological
isolation as Romanes has done; while he asserts, con-
trary to Romanes, that not only species, but larger
groups as well require isolation as a factor in their
origin. His central idea is this: no two groups of
individuals of one species contain exactly the same pro-
portion of variations in their aggregate number; accord-
ingly, if two or more miscellaneous groups are isolated,
inasmuch as free intercrossing is no longer practicable
to preserve the average, each division will necessarily
start off on an independent line, irrespective of its
environment. Futhermore, natural selection without
isolation could not produce divergent, but only linear

126 CALIFORNIA ACADEMY OF SCIENCES.

evolution. Natural selection implies a contest between
a more and less favorable class of variations, in which
the less favorable class perish. As an example, a race
horse might be evolved by artifical selection from a
common stock, but in order to produce a dray horse
from the same stock, the two varieties would have to be
isolated from the earliest stages of their divergence.

The tendency of this view of Mr. Gulick’s is to over-
estimate the self-sufficiency of the organism to originate
new forms by virtue of an assumed inherent tendency
to vary indefinitely. In other words, he sometimes
appears to assume, in conformity with Nageli, that each
organism contains within itself the potentiality of devel-
oping all the forms which may be subsequently derived
therefrom; that free intercrossing suppresses this ten-
dency to vary, but isolation enables each group to
develop any little idiosyncrasies toward which it may
have a leaning, and by successive stages of isolation
these are piled up into new characteristics or structures.
Thus, in summing up the result of his survey of the
various forms of selection, he finds: *‘‘ First, that all
the forms of Reflexive Selection are due to the relations
of members of the same species to each other, and are
liable to change without any change in the environ-
ments. Second, that Active Natural Selection is due to
change in the successful use of the powers of the organ-
ism in dealing with the environment, and is not depend-
ent on change in the environment. Third, that Passive
Natural Selection, which is due to the exposure of the
organism to a different environment, is often produced
by the organism’s entering a new environment without
there being any change in either the new or the old
environment. Fourth, that when Passive Natural Selec-

* Journ, Linn. Soc. xxiii, p. 337.

EVOLUTION OF THE COLORS OF BIRDS. 127

tion is produced by change in the environment, the
more effective forms of Selection do not appear till the
organism has so multiplied as to produce what I call
Superlative Natural Selection through intense competi-
tion between rival individuals of the same species in
gaining possession of limited resources. And, fifth,
that Passive Comparative Natural Selection, which de-
pends on change in the environment, without special
rivalry between the members of one species, also depends
on variation in the adaptations of the organism, many
of which variations do not depend on that change in the
environment which has produced the change in the
Natural Selection, nor, indeed, on any change in the
environment except those physical changes by which
the world has passed from its primitive gaseous to its
present partially liquid and solid state, rendering it a fit
abode for organisms.”

He thus entirely ignores the origin of variations and
assumes that inasmuch as the environment in the vari-
ous forms of selection cited does not produce the varia-
tions definitely and directly, it can have no influ-
ence whatever in their origination. If the discussion
of variation in the preceding pages has any force what-
ever, this conclusion certainly should not seem an in-
evitable one.

An attentive consideration of Mr. Gulick’s views seems
to make one fact undeniable, viz. that some form of
isolation is indispensable to divergent evolution. His
elaboration of the various forms of isolation is so exten-
sive that anything more than a brief outline of it
would be impossible in the present connection. This
much is, however, necessary to even a general under-
standing of the factors of organic evolution. The fol-
lowing is Mr. Gulick’s classification table of the forms
of segregation:

128

CALIFORNIA ACADEMY OF SCIENCES.

A. Environal Segregation.

(a) Industrial Segregation.

Sustentational.
Defensive.
Nidificational.

(b) Chronal Segregation.

Cyclical.
Seasonal.

(c) Spatial Segregation.

Geographical. /
Local.

Migrational.
Transportational.
Geological.

(d) Fertilizational Segregation.
(e) Artifical Segregation.

B. Reflexive Segregation.

(a) Conjunctional Segregation.

Social.
Sexual.
Germinal.
Floral.

(b) Impregnational Segregation.

Segregate Size.

Segregate Structure.

(c)

Prepotential Segregation.
Segregate Fecundity.
Segregate Vigour.
Institutional Segregation.

C. Intensive Segregation.

(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)

Assimilational Intension.
Stimulational Intension.
Suetudinal Intension.
Correlated Intension.
Integrational Intension.
Selectional Intension.
Fecundal Intension.
Eliminational Intension.

EVOLUTION OF THE COLORS OF BIRDS. 129

The following brief definitions will explain the above
terms:

A. EwnvironaL Srcreaation—lIsolation due to the
relations of organism to environment.

(a) Industrial Segregation. ‘‘ Aetivities by which
the organism protects itself against adverse influences
in the environment, or by which it finds and appro-
priates special resources in the environment.”

1. Sustentational. ‘‘ Different methods of obtaining
sustentation by members of the same species.”
2. Defensive. ‘‘ Different methods of protection

against adverse influences in the environment.”

3. Nidificational. Differences in nesting habits in
the same species.

(b) Chronal Segregation. ‘Segregation arising from
the relations in which the organism stands to times and
seasons.”

4, Cyclical Segregation. Among certain insects
‘the life cycles of the different sections of the species
do not mature in the same years.”

5. Seasonal Segregation. Change in time of flower-
ing or breeding of a section of a species.

(c) Spatial Segregation. Isolation in space.

Geographical. Physical or climatic barriers to free
intercrossing.

Local. Due to the wide distribution of species with
inadequate powers of locomotion or migration.

Spatial segregation may also be subdivided in con-
formity with the causes producing it, thus:

6. Migrational, ‘‘caused by powers of locomotion in
the organism.”

7. Transportational, ‘‘ caused by activities in the en-
vironment that distribute the organism in different dis-
tricts.”

8. Geological, ‘‘caused by geological changes divid-
9

130 CALIFORNIA ACADEMY OF SCIENCES.

ing the territory occupied by a species into two or more
sections.”

(d) 9. Fertilizational Segregation. Repugnance of
insects to. visiting different plants interchangeably pre-
vents hybridization.

(e) Artificial Segregation. Artificial selection.

B. REFLEXIVE SEGREGATION. ‘‘ Segregation arising
from the relations in which the members of one species
stand to each other.”

(a) Conjugational Segregation. Due to ‘‘the in-
stincts by which organisms seek each other and hold
together in more or less compact communities.”

10. Social. ‘‘ Produced by the discriminative action
of social instincts. The law of social instinct is prefer-
ence for that which is familiar in one’s companions;
and, as in most cases the greatest familiarity is gained
with those that are near of kin, it tends to produce
breeding within the clan,” etc.

11. Sexual. ‘ Produced by the discriminative action
of sexual instincts.”

12. Germinal. ‘Caused by the propagation of the
species by means of seeds or germs any one of which,
when developed, forms a community so related that the
members breed with each other more frequently than
with the members of other communities.”

13. Floral. Self-fertilization of flowers.

(b) Linpregnational Segregation. Physiological se-
lection par ercellence,

14. Segregate Size. Inability of varieties to inter-
breed, owing to difference in size.

15. Segrevate Structure. Due to “lack of correla-
(ion in the proportionate size of different organs, and
by other incompatibilities of structure.”

16. Potential and Prepotential. ‘Caused by more
vt less free distribution of the fertilizing element to-

EVOLUTION OF THE COLORS OF BIRDS. 131

gether with the greater rapidity and power with which
the sexual elements of the same species, race, or indi-
vidual combine, as contrasted with the rapidity and
power with which the elements of different species,
races, or individuals combine.”

17. Segregate Fecundity. Relation of species or
varieties inter se, when free crossing between ‘‘ members
of the same species or variety results in higher fertility
than the crossing of different species or varieties.”

18. Segregate Vigor. ‘‘ Relation in which species
or varieties stand to each other when the intergenera-
tion of members of the same species or variety produces
offspring more vigorous than those produced by crossing
with other species or varieties.”

(c) Institutional Segregations. ‘‘ Produced by the
rational purposes of man embodied in institutions that
prevent free intergeneration between the different parts
of the same race.”’

C. Intensive SecreGATION. When two or more
sections of a species are mutually isolated by any of
the above forms of segregation and are subject to the in-
fluences of a highly complex transforming agency, there
is a reasonable certainty that the different divisions will
be differently acted upon. In succeeding generations
there will be increasing degrees of divergence and seg-
regation, which is termed the law of Intensive Segrega-
tion. The different forms of intension are due to
difference in food, influence of the environment, use
and disuse, correlation of growth, selection, etc. In
this case, it will be noticed that Mr. Gulick takes due
notice of the influence of the environment, despite his
assertion that variations may, in many instances, be
wholly independent of it.

It is time to bring this introduction to a close
and hasten on to the more special considerations for

132 CALIFORNIA ACADEMY OF SCIENCES.

which it is a preparation. The idea which I have
attempted to emphasize in the preceding pages is that
life has evolved in accordance with tolerably definite
and unvarying laws, and that the element of chance, if
any such there be, is a very limited one. The living
forms of to-day are looked upon as the result of the
interplay of organism and environment, progress being
urged on by the laws of development and held in check
by the laws of heredity; while the organic cosmos is
under the ceaseless and untiring scrutiny of natural
selection, which, like a wise mother, is perpetually pass-
ing judgment on the acts of her children and abmonish-
ing them when they go astray.

JI. THe Cotors or Norta AMERICAN Brirps.
ON MODES OF PLUMAGE CHANGES.

Tam not aware of any important addition to the sub-
ject of the changes of plumage in birds since William
Yarrell, in 1835, published a paper entitled ‘“‘ Observa-
tions on the laws which appear to influence the Assump-
tion and Changes of Plumage in Birds.”* He there
distinguished four ‘modes by which changes in the
appearance of the plumage of birds are produced ”’:

(1.} ‘By the feather itself becoming altered in
color.”

(2.) ‘* By the bird’s obtaining a certain number of
new feathers without shedding any of the old ones.”

(8.) ‘By an entire or partial moulting, at which old
feathers are thrown off and new ones produced in their
places.”

(4.) ‘‘ By the wearing off of the lengthened lighter
colored tips of the barbs of the feathers on the body, by
which the brighter tints of the plumage underneath are
exposed.”

* Trans. Zool. Soc. of London, i, pp. 13-21.

EVOLUTION OF THE COLORS OF BIRDS. 133

This first mode of change is doubted even by many
ornithologists of to-day, and yet the testimony presented
in Yarrell’s paper seems to be conclusive on the point.
To be sure, it does seem difficult to understand how a
feather, in which there is no circulation, in fact no life,
apparently, can alter its color. Mr. Yarrell tried the
experiment of marking certain feathers in a bird which
he suspected to be changing color, and succeeded in
actually observing the addition of pigment. He also
says: ‘‘ On the breasts of several golden Plovers, some of
the feathers were entirely white, the colour peculiar to
all the feathers of that part of the bird in winter; some
were entirely black, being the colour assumed at the
breeding season; while others bore almost every possible
proportion of well-defined black and white on the same
feathers; from which it appears that the same cause of
particular colour in new feathers can also partially or
entirely change the colour of old ones.”

Mr. F. O. Johnson first called my attention to the
tails of some Arizona hooded orioles (Icterus cucullatus
nelsoni) in his collection. They represent various
stages of transition from yellow to black. I have since
examined all the specimens of this bird in the National
Museum and the American Museum of Natural History
in New York, besides a number in private collections,
and from this ample material cannot but conclude that
the transition in this species is by an addition of pig-
ment without moult. Plate VI shows a tail in a state
of change. The primitive color of the tail in this
species is yellow, but in the adult male entirely black.
The tail figured was of an immature male in changing
plumage. The feathers are still yellow for the most
part, but one has become almost completely black, only
apart of the tip being left yellow. In other feathers
little spots of black were visible, while in still others the

134 CALIFORNIA ACADEMY OF SCIENCES.

general shade of a part of the feather was dusky, as if
only a thin layer of black pigment had been deposited.
In other specimens various stages of transition were
observable, from the example figured to instances
where the entire tail was black with simply a few little
isolated irregular patches of yellow.

Dr. C. Hart Merriam has published an instance of a
seasonal change of color occurring by the introduction
of pigment without moult, although his views are not
accepted by Mr. Ridgway in his Manual. With regard
to Allen’s ptarmigan (Lugupus lagopus alleni), the bird
in question, Dr. Merriam writes:* ‘‘The large series
of wings sent by Mr. Comeau demonstrates beyond a
question that individual feathers do change color. Most
of them are already pure white excepting the shafts of
the six outer primaries, which, as usual in winter speci-
mens of Lagopus albus, are black. The quantity of
black varies greatly in the different wings. In thoxe in
which the change is most advanced it is merely a nar-
row strip of pale sooty-brown, extending along the
middle of the upper surfaces of the shafts of the six
outer primaries, and is confined to the middle half of
the exposed part of each, so that the basal half, and a
considerable apical portion, together with all the rest of
the wing, is pure white,” etc.

Mr. Witmer Stone called my attention to specimens
of Terpsiphone cristute trom Western Africa in the col-
lection of the Philadelphia Academy of Sciences, in
which an apparent transition of color of the tail without
moult is shown, but of an opposite character from the
instances above cited, viz.: from dark to light. The
tail in the first stage is colored a reddish brown or
rufous, this color changing to black, and the black to
white, apparently without any moult. In the orioles

* Auk. ii, pp. 202-203.

EVOLUTION OF THE COLORS OF BIRDS. 135

this change does not appear to be confined to the tail,
but the black patches of the head and throat seem to be
developed in the same manner. Thus figure 11 of
Plate I shows a feather from the throat of Icterus
cucullatus nelsoni which is changing from yellow to
black. Figure 12 represents a feather from the back of
the head of J. bullocki, taken from an immature male of
the second year, in which the black was appearing in
spots upon a field of yellow.

It is impossible to say without more complete investi-
gations, and perhaps some experiments upon living
birds just how general this change by increase or de-
crease of pigment is, but it is probably quite a limited
mode of transition.

The fourth mode of change, 7. ¢., by the wearing off
of the dull tip of the feather, leaving the bright under
part exposed, is a very common one, among land birds.
Dr. Coues has called my attention to a term in the Cen-
tury Dictionary covering these two classes of change—
aptosochromatism, which is defined as change of color
of the plumage without loss or gain of any feathers.
He very kindly suggested two new words to indicate
these two forms of aptosochromatism; acraptosis, mean-
ing the falling off of the tip of the feather, and acropto-
sis, implying that the tip is not cast. Plate I shows a
number of acraptotic feathers. Figure 6 isa frontal
feather of the western yellow-throat (Geothlypis trichas
occidentalis) in winter plumage. The basal portion of
the feather is much darker than in most feathers, in fact
almost black, the median portion a clear ash, and the
tip brown. This brown terminal third is inclined to be
worn and very brittle. It is so broad that it completely
conceals the under portion, but in spring it all crumbles
away leaving nothing but the clear ash. Along the line
of demarkation of black and ashy the black base is

136 CALIFORNIA ACADEMY OF SCIENCES.

sometimes broader and the ashy edging so narrow that
the tip wears down to the black. Figure 7 is a feather
from the back of the head of the western warbler
(Dendroica occidentalis), in winter plumage. In this in-
stance the brown edging wears off leaving a yellow
feather with a black dot at the tip. In full plumaged
birds all this black wears away leaving the head uni-
form yellow. Figure 8 is a feather from the breast of
the rusty blackbird (Ncuolecophayus carolinus), in winter
plumage. In the above two instances the brown was so
complete as to produce a uniform color, but with the
rusty blackbird it is less regular, causing a mottled effect.
The brown tip wears off in spring leaving the plumage
clear black. Perhaps thetwo most interesting examples
of all, however, are represented in figures 9 and 10 of
feathers from the backs of the two species of snow-bunt-
ings (Plectrophenury nivalis and P. hyperboreus), nearly
in breeding plumage. In P. nivulis (fig. 9), the broad
white terminal portion has almost entirely worn uway
leaving the black base exposed, but it does not wear off
thus in P. hyperboreus, the black remaining white
through the breeding season.

Mr. Yarrell says: ‘‘ Young birds of the year in vari-
ous species, after the autumn moult, continue through
the winter to assume, by degrees, the more intense
colours characteristic of adults, without changing the
feather. This colour commences generally at that part
of the web nearest the body of the bird, and gradually
extends outwards till it pervades the whole feather.”
He calls attention to the influence of the sexual organs
in changing plumage. Many birds appear to become
more brilliant in color as the breeding season ap-
proaches without either a moult or the wearing away of
the tips of the feathers (vcraptosis). This I believe is
the case with the house finch (Curpodacus inesicunis
frontalis).

EVOLUTION OF THE COLORS OF BIRDS. 137

Changes of plumage, then, may be classified as fol-

lows:
I. Substitution of feathers.

1. By renewal (moult).

2. By addition.
Il. Modification of feathers (aptosochromatism).

3. By shedding of tip (acraptosis).

4. By change of pigment (acroptosis).

Different pigment.

a. By addition. : : ,
- "oO" ) Intensification of same pigment.

b. By loss.

The first class, namely, the substitution of feathers,
has not been discussed because its bearing upon our
subject is less direct, and because it is more familiarly
known. It need only be noted here that all birds have
an autumn moult, while some have a more or less com-
plete spring moult in addition, either in both sexes or
in the male only.

GENERAL PRINCIPLES OF COLOR IN BIRDS.

Having devoted so much space to the fundamental
problems underlying all transitions in animals, it will
not be possible to consider at any length the general
principles of coloration in nature. Indeed, the subject
has already been touched upon in discussing sexual
selection. However, a few words more are necessary.

Colors may be classed, as Poulton has suggested, as
structural and pigmental, depending upon the manner
in which they are produced. The structural colors are
of three sorts—those due to thin plates, one upon an-
other; those due to diffraction, where the surface is
broken by fine parallel grooves; and those due to re-
fraction, where the light passes through a transparent
wedge-shaped substance or prism. All of the above

1388 CALIFORNIA ACADEMY OF SCIENCES.

classes are due to the reflection and interference of light
waves, and probably all occur to a greater or less extent
among birds, although authorities disagree as to which
form is the commonest. The iridescent colors are due
to these structural peculiarities. White is due to the
presence of minute bubbles of gas contained in the
feather which breaks the structure so that the light is
refracted in all directions, much on the principle of
pulverized glass. The worn, dull plumage of birds, at
the close of the breeding season, is due to a similar
though coarser form of this same agency. Frequently
the outer edge of the wing feathers becomes dull brown-
ish, or even nearly white, from becoming worn and
abraded, even though its natural color be black.*

The vast majority of color effects, particularly among
birds, are due to chemical pigments. Many of these
pigments have been chemically analyzed and named,
and it is found that the same color in different species
is not always due to the presence of one pigment, but
to different ones. ‘‘ Thus,” says Beddardt ‘“ the brown
colour of birds is chiefly due not to one pigment, but to
two apparently distinct pigments, which give different
chemical reactions; to these two pigments their discov-
erer, Kriikenberg, has given the name Zoorubin and
Pseudozoorubin. An inspection of the feathers would
not enable one to tell with certainty which of the two
substances was the cause of the colour; but un extract of
zoorubin can always be detected by its change to a beau-
tiful cherry red on the addition of the minutest trace of
blue sulphate of copper.”

The theory of bird colors which I shall attempt to
elaborate in the succeeding pages is as follows: Pig-

* For fuller discussion of structural colors, sce Dr. Gadow, Proc. Zool.
Soc., 1882, p. 409; Poulton, Colors of Animals, pp. 3-11.
+ Animal Coloration, pp. 2-3.

EVOLUTION OF THE COLORS OF BIRDS. 139

ment is a chemical composition thrown off from the
system of the bird, probably as a product of waste, and
lodged in the integument. The chemical substance thus
generated varies in different groups of birds, but is
probably generally constant in the species of one genus,
or frequently in an entire family. A certain genus
would thus be capable of generating only a given num-
ber of fundamental colors, but natural and sexual
selection by combining and rearranging this limited
assortment, can produce a great variety of effects. Be-
sides black and white, most North American genera
appear to contain but two fundamental colors, but in the
tropics, three frequently occur in a single genus. If
the pigment be regarded, to some extent at least, asa
product of waste due to superabundant vitality, as Wal-
lace maintains, it is not difficult to understand how
among some tropical birds a. third chemical product
might be generated in addition to the usual two. As an
illustration of this view, let color be compared to a
variety of skin diseases. Hach genus of birds in gen-
eral, we may say, developes two kinds of skin disease
which are bound to be in the system of every species.
One of them may sometimes be suppressed or they may
be combined in such a way as to be no longer recogniz-
able, but they will still be present.

This theory could not be demonstrated without further
study of the chemical properties of pigment. If it
could be shown that in closely allied species the color,
although the same, was produced by different pigments;
or that in the same or closely allied species, where one
color gave the appearance of being a combination of two
other colors which occurred in the same or allied species,
there was no real connection in the pigments, chemi-
cally; the theory would certainly become very weak.
Until such experiments have been made, however, it is

140 CALIFORNIA ACADEMY OF SCIENCES.

necessary to depend upon appearances, and here there
are many facts that seem to support the view. Thus
Plates XVIII and XIX seem to be in accordance with it.
It will be noticed that in the genus Dendroica olive green
is, especially in the female and young, the prevailing
color of the group. It is not a little significant that in
coloring these two plates it was found that in order to
produce this color all that was necessary was to mix
black with gamboge yellow, in about equal proportions
—black and gamboge being the two most characteristic
colors of the specialized males.

It might at first sight appear as if this was a modifi-
cation from complexity to simplicity, but this is not
really the case. If the system of the bird from which
the present genus Dendroica was evolved normally pro-
duced two pigments which were lodged in the integument,
unless some controlling and distributing force were
brought to bear upon them, they would naturally be
mixed and combined at first, and only after a long pro-
cess of selection would the real colors become apparent.

This principle may be termed the Law of Assortment
of Pigments. If true at all it will be found to have a
very wide applicability in the evolution of colors of
birds, and, indeed, to underlie all the other principles
of color differentiation, although, of course, conditioned
by all the more general laws of evolution. The best
examples of it are to be found, as might be expected,
among the most highly specialized genera, where the
colors have changed from some dull hue to distinctly
colored patterns. Thus woodpeckers are generally col-
ored black and white and scarlet. The combination of
the black and scarlet would produce a brown, which is
still the body color of some species, as Dryubutes arizonw.
In the genus Tyrannus the original color was probably
an olive green, although in many instances where the

EVOLUTION OF THE COLORS OF BIRDS. 141

bird is found in arid districts, this color has been
bleached out to a brownish or grayish. The more
special colors are black and yellow. The male of
Pyrocephalus rubineus mexicanus is scarlet and brownish
black. The female is colored a plain brown olive,
which is a mixture of red and black. The blue jays are
generally colored some shade of blue in the adult,
together with black and white, but a mixture of these
shades would produce the grays of the young and of the
less specialized forms as Perisoreus and Picicorvus. The
adult male red-winged blackbirds are black and scarlet,
the female brown. The meadow lark is brownish or
grayish in its generalized color, and black, white and
yellow in the specialized shades. I am not certain that
the combination of black and yellow (olive-greenish)
together with white would produce the grayish-brown of
the back, but if this instance be an exception, it is not
a very glaring one. The term brown is a very loose one
and may cover a great variety of color effects. It may
be produced either by a combination of black and red,
or of black and green; or it may be due to the structure
of the feather. If a certain amount of gas bubbles are
contained in a feather where the pigment is dark, it
will produce a brown effect, or a black feather with a
frayed edge may appear brown. There is probably no
white pigment in birds’ feathers, this effect being
structural or due to the absence of pigment.

Among the orioles, the colors of the specialized males.
are black and yellow (the orange being merely an inten-
sification of this) while the primitive color as exhibited by
the females and young is olive green. Plates XVI and
XVII showing the heads of a number of representative
species in the generalized and specialized stage illustrates
this instance. It might appear that Jcturus spurius with
its reddish brown color was an exception to this law, but

142 CALIFORNIA ACADEMY OF SCIENCES.

an attempt will be made later to explain this instance
in accordance with it.

Enough examples have now been given, I trust, to
show that, as a rule, the shade of color which we agree
to call the primitive color is a composite which, when
more or less completely resolved into its component
elements, gives the two specialized tints of the species
or genus. A word may be said as to the criterion of a
general and special color. In the first place, in accord-
ance with the law of the correspondence of the onto-
genic and phylogenic series, or the tendency of the indi-
vidual to repeat ancestral stages, if the young differ from
the adult in color (unless the latter be a degenerating
form), it will represent an earlier and more generalized
type. Furthermore, in an adult male bird the color
which is spread over the greater part of the body is
ordinarily the generalized color, while the detailed
markings represent the more specialized hues. This is
directly analogous to the geovraphical distribution of
animals. If we have two allied species of animals, the
one found upon the mainland, the other upon an island,
or the one spread over a wide area and the other in a
limited section, we should conclude that the species
having a restricted range had sprung from the species
having a wide range. More especially would we con-
clade this if the limited form had some mark of superi-
ority to the other, It is found that among birds the
limited markings are, as a rule, of a purer or more
sharply defined color than the body color. Notice, for
instance, the crest of the kinglets or the breast of the
rose breasted grosbeak. In cases of high specialization,
as with the orioles and tanagers, for instance, the gen-
cralized colors may be completely superseded by the
specialized, in which case the female and young, alone,
would show any trace of them.

EVOLUTION OF THE COLORS OF BIRDS. 143
Specialized colors may be classed under four heads,
according to the degree of their development: (1) local
in the male sex only; (2) local in the species; (3) uni-
versal in the male sex only; (4) universal in the spe-
cies. As an example of the first may be mentioned the
ruby-crowned kinglet (Regulus satrapa olivacea), red-
winged blackbird (Agelaius), etc.; of the second, the
kingbird (Tyrannus tyrannus); of the third, the scarlet
tanager (Piranga erythromelas); and of the fourth, the
California woodpecker (Melanerpes formicivorus bairdi).

THE PROPORTION AND DISTRIBUTION OF THE COLORS IN
NORTH AMERICAN GENERA.

It may be well at the outset to take a comprehensive
view of the various colors as they appear among North
American genera. There are two hundred and nine
genera represented among North American birds. Black
is present to a greater or less extent in a hundred and
thirty-five, and white in a hundred and sixty-nine of
them. Pure red occurs in only thirty-three genera, al-
though there are eighty-three besides in which it is
present in some such combination as reddish brown,
vinaceous, pink, etc., making a total of a hundred and
sixteen genera in which some red pigment appears.
Pure orange occurs in but ten genera, while yellow is
found pure in fifty, and in combinations such as ochra-
ceous in twenty-nine others, making a total of seventy-
nine genera in which yellow pigment is found. Green
is present in twenty-two genera, but in fourteen of them
it is metallic, and hence may not be due to a green pig-
ment. Green in combination, particularly with olive,
is found in twenty-seven genera, being metallic in only
two or three instances. There are accordingly about
thirty genera that have green pigment. Blue occurs
pure in twenty-three genera, in seven of which it is me-

144 CALIFORNIA ACADEMY OF SCIENUES.

tallic or iridescent, while bluish-black or bluish gray is
present in thirty-five other genera, giving a total of
something like fifty-one genera with blue pigment.
Purple is by far the rarest of the colors, only six genera
showing it pure and seven in combination, while out of
these thirteen instances it is iridescent in ten. Gray
and brown are the most difficult colors to assign toa
given number of genera, because they frequently occur
only in the young, while in a large number of instances
the two hues are more or less combined. In general,
however, it may be said that gray occurs pure in a hun-
dred and thirteen genera, and in combination in thirty-
nine others, making a total of a hundred and fifty-two.
Brown is found pure in a hundred and twenty-nine in-
stauces, while in combination it exists in forty-three
additional genera,—a hundred and seventy-two in all.
These somewhat dry statistics of the relative abundance
of the different colors may be shown more clearly in the
following condensed table:

Total number of genera. ots . 209
Brown ..... ji Yellow... 79
White... ..169 Blue... iaeol
Gray .. - 152 Green... be 4 hol)
Black... .185 Orange..... .10
Red. abs .116 Purple .. 3

The above facts are susceptible of more than one in-
terpretation. The very large number of genera in which
brown is present may be accounted for by the fact that
brown affords better protection, and that where it ap-
pears it is a protective color which has been developed
by natural selection. The same argument might be ap-
plied to gray, while black and white usually occur in
such limited areas that they may be looked upon simply
as recognition markings. There is, however, another

EVOLUTION OF THE COLORS OF BIRDS. 145

view which seems to be more in harmony with the facts.
Brown and gray are generally present in species in

_ which the markings are not highly specialized. More-
over, these colors very frequently occur in the young of
species which are highly specialized in the adult, as, for
example, in the red-headed woodpecker (Jelanerpes
erythrocephalus). The presence of brown or gray in al-
most every genus of North American birds, either in the
young or the adult stage, would seem to indicate that
these are the most primitive colors, and that where they
still occur it is due to a lack of specialization in this
respect. To be sure, in many cases this specialization
may be retarded in many instances by the positive ad-
vantage of protection, or protection on the contrary
may enable it to continue. Asan excellent example of
these two cases may’be mentioned the goldfinches. The
pine finch (Spinus pinus), retains the primitive plum-
age, perhaps from the need of protection, while the gold-
finches (Spznus tristis for example), have become special-
ized in color without sacrificing protection. They
frequent during the summer the yellow composite, in
the vicinity of which they can be detected only with
great difficulty.

This view would be in harmony with the law of assort-
ment of pigments. The tendency would be to progress
from such mixed shades as brown and gray to the pure
colors. The need of recognition would in any event be
sufficient to account for the early and universal tend-
ency to produce limited areas of black and white. It
might at first sight appear as if the fact that green is
less frequently found than either of its constituent ele-
ments, yellow and blue, would militate against the law
of the assortment of pigments. This discrepancy is only
an apparent one, however. The green pigment of the
bird is a different chemical substance from either the

10

146 CALIFORNIA ACADEMY OF SCIENCES.

yellow or the blue. It might be that in certain in-
stances, especially among tropical birds, that a green
color was produced simply by a combination of yellow and
blue pigments, in which case we shall have the primitive
color of the bird, as indicated either by the young stage,
or by the body color of the adult, green, and the special-
ized colors, yellow and blue. In general, however, the
green is probably either a different pigment, or, as is
frequently the case, in olive greens, produced by acom-
bination of yellow and black.

It may be well to consider next each of the primary
colors (together with black and white) more particularly.
More species are wholly or largely black than any other
of the primary shades. The following are exclusively
or almost completely black:

Catharista atrata. Scolecophagus.
Crotophaga ant. Quiscalus.
Crotophaga sulcirostris. Progne subis.
Corvus. Phainopepla nitens.

Molothrus ceneus.

Still larger is the list of species in which black covers
a large proportion but not the entire surface of the body:

Colinus virginianus cuban- Pica pica hudsonica.

ensis. Dolichonyx oryzivorus.
Pseudogryphus californianus. Molothrus ater.
Cathartes aura. Agelaius.
Urubitinga anthracina. Spinus psaltria mexicana.
Canvpephilus principalis, Sprophila morelletts.
Nenopicus albolarvatus, Evetheia bicolor.
Sphyrapicus thyroideus. Calamospiza melanocorys.

A number of species are colored a very dark brown
or gray, showing a strong tendency towards black. The
list of these is as follows:

EVOLUTION OF THE COLORS OF BIRDS. 147

Parabuteo wnicinetus har- Archibuteo lagopus sancti-

Tisi, johunnis.
Buteo harlani. Ceophleus pileatus.
Buteo abbreviatus. Leucosticte atrata.
Buteo swainsont. Junco hyemalis.
Buteo fuliginosus. Sayornis nigricans.

Falco columbarius suckleyt.

There are thus something like forty-five or fifty North
American species which show a very strong tendency
toward melanism. The relation of geographical distri-
bution to this subject will be discussed at a later stage of
our inquiry. Besides these species in which black is
the sole or predominant hue, there are a number in
which it figures quite largely. Thus the genus Pipilo
contains two groups, erythrophthalmus and maculatus in
which the back, throat and breast are mainly black.
The American redstart Setophaga ruticella has the black
distributed in precisely the same manner, while in the
painted redstart (S. picta) there is a still greater pre-
dominance of black. Indeed, there is a curious paral-
lelism between the colors in these two widely separated
genera which may be only a coincidence or may be a
matter of profound significance. At any rate it is a
subject of great importance in an investigation like the
present, to scrutinize all these apparent coincidences
and see if they may not have some real meaning. In the
first place, then, Pipilo erythrophthalmus and Setophaga
ruticilla both have the black of the male distributed
over the same parts of the body. There is the same
abrupt contrast between the black of the breast and the
white of the belly. The sides of the breast are red in
both species, although in the one it is a bright flame
reddish-orange (Setophaga) while in the other it is chest-
nut or rufous. There is a light patch on the wing,
orange in Setophaga and white in Pipilo. The tail

148 CALIFORNIA ACADEMY OF SCIENCES.

markings are decidedly different, however, being basal
in Setophaga and terminal in Pipilo. In both species
the black of the male is replaced in the female by brown.
Perhaps the most curious point of resemblance, how-
ever, is the fact that both species which belong to the
eastern states, have western or southwestern represent-
atives (Setophaga picta and Pipilo maculatus) in which
there is a greater amount of black, and in which the
sexes are colored alike. To be sure, the difference in
structure, size and general appearance between these two
birds is so great as to apparently preclude any possi-
bility of kinship, yet the parallelism is complete in so
many points that it is difficult to believe it to be a mere
coincidence.

There are, however, several species in which the color
of the back, head, throat and breast is black or some
very dark shade, in abrupt contrast to the pure white of
the breast. Such, for instance, is the case with Junco
hyemalis and Sayornis nigrescens.

Again, the area of black may be the same, but the un-
der parts some color, such as yellow or brown, instead of
white. As examples of this Jcterus parisorum and I.
spurius may be cited. The general style of coloration
in the rose-breasted grosbeak./ Habia ludoviciana, is the
same, although the black is interrupted on the back
by a patch of rose color. This species forms a connect-
ing stage between such forms as Pipilo, Setophaga, Junco
and Sayornis, where the black extends from the back
around the throat and breast, and such forms as the
Blackburnian warbler (Dendroica blackburnic), where the
entire upper parts are black and the entire lower parts
light colored. In the magnolia warbler (D. maculosa),
the black is still an important feature of the back, but
less evenly distributed, not covering the top of the head,
and frequently broken by olive green. It also appears
in decided streakings upon the breast and sides.

EVOLUTION OF THE COLORS OF BIRDS. 149

In general, it may be said that the tendency is for the
dark markings, whether black or some other shade, to
occur in the greatest masses upon the back. As will be
seen later, this might be accounted for by supposing
that light is inclined to favor the deposit of pigment,
and as the back has always received a more considerable
share of it than the breast, a rhythm had in the course
of ages been established, inclining the larger amount of
pigment to those parts of the body. As if to bid defiance
to any such laws as we may make upon the subject, a
number of species are found in which this order is
directly reversed. Thus, in the chestnut-colored long-
spur (Culearius ornatus) the top of the head and entire
lower parts, except throat are black. The same parts
are colored black in the grassquit (Huethera bicolor) and
in the bobolink (Dolichonyw oryzivorus). In all these
cases the upper parts contain the lighter markings.

These black markings seem asa rule to spread from
certain well marked areas. Those on the lower parts
appear to spread from the throat or breast patch, while
those of the upper parts usually come first on the head.
This is by no means universally the case, as, for in-
stance, with the genus Icterus (Plates XVI-XVIT). In
the females the only patch of black is the one on the
throat. It does not extend from there at first, however,
but the back darkens, as in the adult of Icterus cucullatus.
In Icterus bullockt the black has extended over the head,
while in the next stage, Icterus galbula, the patches on
throat and back of head become confluent. The point to
be noted is that there is no species in which the top of
the head is black while the back remains light colored.

Sometimes when the entire bird is black or some dark
shade, its antecedent stage is black above and lght
below. <A good example of this is the purple martin
(Progne subis), the male being entirely dark glossy blue

150 CALIFORNIA ACADEMY OF SCIENCES.

black, while the female has the under parts gray and
white. Among the blackbirds, the females frequently
have the under parts lighter than the back. This is
noticeable in Scolecophagus and extremely pronounced
in Agelaius. In Xanthocephalus, the breast of the
female is mixed with white. In the hawks it is fre-
quently the case that a melanistic phase will occur in
which the bird is dark above and below, whereas, in the
normal phase the breast is light colored. Such is the
case, for example, with Buteo borealis and B. swuinsone.

Some of the tanagers (Pirunga erythromelas, for in-
stance), present a rather unusual localization of black,
the wings and tail alone being shaded thus. In the
Louisiana tanager (Piranga ludoviciana) the back as well
as the wings and tail is black. LP. erythromelis un-
doubtedly represents a more advanced stage, in which
the black has become restricted to these regions.

Perhaps the most instructive example of the way the
black spreads from the breast, as a center of pigmenta-
tion, is the female of Williamson’s sapsucker (Sphyrupi-
cus thyroideus). The final color of the male isan almost
uniform black, varied on the breast by only one patch
of red on the throat. Every stage may be traced from
the young female with white under parts, through the
adult female with a black patch upon the breast, and
the different stages of its extension in the young males
to the final plumage of the adult.

The most complete example with which I am ac-
quainted of the spreading of black upon the back is the
erecn-backed goldfinch and its allies (Plate VIL). Sprnus
psaltvia has a black cap upon the top of its head, and
the shoulders are usually dark. In YS. psultriu urizonw
both of these areas of color have become more extended
and the dark on the shoulders is a more pronounced and
uniform black. In addition to this, black feathers are

EVOLUTION OF THE COLORS OF BIRDS. 151

interspersed throughout the back. In the third variety,
S. psaltria mexicana, the black is the predominant color
of the back, while in still more extreme races no trace
of the green can be detected.

Three methods of turning black have been observed
among the examples above cited. (1) The general tone
of color may darken uniformly, approaching a black,
as in Jwnco hyemalis and Sayornis nigricans; (2) masses
of color may spread until they meet, as in Icterus; or
(3) dark spots and irregular mottlings may become fused
into continuous patches, as in the Spinus psaliria group.
This latter is the most universal method, and the other
two may be combined with it to a greater or less degree.

As we are not concerned at present with the causes of
colors, but rather with the facts of the distribution and
localization of the different primary shades, we may
rest content for the present with this survey of black
as it appears among North American birds, and take up
white. ©

One of the first points which is noticeable with regard
to white is its close interrelation with black. Next to
brown, white is more universally present among North
American genera than any other color. In spite of its
prevalence, however, there is not a single species which
is pure white, and only a very few in which it is pre-
dominant. The fact that black and white both occur
in the same species so frequently might first be consid-
ered as a mere coincidence, due to the fact that both are
very prevalent, or it might be regarded as a question of
utility, white setting off black, or vice versa, in a con-
spicuous manner, and affording a useful recognition
mark. This may indeed be a partial explanation, but
I think not the fundamental one. We have such species
as the dusky seaside sparrow (Ammodramus nigrescens),
and the black and white warbler (dA/niotilta varia), in

152 CALIFORNIA ACADEMY OF SCIENCES.

which the black and white are about evenly distributed
in streaks over the body. Among the woodpeckers a
considerable number have black and white markings in
about equal proportions. The general black color of the
lark bunting (Calamospiza melanocorys) is relieved by a
broad white patch on the wing. The bobolink (Doli-
chonyxz oryzivorus) has almost as much white and buff in
its make-up as it has black. The shiny-crested flycatcher
(Phainopepla nitens), although for the most part black,
has the inner webs of the primaries white. Perhaps
the most remarkable case, where no utility can be con-
ceived, is the white-necked raven (Corvus cryptoleucus),
which has the feathers all around the neck pure white
at the base. It would be difficult to understand how this
could be an advantage, being entirely concealed by the
overlapping tips of the feathers. The tendency toward
albinism is common among the crows, while in the mag-
pies (Pica) and the flute bird of Australia (Gymnocitta)
white figures largely in the plumage.

As a recognition of some sort of interdependence be-
tween black and white, as they exist among birds, I
would suggest that they be termed Cognate Colors.

Although, as before stated, there are no exclusively
white birds in North America, the winter dress of the
ptarmigans (Lagopus), approaches very close tothis. In
the following species white is the prevailing color:

Elanoides forficatus. Bubo virginianus arcticus.
Elanus leucurus. Nyctea nyetea.
Falco islandus. Plectrophenax nivulis.

Scotiaptex cinereum lapponi- Plectrophenax hyperboreus.
cum.

In no instance is a species colored in some uniformly
pale shade approaching white, the anthesis of the dark
color of Sayornis nigricans. There are thus only about
four species of ptarmagin (Lagopus), which are almost

EVOLUTION OF THE COLORS OF BIRDS. 1538

exclusively white and eight other species in which it
figures as the prominent color. Among nearly all North
American genera some white is present, and in a large
proportion of species. When occurring in large masses
it is usually on the breast and under parts, as previously
intimated. It generally persists longest on the abdomen
and under tail coverts. Plate XIX of the genus Den-
droica furnishes illustrations of the way in which the
color fades out into white on the under tail coverts. I
know of no instances in which large masses of white
occur upon the upper surface of the body except such as
the bobolink and a few others in which the under parts
are largely black.

The only North American species which is exclusively
red is the summer tanager (Piranga rubra). In the fol-
lowing species red is the prevailing color:

Pyrocephalus rubineus. Lewcosticte.

Pinicola. Cardinalis.
Curpodacus. Piranga erythromelas.
Ergaticus ruber. Piranga hepatien.

Two classes of red birds may be distinguished—those
in which the color occurs in clear, well defined patches
as in Pyrocephalus, Piranga and Cardinalis, and those
in which the plumage is everywhere suffused with red,
but the color not sharply marked out, as in Pinicola,
Carpodacus and Leucosticte. Carpodacus is more or less
intermediate between these two styles. When pure red
is present in smaller amounts it generally occupies well
defined localareas. Red crests are frequent, 7. e., in most
of the woodpeckers where the color sometimes spreads
over the entire head as in Melanerpes erythrocephalus.

Among the species having a crest of scarlet or some
bright red may be mentioned Milvulus forficatus,
Tyrannus verticalis, Passerina versicolor, Acanthis and
Regulus calendula. Red or rose colored throat patches

154 CALIFORNIA ACADEMY OF SCIENCES.

are found in Sphyrapicus, Platypsaris and Hubia ludo-
viciunw. There is a curious resemblance in the col-
ors of this latter species with the becards (Platy-
psaris). In both cases the tendency is toward black on
the upper parts of the body, white on the lower parts,
and arose colored breast. The red-winged blackbirds
(Agelaius), are unique in the red shoulder patch. It is
a noteworthy fact, and one which is too apt to be looked
upon, as a matter of course, that scarlet, or any other
brilliant color, when occurring only in limited areas, is
never in an inconspicuous place, as upon the abdomen,
for instance.

The presence of red in connection with slaty gray in
a few instances, as in Pyrrhulowiu sinuatu and Pinicola
enucleator, may be worthy of note. The most significant
interrelation of colors, however, and one which I
believe to be of wide application in the explanation of
bird colors, is that between red and yellow. Whether
these two colors are produced by the same or a different
pigment I am unable to say; but, however this may be,
there is a high degree of probability that the red is
simply an intensification of the yellow. There is much
evidence to show that yellow is a more primitive stage
than red, and that the latter has always, or nearly
always, been developed from the former. What the
factors in this intensification may be I will not attempt
to state, merely suggesting that heat, moisture and food
may play an important part. The term CoRRELATIVE
Cotors will be an appropriate one by which to indicate
this interrelation.

Now, as to the evidence of this dependence of red
upon yellow. Plate V indicates a number of instances
of correlative colors. What I have attempted to show is
that when red occurs in a group of birds, yellow will be
found in the same group, and that the yellow represents

EVOLUTION OF THE COLORS OF BIRDS. 155

a more primitive stage of development than the red.
For instance, in the family Icteridee we have the red
winged blackbirds (Agelaius) with a scarlet shoulder
patch. In the same family is the yellow-headed black-
bird (Nanthocephalus xanthocephalus). This fact in itself
would be of little significance, but may be explained in
accordance with the law of assortment of pigments
together with the correlation between red and yellow.
Of far more significance are the colors in the grosbeaks
(Habia). In the male rose-breasted grosbeak (H. ludovi-
cana) the breast patch and under wing coverts are
bright rose red, while exactly the same areas in the
black-headed grosbeak (H. melanocephala) are a clear
gamboge or lemon-yellow. As if still more emphatically
to emphasize the fact that this yellow was really a more
primitive stage of the red, the females of both species
have these parts colored a paler yellow. The fork-tailed
flycatcher (Milvulus tyrannus) has the crown patch a
bright lemon-yellow, while the crown patch of the
scissor-tailed flycatcher (JV. forficatus) is scarlet. So also
the kingbird (Tyrannus tyrunnus) has a crown patch of
orange-red. In the gray kingbird (7. dominicensis),
according to Ridgway’s Manual, it is orange colored,
while in the thick-billed kingbirds (7. crassirostris) it is
lemon-yellow. In this genus then, the stage of transi-
tion from red through orange to yellow is present.
Among the woodpeckers the crown patch is ordinarily
red, but in three-toed woodpeckers (Pcoides) it is
yellow.

The brilliant red of the scarlet tanager (Piranga ery-
thromelas) is replaced in the western tanager (P. ludowi-
ciana) to a large extent by yellow, the scarlet being
developed in the latter species in limited areas. The
females and young of both are yellow. In the flickers,
which are so intimately related, as shown by their
habitual hybridization, the one species is yellow (Co-

156 CALIFORNIA ACADEMY OF SCIENCES.

laptes aurutus) where the other one is bright red (C.
cafer). The ruby-crowned kinglet (Regulus calendula)
has a cap of scarlet while in the golden-crowned kinglet
(R. satrapa) it is orange and yellow. The meadow-lark
of South America (Stuinella defilippii) is rose-red where
the North American species (S. magna) is yellow.

Having now cited instances of different genera of
the same family and different species of the same
genus in which red and yellow are correlative, let
us examine some of the closer genetic relationships.
The Arizona hooded oriole (Jelerws cucullatus nelsoni) is
colored yellow, varying from saffron to gamboge. In
the hooded oriole (J. cucullatus) the color is ‘‘ orange or
dull orange red,”’* while in the fiery oriole (J. cucullatus
igneus) itis ‘intense orange-red, sometimes almost scar-
let.” The male redstart (Setophuga ruticilla) is marked
with rich orange-red, the same areas in the female
being yellow. In the genus Sphyrapicus and also in
Melanerpes, the markings are largely bright scarlet, but
supplemented by yellow, as if a part of the color had
been intensified by sexual selection, for example, and
the rest left unaffected. In the winter plumage of the
red-winged blackbirds the shoulder patch is yellow
instead of red. The house finch (Carpodacus mevi-
cunus frontalis) when kept in captivity loses its bright
red tints, which are replaced by yellow. The same is
true of the crossbills (Loviw), and would probably be
found to hold good for Pinicola and Leucosticte.
Canaries fed upon cayenne pepper turn red, although
this may possibly not be a converse instance.

I have now given a considerable number of examples
covering all degrees of genetic affinity, of this correla-
tion of red and yellow. One of the strongest points in
favor of the validity of this correlation as an explana-

* Ridgway’s Manual.

EVOLUTION OF THE COLORS OF BIRDS. 157

tion of color changes from yellow to red is.the fact that
there are so few exceptions to it. The vermilion
flycatcher (Pyrocephulus rubineus mexicanus) is a partial
exception to it, although the female is sometimes tinged
with a salmon color which is not far from yellow. The
female cardinal grosbeak (Curdinulis virginianus) is dull
brownish or reddish, but the head and under parts are
more or less tinged with yellowish. Acanthis with
its red head is not far removed from the goldfinches
(Spinus). The becards (Platypsaris) and the red warb-
ler (Frgaticus ruber) form the only complete exceptions
to the rule with which I am familiar, but a knowledge
of allied South American forms might bring them also
under the law of correlation.

There are no North American birds which are ex-
clusively yellow in color. In the following species it is
the predominant hue, or figures very conspicuously:

Nanthocephalus wanthocepha- Pirwnga ludoviciauna.

lus. Dendroicu cestiva.
Sturnella magna. Sylvania pusilla.
Leterus. Geothly pis trichus.
Coccothraustes vespertinus. Vireo.
Spinus. |

It is conspicuously present among the warblers
(Mniotiltide), and in such of the vireos as Vireo jlavi-
frons and V. flavoviridis. As a rule it is found upon
the under surface of the body, especially when in large
masses. In fact, when the tendency is for the whole
bird to be thus colored, it will be noticed that the yellow
upon the back is apt to be less pure, with more of an
olive or greenish tinge, as in Dendroica cestiva, the fe-
males of Piranga, etc.

Two North American species are almost exclusively
blue in color—the indigo bunting (Passerinu cyanea),
which has the lores black, and the arctic bluebird

158 CALIFORNIA ACADEMY OF SCIENCES.

(Stalia areticu), in which the belly and under tail coverts
are white. In the following, blue is the conspicuous
color:

Cyanocitte, Guiraca cruled,
aAphelocomu. Passer ind Gainer ne.
Cyanocephalus eyanocephalus, Huphonia elegantissinu,
Dendroica cvrulea,

It is a fact worthy of note that blue very rarely occurs
in limited patches or markings. There are a number
of species in which the general shade of color is ap-
proaching a clear blue, as in the blue-gray gnatcatcher
(Polioplila carvwlea), the yellow-rumped warbler (Den-
droice coronutu), and a number of others of the same
genus; but where do we find, among North American
birds, species with blue heads, ear coverts, throat patches,
wing markings, or spots in general? The green-jay
(Nanthowra luxuosu) with its bright blue head is a strik-
ing exception.

It will not be necessary to consider green at any
length, for the simple reason that there are hardly any
species in which a pure green figures at all, the above-
mentioned jay and the parrots being exceptional cases.
The metallic greens, as in the humming birds, are prob-
ably not pigmental in nature, and hence must be ex:
cluded. As an olive green it is frequently found, how-
ever, but we are at present concerned only with the pure
colors.

A few generalizations from the forgoing facts may
now be in order. It will be noticed that with a very
few exceptions (Buteo and Dendroica) pure color in
large masses occurs in small genera. Black forms are
scattered through various families, from the lowest to
the highest. White reaches the climax of its develop-
ment among the Falconide and Bubonide; red among
the Fringillide and the allied Tanagride; yellow among

EVOLUTION OF THE COLORS OF BIRDS. 159

the Mniotiltide, and blue among the Corvide. When
any of these colors occur in considerable masses in
combination with some other color, it is generally with
brown, black or olive green, and seldom with any other
very pronounced shade.

PATTERN OF MARKINGS.

In the preceding discussion of the distribution of color
masses it has been found difficult to keep clear of a
closely related subject, to which attention must next be
directed, viz.: the pattern of markings, both as distin-
guished from and as related to their colors. These
markings must be regarded in two ways; first, as to the
pattern, mode of development, etc., of the markings
upon individual feathers, and second, the general effect
of feather markings in the pattern of the entire bird.

In an investigation of color patterns, the laws of growth
and structure stated in the introduction, should be kept
in mind. The law of growth force or bathmism is
especially important in this connection. Pigment is a
definite chemical substance which travels through the
various branches of the feather, advancing farthest
and most rapidly along the lines of least resistance and
accumulating in masses where the resistance is greatest.
Now the pigment cells must reach the various parts of
the feather by way of the shaft, and we should a priori
expect to find that the resistance would be least right
down the shaft. It might spread out a very short
distance on the barbs but the main tendency would be
toward the tip. This would produce a streaked feather
as the most primitive form. Having reached the tip it
would encounter an obstacle, and then would develop in
mass. This route of the pigment might be illustrated
by a panic-stricken mob running down a street. They
would naturally keep in the road where travel was easiest

160 CALIFORNIA ACADEMY OF SCIENCES.

rather than branching off into the fields where their pro-
gress would be impeded by the unevenness of the
ground, fences, ete., but if a large body of water should
finally interpose a barrier to their course they would,
upon reaching it, come to a halt at first while the multi-
tude would press on from behind causing a mass at the
end.

This phase would represent a feather with a terminal
spot. IEf the crowd ceased to press on in the rear they
would spread along the margin of the water correspond-
ing to a feather with a black band at the tip. In this
instance the pigment would desert the base of the barbs
and spread out into their ends. Ifa second invasion of
the mob occurred later, and it happened that the road
along which they were escaping traversed the center of
peninsula, the last resort of the people would be to flee over
the rough ground in all directions from the road, and
we should have first a stage corresponding to a feather
with a pigmented border and pigmented center but a
clear space between the two, and finally a uniformly pig-
mented plume, when the crowd had filled the entire
space. Nothing more would remain but for them to
pack in tighter and tighter which in the case of the
feather would result in the intensification of the color.
The accompanying diagram is a conventional represen-
tation of these feather changes.

This example of a mob may seem a fanciful one, °
and not calculated to prove anything, but indeed it is

EVOLUTION OF THE COLORS OF BIRDS. 161

not so. If the laws of growth have any sort of univer-
sal validity, and I am firmly convinced that they have,
the line of movement of a mob of men along the lines
of least and greatest resistance would typify any form of
involuntary movement along the same lines. The ques-
tion simply is to determine which are the real points of
greatest and least resistance. The sequence of color
assumption here exemplified may be infinitely varied in
detail, but will, I think, be found tolerably uniform in
general. Different forms of feathers may offer different
degrees of resistance in the various parts of their struc-
ture, but I think it will be found true in the main that
the pigment will pass down the shaft most readily, while
there can be no possible exception to the obstruction at
the tip. The channel of pigmentation may be diverted
along the sides, however, and thus not give rise to any
terminal spot or band.

I have thus far assumed that there was some force
analogous to the blind terror of a mob which urged the
pigment cells away from the source of danger, the body.
This is probably not strictly true, however. It is more
likely thatit is the warm sunlight which, with its genial
touch, causes them to crowd forward; to leave the prison
in which they were born, and press on joyfully into the
free air of the outer world. Capillarity or a variety of
undetermined forces might also tend to draw them out-
ward. The number which were thus enticed to thesur-
face would depend simply upon the amount of surplus
energy, or whatever else it may be, which results in the
formation of pigment in the system of the bird. As a
species grows older and more vigorous, and more com-
pletely adapted to its environment, the sum total of pig-
ment would increase,and accordingly, if no other ten-
dency were at work in a contrary direction, there would

il

162 CALIFORNIA ACADEMY OF SCIENCES.

be a development towards a uniform pigmentation of
each feather:

The subject of the lines of pigmental development
has receiyed more attention at the hands of the Ger-
man scientists than from any others.* The re-
searches of Eimer in particular are in accordance with
the view here stated. It seems that among nearly
all animals (at least among the vertebrates), the earl-
iest form of color is the longitudinal streak. These
longitudinal markings are finally broken up into spots
(when the tendency to assume a uniform color does not
assert itself) and the spots ultimately fuse into cross
bars.

This view of the general succession of patterns has
been commonly accepted, although denied by Kerschner,
who asserts that the longitudinal barring is the oldest
formof marking. As his conclusions were based upon
a limited number of forms, the eye spots of the pea-
cock’s tail having claimed his special attention, they
cannot be considered to be of general validity. Hacker
has devoted special attention to pigmentation in the
thrush family, although he considers the water birds
also to a certain extent in the paper cited below. With
the exception of the primitive streak he finds the clear
feather with a pigmented border to be the earliest form,
which may be modified into a spot as in the thrushes.
The coloring of the interior of the feather he considers
to be due to a secondary pigmentation.

~On this subject see Th. Eimer, Untersuchungen tiber das Variiren der
Mauereidechse, etc., 1881. Ueber die Zeichnung der Vivel und Saugethiere.
Jareshefte des Vereins fur vaterld. Naturk. in Wurttemberg, 1853, p.
556. Zoolog. Anzeiger, 1852-83. Humboldt, 1885-1857.

Dr. Kerschner, Zur Zeichnung der Vogelfedern, in: Arbeiten aus dem
Zoologischen Institut zu Graz, I. Band, Nr. +. Leipzig, 1886.

V. Hacker, Unter nichungen uber die Zeichnung de Vogelfedern. Zool.
Jabrb., III, Jan. 15, 1888, pp. 309-316.

EVOLUTION OF THE COLORS OF BIRDS. 163

With regard to this secondary pigmentation, meta-
bolism may be introduced as an explanation. When a
species is katabolic the surplus of energy might be ex-
pended in the formation of pigment. This would be-
come distributed in the feathers in accordance with the
laws determining the location of growth force. It would
become fixed there by the conservative laws of heredity,
and then a state of anabolism might follow. At the
beginning of the next katabolic period, in accordance
with the rhythm of metabolism of Geddes and Thomson,
more pigment would be produced and again distributed,
this being the secondary pigmentation of Hacker.

Let us see now how the facts agree with these theories.
The first group of feathers on Plate IT is a series from
the under parts of a Californian woodpecker (Melunerpes
formicivorus bairdi) commencing on the sides and
extending up on the breast. It will be noticed at once
in how many respects it tallies with the ideal instance
previously considered. The feathers of the belly are
white, but along the sides and the upper portion of the
belly there is a black streak down the shaft. In some
instances the shaft alone is thus marked, but generally
the base of the barbs is similarly affected. On the
feathers located on the lower part of the breast the line
has become broader, and black pigment has appeared on
the borders of the feathers. Both areas of black next in-
crease, although the inner one spreads most rapidly, until
the white area has been confined to two small islands
which ultimately disappear in the clear black feathers
which form the patch upon the upper chest. This
sequence of stages is in accordance with the law of suc-
cessional relation (see ante, pp. 77-78), and undoubtedly
indicates the phylogeny of the black feathers. Certain
it is that the feathers of the abdomen are less developed
with regard to pigmentation than those on the flanks;

164 CALIFORNIA ACADEMY OF SCIENCES.

the flank feathers are less developed than the feathers
of the lower breast, and these in turn less advanced than
a stage higher up on the breast. Through all this a reg-
ular sequence of successional relation is maintained,
and we, consequently, feel justified in assuming that
the different parts of the body of the bird represent
different stages in the evolution of the feathers. Being
unfamiliar with any word expressive of this relation so
closely akin to ontogeny and taxonomy, I would suggest
the term SuccessionaL Ta xoLoey.

As before stated, the assumption of a uniform pig-
mentation by a feather is by no means universally along
the ideal development of Melanerpes. Thus, in the
yellow-breasted sapsucker (Sphyrupicus varius) the
transition is from the original streaked form through a
barred type to a uniform black. The successional tax-
ology in this species as indicated by the feathers along
the border line of the black throat-patch is represented
in a tolerably typical series in Plate II. The streaked
type generally occurs farthest from the black patch,
but these feathers are inconspicuous and largely con-
cealed, so that no streaked pattern appears in a general
survey of the bird. In the next phase the streak becomes
slightly tilted to one side (fig. 8), or it may divide
and extend out on both sides enclosing a white spot
(fig. 9). Continuing to descend, it approaches a posi-
tion in which it is at right angles to the shaft, forming
a bar (fig. 10) and presently a second bar makes its
appearance along the edge of the downy basal portion
(fig. 11). The two bars next fuse, frequently passing
through the stage indicated in figure 12 and end by
becoming perfectly black.

I cannot see that this series can be explained by the
general laws of growth previously used, but would sug-
gest that it may not improbably be a case of correlation.

EVOLUTION OF THE COLORS OF BIRDS. 165

The back of Sphyrapicus varius is barred, and this
barred stage in the progression from the primitive
streak to the uniform black pigmentation may well be
due to correlation of pattern. The bars may be of utility
on the back, whereas they cannot be on the breast, be-
cause they are so concealed as to be almost invisible. In
Williamson’s sapsucker (Sphyrapicus thyroideus), the
barred markings surrounding the black patch on the
breast of the female assert themselves very strongly, but
this may be simply an advanced form of correlation.
They are also much more conspicuous on the back in
this instance, and we should therefore expect to find
them correspondingly developed on the breast if it was
a case of correlation. In the Californian woodpecker, in
which the breast feathers do not pass through the barred
stage in becoming black, it is to be noted that the back
is never barred at any stage of its development. Figures
14-17 of Plate II show the transition from the barred
to the uniform black type taken from the border of the
black throat-patch. It may be remarked that these in-
stances illustrate Cope’s law of parallelism previously
stated (see ante, p. 78), this last instance being especi-
ally instructive in this respect, because in the male
these parallel stages are no longer repeated as in the
female.

The most systematic study of successional taxology in
the pigmentation of feathers can be made upon the wing
and tail,where the exact relation of feathers is fixed. Here
the interesting fact is disclosed that all changes in the pat-
tern occur according to a defininite orderly sequence.
Plate III represents the feathers from the wing of a spar-
row hawk (Falco sparverius) in their natural order. Fig-
ures 1-5 are the feathers of the spurious wing. In fig. 1 the
barring is evident, but has not yet strongly asserted itself,
being irregular on the upper part of the feather. The sec-

166 CALIFORNIA ACADEMY OF SCIENCES.

ond feather has two complete bars and a triangular sub-
terminal dot. The tendency, however is towards a more
complete pigmentation, and in the third feather the
bars have run together somewhat along the line of the
shaft. The triangular patch has become extended up-
ward along the shaft also. In the fourth feather this
darkening of the feather has proceeded another stage,
while in the last of the set (fig. 5), the outer web is
almost entirely black, and the inner web has succeeded
in closing in an island of light, a tendency which had
been developing through the whole series. In the
primary coverts no such transition is observable except
the gradual loss of one of the bands (figs. 6-15). The
primaries themselves, however, present an interesting
series of changes (figs. 16-25).

In the first place it will be noticed that the number
of bands is largely dependent upon the size of the
feather. If there is a little more room at the basal por-
tion another bar will be formed. In the first feather
(fig. 16) there are eight white bars. In the next two
which are the longest there are nine (counting the spots).
In the fourth (fig. 19) there are only eight, this number
persisting in the fifth and sixth, although the basal bar
is very indistinct. In theseventh feather (fig. 22) there
are but seven bars, this number being reduced to six in
the last three—and the sixth one very inconspicuous.

It seems to me we have here a fact of considerable
significance. This basal bar, or even two or three
of them perhaps, could never be of any use because
they would be concealed in life by the coverts, and must
therefore be repeated fron: some general law. Just as
soon as there is a little space at the base of the
feather another bar of pigment crowds in. It seems
probable that this is to be accounted for by the law of
repetition, so closely akin to the law of correlation. In-

EVOLUTION OF THE COLORS OF BIRDS. 167

stead of the repetition of parts we have here the repeti-
tion of color pattern.

The two fused spots of the second feather (fig. 17)
are probably not homologous with the two bands of the
first feather, but are two additional spots. This may be
due to the fact that the terminal portion which contained
them has become aborted. Atany rate, the character of
this first feather is so different from the second that an
abrupt transition of pattern might be anticipated. It
will be noticed that there are two other tendencies man-
ifested in this series—for the lowest spot to increase in
size and for the white bands to be crowded toward the
edge of the feather; both of which tendencies take place
gradually and regularly. On the opposite wing the
markings are very similar, feather for feather. Merely
in slight details do they differ, as for instance in regard
to the fusion of spots, which is not so decidedly pro-
nounced in the second feather and more nearly ap-
proached in the third. Again there is a tenth white
bar, which is barely indicated in the second feather of
the wing figured but well marked on the same feather of
the other wing.

There is a more marked change between the last pri-
mary and the first secondary (fig. 26). The subterm-
inal spot which has been gradually enlarging suddenly
breaks out into a broad terminal band of bluish with a
white spot still left. A trace of black has also persisted
along the line of the shaft. As a sort of premonitory
symptom of the coming light band, a small spot of
white is noticed at the base of the last three primaries.

The next important change occurs in what I take to
be the first of the tertiaries (fig. 34) where a decided
tendency toward the elimination of the black bars first
asserts itself. This rapidly advances until the simple
stage of figure 39 is reached. The same sort of

168 CALIFORNIA ACADEMY OF SCIENCES.

e
transition occurs in the secondary coverts, the last five
of which are figured (figs. 40-44). It is difficult to
say whether the spot which first appears near the tip of
the fifth secondary (fig. 30) spreads toward the shaft
forming the spot in fig. 836 and afterwards becomes re-
duced to a spot on the shaft as in figs. 88-39 or whether
the marking in fig. 36 is of distinct origin. The former
view, however, is the more probable one.

This same sequence of stages seems to hold for all the
forms which I have examined, and the uniformity of
successional taxology will doubtless be found to be the
normal condition of wing markings. The wing of the
Californian woodpecker affords an excellent illustration of
this. Some of the changes from feather to feather are
rather sudden, but an examination of any particular
feather would enable one to predict in general what the
pattern of the next one was to be, so inevitably is cach
new character foreshadowed in the antecedent feather.
The general color of the wing feathers in this species is
dark brown, with a conspicuous band of pure white
crossing it. The first primary is uniformly dark, with
the exception of a small gouge of white close to the base
of the inner web. In the second primary the white has
spread into a patch about thirty mm. long, and extend-
ing from the inner edge almost to the shaft. In the
third it has become still more elongated, and spread not
only to the shaft, but across it on the outer edge. In
the fourth and fifth primaries the white continues to
spread until it occupies almost the entire basal half of
the feather. <A little dark pigment at the extreme base
of the fifth feather asserts itself in the sixth, forming a
well marked patch of brown. This spreads down the
shaft in the seventh feather, and in the eighth has iso-
lated the white in two patches, a long one along the
inner edge of the feather and a shorter one midway

EVOLUTION OF THE COLORS OF BIRDS. 169

along the outer edge. In the ninth primary this outer
white patch has vanished, the one on the inner edge
persisting in a long line on the edge of all the second-
aries. The first secondary is well marked off from the
last primary by a spot of white at the terminal end of
the white border. A second spot is gradually constricted
off from the end of the border, and the first one dwindles
into insignificance.

It is time now to ask the meaning of this successional
taxology in the color pattern. If the successional tax-
ology represents different evolutionary stages of pro-
gression, as I believe it does in some cases, it is easily
understood. For example, in the feathers of Melanerpes,
previously described, it is only necessary to assume that
the under part of the body is becoming black, but the
tendency has been stronger on the upper part of the
breast than lower down, to understand this successional
relation perfectly, as different degrees of complete-
ness in evolution in different parts of the body. This
is a beautiful example of Cope’s law of acceleration and
retardation. The growth force has been accelerated on
the upper breast and retarded on the belly. With re-
gard to wing feathers, however, the explanation is not
so simple. Here, instead of one part of the body ex-
hibiting a general stage of development, and other parts
successively higher stages, we find feather following
feather in uniform steps of transition, when in many
cases it is simply impossible to say that one stage is
higher than another. Certainly there is not an ideal
mode of coloration being approached through successive
stages, as is the case with feathers becoming black, but
rather a variety of individual patterns combined to
produce one effect.

Thus far little has been said of general effects. The
question has been discussed as if it were supposed that

170 CALIFORNIA ACADEMY OF SCIENCES.

all colors were produced strictly in accordance with
these laws and utterly regardless of any external in-
fluences which might modify the patterns. We must,
however, frankly acknowledge that, while all internal and
constitutional laws of development must rigidly limit
and condition the assumption of pigment by a feather, a
large proportion of color markings are produced by ex-
ternal causes—natural and sexual selection, direct action
of the environment, etc. For example, the general laws
of the location of growth force might have caused the
pigment to have lodged in the tips of the feathers of the
thrush’s breast, but the effect would have been one of
mottling. Then natural selection might have taken
advantage of this and emphasized and defined the mark-
ings, and left each one distinct and isolated as a spot.
And thus it is with the wing bars and bands under con-
sideration. They have doubtless been produced by
selection for some effect, and indeed it might be argued
that the orderly sequence from feather to feather was
due solely to the fact that in this manner the general
effect might best be secured. This view, however, can,
I think, easily be shown to be untenable. For example,
upon the last primary of a wing of the Californian wood-
pecker (Melanerpes formicivorus buird/), which I ex-
amined, was a mere suggestion of a white dot below the
strip of white on the inner side of the feather. It was
so small that it might easily have been overlooked in an
examination of the feather, and most certainly could
have had no more influence on the general appearance
of the feather than a speck of dust. Upon the feather
next it, the first secondary, a well marked spot appears
in exactly the same place, while in the secondaries suc-
cessively following, the first this spot slowly dwindles
away. Now it might of course be argued that this spot
had been a useful character at some past time, but was

EVOLUTION OF THE COLORS OF BIRDS. 171

becoming obliterated. This objection wholly misses the
point, however, which is, not why does this little dot
which can be of no utility exist but why does it occur as
anticipating a larger spot? This is only one instance out
of an innumerable number of cases where a well marked
character in one feather is led up to by anticipatory
stages. I would suggest that the term REPETITIVE
Marks be used to designate these characters. These rep-
etitive marks, from their very nature, cannot be useful
characters nor can they be rudimentary. The most
probable explanation of them appears to be that they
have béen developed in accordance with the law of repe-
tition. This is a form of correlative development, but
it does not appear by any means adequate to completely
explain the case. In a general way it might be under-
stood that if a definite manifestation of growth force
were located in one feather, there would a tendency to
repeat this less and less completely in succeeding feath-
ers. It might seem, however, that this repetition pre-
cedes and leads up to the complete fori rather than fol-
lows it. Of course it is difficult to say which end is the
beginning of the series, but it might seem at first glance
as if it should begin with the first primaries and end
with the tertiaries, these latter being less specialized.
In reality, however, it seems more rational to suppose
that the tendency works from the simple to the complex.
Going from the secondaries toward the primaries it is
found that while certain characters are becoming more
developed others are fading out. Thus in wing of the
sparrow-hawk (Plate III), proceeding from the tertiaries
towards the primaries the black bars are found to be
progressing in complexity, while at the same time the
black subterminal spot of fig. 39 diminishes and lingers
on as a repetitive spot to fig. 30. All that can be said
of this successional taxology in marks as exhibited on

L772 CALIFORNIA ACADEMY OF SCIENUES.

the wing feathers is that it is not due to selection or any
external force, but rather to some unknown principle of
repetition or correlation.

Thus far the discussion has been confined to black and
white, or brown and white markings. As soon as
colored feathers are brought into consideration the
factor of selection immediately becomes more prominent.
lt is a noteworthy fact that along the line of division of
{wo patches of color the feathers are not uniformly
colored, some the one shade and others the other, on
the contrary the two colors in a large number of cases
appear on the same feather. It will be found con-
venient to designate these as Hyprip Fearuers. Take
for instance the rose-breasted grosbeak where the black,
white and rose color come together on the breast and
sides of the head. Some of the feathers are colored
black and white, the shaft dividing the two, and each
color contiguous to the general patch of that color.
Others are colored rose and white, the rose generally at
the base of the feather and adjacent to the rose patch,
the white at the tip of the feather and helping to cut the
white line of the breast from the rose of the throat.
Many rose and black feathers are to be found, the shaft
generally dividing the two colors, although sometimes
there is merely a black spot at the tip. The point to
he noticed is that the colors are not as a rule distributed
in accordance with any law of growth force, but in any
way which will produce the general effect of a rose
colored patch with moderately sharply marked limita-
tions, bordered with black on the sides and white below.
Occasionally a very interesting feather is found which
beautifully illustrates the interplay of internal and ex-
ternal effects. Such a one, for instance, was marked
according to the ideal mode of pigment deposition with
a dark tip and border, but only on the side nearest the

i

EVOLUTION OF THE COLORS OF BIRDS. 173

black patch, the pigment having been directed to that
side of the shaft by natural selection but still insisting
upon following the conventional route of travel.

The Californian woodpecker (Melanerpes formicivorus
baivdc) affords several especially fine illustrations of color
for effect regardless of the laws of growth. Thus at the
line of demarcation of the white of the frons and the
scarlet of the occiput, which is quite clearly cut, nearly
all the feathers are literally cut in two by the color, the
basal half being white and the distal end scarlet. Along
the sides of the head many of the feathers are scarlet
terminally and black basally, but’ I fail to discover any
feathers at the base of the scarlet patch which are black
terminally. Here again we have an illustration of how
natural selection must still be limited. The scarlet was
able to come in upon the white pigmentless feathers or
the white to encroach upon the scarlet, but the black did
not seem able to get a foothold upon the tips of the
scarlet feathers in a corresponding manner.

The throat markings are interesting as being remark-
ably free from hybrid feathers. This is particularly the
case with the feathers of the black jugular patch where
it meets the yellowish crescent of the throat, hybrids
being almost wholly absent here. They also play a very
subordinate role at the lower end of the throat crescent
where it joins the black of the breast, although some of
the feathers here have a narrow edging of black. The
red-shafted flicker (Colaptes cafer) presents some interest-
ing features with regard to hybrid feathers. The red
malar streaks, or moustaches, occupy a tolerably dis-
tinct pterylographical area, and are well separated from
the feathers of the throat by a bare tract. Accordingly
hybrid feathers are almost or wholly wanting on the in-
side edge and rare on the outside of the streak. The
black crescent on the breast is of especial interest be-

174 CALIFORNIA ACADEMY OF SCIENCES.

cause strongly marked hybrid feathers form both the
upper and the lower line of the patch. The upper line
is formed by white feathers with the distal half sharply
black, and the lower edge by black feathers with the
distal end edged with white. This is rather unusual,
for the rule is that when the base and tip of a feather
are differently colored on one side of a patch they pre-
serve the samme relative positions upon the other side.

In order to study these hybrid feathers more system-
atically I shall make the following classification:

I. Hybrids (feathers of two or more colors helping
to define a patch).

1. Symmetrical.

a. Lateral,
Relative positions uniform.
b. Transverse : Meet oe
Relative position insterchangeable
2. Asymmetrical.
a. Sharply marked.
b. Indistinctly defined.

II. Pseudohybrids (feathers of two or more colors
not helping to define a patch).

1. Sharply marked.

a. Both colors showing.
b. Only one color showing, the other concealed.

2. Tinged with some foreign color.

I will now explain each of these divisions and give
examples. I have restricted the term hybrid feathers to
such as are divided into two or more parts by color and
in which each color shows in the general pattern of the
bird. Where two colors show in a feather but have no
significance in defining a patch, I have termed them
pseudohybrids. Ina large proportion of cases patches
of color are marked on one side at least by true hybrid
feathers. When the feathers are divided into two toler-
ably uniform parts they may be considered as sym-

EVOLUTION OF THE COLORS OF BIRDS. 175

metrical. Then the question is are the divisions lateral
or transverse? This is determined in the main by the
position of the patch. If the feathers in question occur
on the side of a patch they are ordinarily divided later-
ally. Thus the white streak down the back of the downy
woodpecker (Dryobates pubescens) is defined along its
sides by feathers which are, in general, white on the
inner side of the shaft and black on the outer. So also
with the streaks on the head of the meadow lark (Stur-
nellu magne), the boundaries of which are fixed by
feathers which are white on one side and brown on the
other. Examples of tranverse symmetrical hybrids have
already been given, no better case being known than
such as the dividing line between the white and red
across the head of the Californian woodpecker (Melunerpes
formicivorus bairdt).

When a-band occurs, as for instance, across the head
or breast, cutting some uniform ground color into two
areas, the line of demarcation is generally made by
hybrid feathers on one side only. Thus the breast of
the belted kingfisher (Ceryle alcyon) is white, but crossed
by a broad, well-defined band of blue. The white feath-
ers along the upper border of this blue strip are edged
with blue, and both the blue and white in a single
feather are visible. Accordingly the line separating the
white from the blue along this edge is not made by the
tips of feathers of one color overlapping another, but
cuts right through the center of the visible part of the
feather. The blue feathers along the lower edge of the
blue patch, on the contrary, were not, in the specimens
examined, edged with white. In other words, the lower
border of the blue patch is marked off simply by the
tips of the blue feathers overlapping the white feathers.
This state of affairs is due to the fact that in all the
feathers of the breast the relative position of the colors

176 CALIFORNIA ACADEMY OF SCIENCES.

is constant, that is, if there be two colors, the white will
occupy the base of the feather and the blue the tip.
This is probably due to the fact that the breast of this
species was white, and that the blue pigment which
came into these white feathers, in following out a gen-
eral law of pigment distribution, sought the tip of the
feather in every case. Or the converse may be true,
that the breast was blue, and that in becoming white
the base of the feather was uniformly affected first.

The black crescent on the breast of the red-shafted
flicker (Coluptes cufer) has already been mentioned as a
case where the relative position of the colors on the
hybrid feathers is interchangeable. In other words,
both the upper and the lower border of the black patch
is defined by hybrid feathers—the upper border by
light feathers with black edgings, the lower border by
black feathers with light edgings.

We come next to the asymmetrical hybrids. It
would be wrong to suppose that there is a sharp
distinction between these two classes. Hybrids which
are not asymmetrical are really quite the excep-
tion. The distinction is made to express the difference
between hybrids where a definite plan is followed to
produce the effect, as by the cutting of the feather in
two, more or less regularly either transversely or lateral-
ly, and hybrids where no regularity is observed, but the
two colors distributed on the feather more or less at
random. Many of the hybrid feathers on the throat of
the rose-breasted grosbeak ( Hubiu ludoviciana) are of this
asymmetrical type. So also are they on the back of the
neck of the golden-crowned sparrow (Zonotrichia coro-
natw) where the black, white and brown are all mingled,
but the area of each color sharply defined.

Asymmetrical hybrids are sometimes not sharply de-
fined as in the preceding instances, but the two colors

EVOLUTION OF THE COLORS OF BIRDS. 177

blend more or less at the point of meeting. This is the
case in the vermilion flycatcher (Pyrocephalus rubineus
meaicanus), where the scarlet feathers along the border
of the crown are brown, basally, but the two colors are
not sharply separated on the feather. This is also the
case with the white and slate-colored hybrids of Junco
hyemalis.

Besides these hybrid feathers, which help to define
patches of color on the bird, there are other feathers
which are of two or more colors, but which are of no
apparent utilitarian significance. These I have called
pseudohybrids. They are especially interesting as
showing the sort of material furnished to natural
selection with which to produce effects. It is evi-
dent that along the line of demarcation of two colors
the pigments are apt to get confused as to which is their
proper route, and hence both come upon the same
feather, by accident, as it were. Natural selection
has frequently used this chance commingling for the
production of effects, but has not always done so.
Thus true hybrids are almost wanting in the crown
patches of the goldfinches, but pseudohybrids are
not uncommon. They are feathers in which the black
of the crown and the olive of the back are both
present, but instead of helping to define the black patch
they rather tend to break up the symmetry of its bound-
ary. This is especially noticeable in specimens of
Spinus psaltria and S. lawrence. In the latter the feath-
ers along the edge of the crown patch are pseudohybrids,
and the same is true of those between the yellow and
gray of the breast.

Another class of sharply marked pseudohybrids are
those in which one color is completely concealed by
overlapping feathers. Such pseudohybrids are very
common and are interesting as being in most instances

12

17s CALIFORNIA ACADEMY OF SCIENCES.

“apparently vestiges of some former color of the bird.
This under color is of no more utility than a rudimen-
tary organ. A good illustration of this class is presented
by the feathers of the black throat patch of the meadow
lark (Sturnella magna), These feathers generally have
a white basal portion anda black terminal portion, with
a more or less decided tinge of yellow upon the white
nearest the edge of the black. This is plainly a relic of
the time when the breast was all yellow, for only the
black is now visible, and the white and yellow bases
could be of no possible utility. These feathers are
very common in patches of bright color, as the scarlet
head of the Californian woodpecker, where the feathers
are black below the scarlet. It is interesting to note
that the extent and distinctness of the subterminal line
line of black on the scarlet feathers decreases in orderly
succession from the feathers of the frons backward
toward those of the nape. This is directly parallel with
the extent of the scarlet on the head of the different
forms now living. The scarlet in these decreases and
the black increases from the frons backward. In gen-
eral, it may be said that true hybrids occur only along
the border line of two color areas, while pseudohybrids
are sometimes the only form found throughout an entire
color patch.

There is but one other form of pseudohybrid to be
considered and this an uncommon variety. In the red-
winged black birds the line of division of white and
scarlet upon the shoulder is made by the scarlet feathers
overlapping the white. In other words, there are no
true hybrids. Frequently in Agelains tricolor an infu-
sion of pink or buffy is noticed upon the white feathers
as if some of the scarlet pigment had run in upon the
white by mistake. A. gubernutor frequently has a blush
of scarlet upon the buffy feathers.

aA

EVOLUTION OF THE COLORS OF BIRDS. 179

We come next to the general patterns of bird colors.
These are so infinitely varied and complex that it will
be almost impossible to bring them under any universal
law or laws, but at least a few generalizations must be
made. Eimer has stated* ‘‘that the appearance of new
characters always takes place at definite parts of the
body, usually the posterior end, and during develop-
ment—with age—passes forwards, while still newer
characters follow after from behind. Thus during life,
e.g. in lizards, a series of markings pass in succession
over the body from behind forwards, just as one wave
follows another, and the anterior ones vanish while new
ones appear behind.” This development from posterior
to anterior is illustrated by many examples among birds,
but seems to me to have far too many exceptions to be
held as as a general law of development. The succes-
sional taxology of markings on the under parts of many
of the hawks, conforms to this rule. Upon the breast
the markings are simple streaks, but from this stage
they pass into spots and from spots into bars which
are most distinctly marked upon the lower part of the
flanks and belly. This is especially well shown in the
western red-tailed hawk (Buteo borealis culurus). It is
also well illustrated by a large number of the wood-
peckers, especially in the genera Dryobates and Picoides
which have the flanks barred and the breast streaked or
plain. The reverse is true with the Californian wood-
pecker, where, as we have already seen, the markings
simplify instead of specialize as we approach the abdo-
men (see ante p. 163 and Plate II, figs. 1-6). So also in
the lark bunting (Calamospiza melanocorys), the specializ-
ation begins at the anterior end. In immature males the
throat first becomes black, and in a series of males it is
found that the tendency is for it to spread from this

* Organic Evolution, p. 28,

180 CALIFORNIA ACADEMY OF SCIENCES.

point downward to the tail in tolerably uniform succes-
sion. The Louisiana tanager (Pirangu ludoviciana), is
doubtless colored somewhat after the fashion of the
primitive color of the scarlet tanager (P. erythromelas),
which had a yellow ancestor; and here the specialized
color, the scarlet, first asserts itself upon the anterior
part of the head and throat, spreading thence down the
back-and breast, to a greater or less degree depending
upon the perfection of plumage of the bird.

Many of the conclusions concerning the feather
patterns will apply also to the pattern of the bird
asawhole. Thus, just as the streaked feather is the
most primitive so also is the streaked plumage. This
is exemplified by the young of a large number of birds.
Thus the breast in many of the sparrows, such as Zono-
trichia, Chondestes, Spizella, Junco, Amphispiza,Peucea,
and Pipilo, which is pure white or buffy in the adult is
streaked in the young, and in many instances the back
also is streaked. The pine siskin (Spinus pinus), seems
to have preserved this primitive plumage with very little
variation, in its adult state. Again, just as the accumu-
lation of pigment at the point or on the edge of the
feather is an early stage, so do we find many young
birds with a mottled plumage such as would be produced
in this manner.

Looking at birds broadly, the endless modification of
pattern and infinite variety in the form and position of
color areas is simply bewildering. Out of this chaos,
however, it is possible to bring some semblance of order
by showing what forms of marking never or seldom
occur and what forms are most often repeated.

In the first place there are no birds marked with an
asymmetrical pattern. It seems hardly fair to explain
this fact wholly by the law of bilateral symmetry, for
this law does not apply in the least to individual

EVOLUTION OF THE COLORS OF BIRDS. 181

feathers. Still there is probably a general tendency to
repeat the same pattern on both sides of the body, just
as there seems to be a tendency to repeat a general style
of marking on the lower that has been produced on the
upper part of the body. The asymmetry of hybrid
flickers is an abnormal case, and would hence not be an
exception to the rule of bilateral symmetry, which is
intended only for normal forms.

Although a streak may occur either just over or
through the eye, I know of no instance where a streak
runs near or through the eye at right angles to the su-
perciliary stripe. I knowof no species which has either
the throat or the top of the head barred, nor can I think
of any instance where any definite pattern of color
occurs on the back proper. Neither does any instance
come to me where a single streak down the middle of
the back (as in Dryobates pubescens) is continued without
interruption over the top of the head to the bill, nor
where a similar streak on the under parts, extending
down the throat and median breast, (as the carmine of
Pyrrhuloxia sinuata) extends also down on the abdomen
and under tail coverts. I can think of no species
in which asingle transverse band or bar crosses the back
below the nape (where it occurs in Dolichonyx oryzivorus,
for example), nor of any instance where such a band or
bar crosses the under parts of the body below the breast.
No instance is recalled where large spots of any kind
occur either on the throat or head.

It may seem like an idle task to enumerate these forms
of marking which do not occur, but by next noting what
styles are most common, the two will be brought out in
strong relief. The head is by far the most complexly
marked part of the body. Of the head markings the
superciliary stripe is the most universal, being char-
acteristic not merely of birds but also of many reptiles

182 CALIFORNIA ACADEMY OF SCIENCES.

and mammals. Next most common is the throat patch
which is not strictly but mainly confined to the ptery-
lographical region lying between the Jower jaw bones
and extending downward along that line. A transverse
stripe on the frons either of light or dark is very com-
mon, as in WMelanerpes formicivorus bairdi, Geothlypis
trichas, Dendroica dominica, etc. So also a transverse
band on the nape is common as in Cyanocittu cristata,
Dolichonyx oryzivorus, A median line on the head is com-
mon, as in Sturnellu magna, Regulus satrapa, Tyrannus
tyrannus, the young of Habia, and many others. A con-
trasted transverse band terminating the throat patch is
common, asin Hesperocichlyu nrveiu, Cyunocittu cristata, etc.
In fact, whenever the breast has any diversity of marking
a breast patch of some sort is the generalrule. The belly
is only very exceptionally marked, and then either
simply streaked or barred, or more rarely, spotted as
in Colaptes, but the flanks are usually marked either a
darker shade than the belly or streaked or barred. The
under tail coverts are frequently streaked, barred, or
colored quite differently from the belly as in the cat bird
(Galeoscoptes carolinensis) and Californian towhee (Pipilo
fuseus crissalis), The rump patch is very often
marked by some conspicuous color as in the yellow-
rumped warbler (Dendroica coronate) and the flicker
(Colaptes wuratus). The wing and tail markings are
strikingly varied and characteristic of species.

In accounting for these patches of color and their
distribution in diverse patterns it will be necessary, it
seems to me, to abandon wholly or largely internal laws
of growth as explanation. Dr. Harrison Allen has pub-
lished a paper on the Distribution of Color Marks of the
Mammalia* in which he elaborates the view that the
distribution of pigment is favored by the presence of

* Proc. Acad. Nat. Sci. Phil., 1888.

EVOLUTION OF THE COLORS OF BIRDS. 183

large masses of muscle lying beneath the surface or by
nerve terminals, which produce great local activity and
excess of nourishment in particular parts of the body.
Inasmuch as the large masses of muscle and important
nerve terminals are more or less constant, especially
among species of the same genus, whereas the areas of
color may be profoundly different, it does not seem
possible that this factor can be a very strong one, if
operative at all among birds. The frequent occurrence
of a patch of color on the breast where the largest
muscles of the body are situated, might be, to some ex-
tent, due to this influence however. With regard to the
great frequency of a strongly marked throat patch Dr.
Stejneger suggested to me the possibility that the con-
stant vibration of the throat in singing might be a
factor in the specialization of its color. There could
not be any direct connection between the motion of the
throat andsinging, however, because sometimes the patch
is distinguished by the presence of pigment as in Harris’s
sparrow (Zonotrichiu querulw) which has a black throat,
while in the same genus even, a species occurs in which
the throat is marked off by the absence of pigment—the
white-throated sparrow (Zonotrichi« ulbicollis). More-
over, the canary, which shakes its throat as if it would
burst, has not produced any perceptible difference in
this region, nor has the mocking bird (.Vimus polyglottos)
in which the throat and breast are uniform white. Dr.
Stejneger’s theory has suggested to me another view
which is not open to these objections. The frequent
vibration of the throat, especially during the courting
season, would make it almost the most conspicuous part
of the body, and it seems highly probable that when-
ever any suggestion of pigment occurred there it would
be retained and encouraged by sexual selection. It
might also prove a useful recognition mark in many
instances.

184 CALIFORNIA ACADEMY OF SCIENCES.

According to this same principle of the conspicuous-
ness of moving parts, other markings may possibly be
explained. A considerable number of birds have the
edge of the wing along the shoulder, decidedly marked
with yellow, white, rose, etc. It is especially on the
under side of the wing and not very readily seen when
the bird is at rest. During the courting season the males
of many species have a habit of rapidly vibrating their
wings in a slightly lowered position. I have noticed
this among the sparrows such as Gambel’s sparrow
(Zonotrichia lencophrys qambelt), and I think the western
savanna sparrow (dAnvunodranus sandwichensis alaudi-
nus). It is probable that this habit obtains with the
grasshopper sparrow (A. suvunnarwie pussercnus) in
which the wing is so conspicuously edged with yellow,
and has been developed by selection in the same man-
ner ag the throat patch. Frequently the under wing
coverts are colored peculiarly and sometimes very beau-
tifully, as in the rose-breasted grosbeak. When the
male bird is paying his addresses to the female this
patch would be sometimes obscured and sometimes visi-
ble, and accordingly more conspicuous and beautiful
than if always in plain sight. Audubon, in his plate of
this species, represents a male facing a female with
wings raised to show this patch, but it may be that he
drew it thus in order that the spectator might see the
marking, and not because he had ever seen the birds in
that attitude. The rump is very frequently colored dif-
ferently from the back and tail (as in Dendruieu corowate),
or when colored the same is more intense (as in Carpo-
dicus meaicunius frontalis). When courting, the wings
and tail are lowered and the rump accordingly is very
conspicuous. The under tail coverts are frequently col-
ored differently from the belly or tail, and generally in
species which lve amongst the underbrush dodging

EVOLUTION OF THE COLORS OF BIRDS. 185

about with the tail frequently thrown up in the air.
This patch of color would accordingly be of great use as
a recognition marking—but this subject will be more
fully discussed later.

It thus seems that the markings of the throat, under
tail coverts, rump and under wing may be explained by
selection. These markings are determined in their
shape chiefly, if not entirely, by the part affected, and
hence present no particular difficulties. With the head
markings the problem is, however, not so simple. We
have already seen that lateral markings are common
upon the head while transverse markings occur only in
special parts and there less frequently than the others.
The superciliary stripe is much the commonest of marks,
and being generally white in color, there seems far more
probability that it is due to some internal principle
than is the case with any other bird marking. The fact
that frequently where no superciliary stripe occurs in
the male it is present in the female or young would also
seem to indicate that it is constitutional rather than
utilitarian in nature. Instances of this sort are the bay-
breasted warbler (Dendroica castanea), red-winged black-
bird (Agelaius pheniceus), bobolink (Dolichonyx oryzi-
vorus), etc. There are at least a hundred and six North
American land birds in which the white superciliary
stripe occurs, from the bob white (Colinus virginianus)
at one end of the list, to the wood thrush (Turdus ius-
telinus) at the other. In the genus Dendroica it is
especially prominent, being present in eleven species
and generally very sharply defined. It is also present
in nine species of the genus Vireo, and among a large
number of warblers of other genera than Dendroica, as
well as many of the Fringillide. It is not always, although
generally,a narrow sharply defined line. In the western
night hawk (Chordeiles virginianus henvyi), for instance,

186 CALIFORNIA ACADEMY OF SCIENCES.

its boundary is not sharply defined, while in the chuck-
will’s-widow (Antrostomus carolinensis) it is a very broad
band. In the California jay (Apheloconue californica) it
is sharply defined but is composed of a series of fine
white streaks or spots, and hence is not a continuous
line. It hardly seems within the bounds of reason to
suppose that all the species which exhibit this mark
are the descendants of a common ancestor which pos-
sessed it, and that it has persisted without significance
for so long a time, although this would not perhaps be
any less conceivable than the retention by man of the
pineal eye. On the other hand, it can hardly be held
that the superciliary stripe is a recognition mark, be-
cause it is very often inconspicuous in life, although
sometimes very noticeable,as in the varied thrush ( Hesper-
ocichlu nwvia), where it forms one of the very best char-
acters for recognizing the bird instantly, even though
at a distance. Among many of the warblers, vireos and
sparrows, where it is present in closely allied species, it
would not serve, however, as a help in detecting the
species.

There is one thing which seems to be of some sig-
nificance—that this marking so universally present in
the most diverse groups is a streak and not a bar. This
fact together with the circumstance that it is frequently
present in the female or young and not in the adult,
and that the reverse is never (?) the case, would indicate
thatit is a marking of great antiquity, and has prob-
ably no very great utilitarian significance, except in
exceptional cases where natural selection has made use
of it for a recognition mark.

In general the markings of the head are lateral rather
than tranverse. They may be reduced, for the most
part, to certain types or modifications and combinations
of these. Plate IV represents the various patterns of

EVOLUTION OF THE COLORS OF BIRDS. 187

black among North American land birds. Nearly all
the characteristic head patterns are produced by the
black and white combinations here figured. There are
five general types of markings, all more or less simple
and rudimentary. They are all lateral, and it seems
probable that all transverse markings have been derived
from them. I have attempted in this diagram to show
how all the head patterns of North American birds are
related to these five types. Of course the relations be-
tween the widely different forms here associated are not
supposed to be genetic, but the diagram is merely in-
tended to show, by means of the arrows, how the various
patterns may have been derived by the modification of
the types. It is not difficult to see why these five pat-
terns should be the simplest and earliest developed. In
the first place they follow the general trend of the
feathers, which would be more natural than to cross it.
Then they follow pterylographical areas more or less.
It is easy to see how types land 2 might be bounded
by the eyes and upper edge of the ear coverts, while
type 3 includes little besides the ear coverts. Types 4
and 5 occupy definite feather tracts separated by bare
spaces, and their origin is accordingly not far to seek.
Polioptilu plumbea is figured as a representative of type
7, which includes also such species as Dendroica striutu,
Sitta carolinensis and Galeoscoptes carolinensis. It is a
simple black cap. Type 2, the superciliary stripe or
band is illustrated by Zonotrichia coronata, Serurus auri-
capillus and Regulus satrapa belonging in the same cate-
gory. The line through the ear coverts, type 3, is the
commonest form existing among North American birds.
The figure represents Saxicola enanthe, but the class in-
cludes also the following: Psaltriparus melanotis, Dendruica
coronata, Dendroica olivacea, Oporornis formosa, Lanius,
Passerina cyanea, besides numerous species where

188 CALIFORNIA ACADEMY OF SCIENCES.

this marking is associated with other head markings,
or where it is present but not black in color. The fourth
type—the malar steak, represented by Coluptes auratus
in the plate, is less commonly met with alone; but type
5, Dendroica vivens in the diagram, which is the throat
patch, is very common. There are only a few forms,
however, such as Dendroica oceidentalis and Amphispiza
bilincatu where it oceurs as the only black patch upon
the head.

The combinations of these five types follow next. Fig.
6is Dendroica tigriny, which presents.a combination,
not of types 1 and 2, but of 1 and modification a of type
2. Fig. 7, Dendroica bluckbuania, is a combination of
1 and 8, the black cap and ear covert. This is a very
common form, being presented with greater or less
deviation from the type by such forms as Sift cun-
ddensis, Certhiolu. bahamensis and Pitungus derbianus. Fig.
S is the combination of types 1 and 4, as shown in
Dendroica striata. Fig. 9 is a combination of types 1
and 5 illustrated by Parus utricapillus. Fig. 10, of
dAmpelis cedrorum is equivalent to 2 plus 5, and fig-
ure 11, Helminthophilu chrysoptera, equals 3 plus. 5.
Big. 100, Parus hudsonicus, is a combination of types
1,2 and 5; while fig. lla, Mnrcotilta varia, is a com-
bination of types 1, 3and 5.

There is little difficulty in understanding these types
and their combinations as markings which have fol-
lowed the feathers along natural areas of the birds’
heads. They may thus be in part accounted for in ac-
cordance with the location of growth force along lines of
least resistence, although natural selection has played a
not unimportant part in shaping and defining them.
Upon leaving the types and considering the various
modifications, it becomes immediately evident that here
the part played by selection must have been a far more

EVOLUTION OF THE COLORS OF BIRDS. 189

important one. Can any lawof growth be suggested,
for example, for chopping off a black line right through
the middle of the ear coverts as in fig. 3a, b andd,
and fig. 10e? By attentively following the arrows, it
will be seen how either by the modification or combina-
tions of patterns, or both, each form may be derived
from some preceding form, more or less completely.
Figs. 10e, 11¢, d, e, alone fail to be reduced to a com-
munity with the other patterns. It is unnecessary to
encumber the text with the names of the various species,
which are not essential to the point under consideration,
and may be found in the explanation of plates. A few
words should be added concerning the nature of some of
the relationships here indicated. By a reduction of the
posterior extent of the black cap in fig. 1, fig. la is pro-
duced. A continuation of this reduction would lead to
fig. 1b; while the cutting off of the front of the black
cap would give fig. le, which process continued would
end in fig. ld. Fig. 2a is hardly more than a narrow-
ing of the band of fig. 2, but fig. 2b is a combination
of this with fig. 16. In fig. 2c the band has become ex-
tended posteriorly abruptly downward, while in 2d
the angle is changed and it is extended not merely
downward but forward.

It will not be necessary to continue an interpretation
of all the transitions indicated in the diagram. I desire
simply to show that all the head markings of birds can
probably be interpreted as modifications of certain fun-
damental types which are probably the parent forms
from which all have sprung. They are in fact closely
allied to the primitive streaked plumage, and may be
looked upon as merely an aggregate of streaks in definite
parts of the head.

It may now be more readily apparent why some forms
of marking never occur while others are so general. It

190 CALIFORNIA ACADEMY OF SCIENCES.

is necessary to keep in mind two general influences, the
internal forces of growth regardless of the effect to
be produced, and external selection, which looks only at
the effect. We have now seen how selection would tend
to produce the most striking effects of form and color of
marking upon those parts which are most exposed, and
especially those parts which are most constantly in mo-
tion—the head, throat, wings, tail, rump and under-
tail coyerts. At the same time, other things equal,
these effects would always be produced in the easiest and
most natural way, rather than in direct opposition to
the laws of growth.

With regard to the wing markings, the laws of devel-
opment apply when the individual feathers alone are
taken into consideration, but the general effect of wing
marking is produced in a great variety of ways which
has no obvious relation to laws of growth or mechan-
ical forces. Wing markings are generally white in
strong contrast to some very dark shade, generally
brown, and are obviously recognition marks of some
sort. It might be thought that the white was due to de-
generation if it uniformly occurred at the tips of the
feathers where the wear is the greatest; but in point of
fact, while it is very often situated thus, it also occurs
at the base or through the middle of the feather. Thus
the cedar wax-wing (dAinpelis cedrorum) has no white wing
bars, while the Bohemian wax-wing (dAmpelis garrulws)
which is so closely related, has a wing-bar formed by
the white spots at the terminus of the primary coverts,
another on the tertiaries, and a vertical line down the
wing formed by the white and yellow spots on the outer
edge of the tips of the primaries. It surely cannot be
held that any internal law of growth, or external me-
chanical force which produced such a variety of effects
in the one species could have been almost or wholly in-

.

EVOLUTION OF THE COLORS OF BIRDS. 191

operative in a species of the same genus so closely allied.
In the rose-breasted grosbeak, the band of white situ-
ated apparently in almost the same place as in the
wax-wing, is produced not by white terminals on the
primary coverts, but by a broad patch of white at the
base of the primaries themselves, which is partly cut off
by the coverts, leaving only a white band. In the Cal-
ifornian woodpecker (Melanerpes forniicivorus baird?) the
white markings on several of the feathers run through
the center of the primaries leaving both the base and
tip uniformly dark.

The tail markings also vary in different forms to such
an extent as to be inexplicable by any factor but selec-
tion. There are, on the other hand, certain points in
regard to them which demand an appeal to internal laws
for explanation. It is a noticeable fact that the two
outer tail feathers have the markings most strongly de-
veloped, and that they decrease as we approach the
central feather. Of course, it would be said that the
outer feathers are the most conspicuous, and the mark-
ings would be of more use here than on the inner feath-
ers, and this is doubtless the true explanation in such
forms as the junco (Junco hyemalis), where only the two
outer feathers are thus marked. In the blue jay (Cyano-
citta cristata), on the other hand, the tail is marked with
a terminal band of white, which diminishes in a regular
sequence towards the center. The two central feathers
have only a minute tip of white, which would be in-
visible even to the closest scrutiny at the distance of a
few feet, and hence could not have been produced by
selection. This is another example of repetitive marks,
it seems to me. Natural selection has developed this
white bar on the outer feathers as a recognition mark,
and it has been repeated according to the law of corre-
lation in a decreasing series toward the center. In the

192 CALIFORNIA ACADEMY OF SCIENCES.

course of time these inner marks might become of suffi-
cient size for natural selection to make use of, and then
they would rapidly increase in size. The tail of the
belted kingfisher (Ceryle alcyon) is an interesting in-
stance of this sort. When the tail is in a normal posi-
tion of rest, the two outer tail feathers almost or quite
touch. The bars on these two feathers are so symmet-
rically placed that they invariably meet, forming one
continuous line. On the outer web, just opposite the
tip of each bar, is a spot of white. This complex ar-
rangement has obviously been due to selection, for by
no law of pigment distribution could the bars on one
feather be made to match the bars on the opposite feather
with such perfect accuracy. On the innermost feather
of the tail the bars are present, but so very faint as to
be scarcely discernable, and on the successive feathers
passing outward they become more and more sharply
defined.

Certain birds display a curiously converse form of tail
marking, having the greatest specialization on the two
inner tail feathers. This is markedly the case with
some of the woodpeckérs, such as Sphyrapicus vurius,
which has the outer tail feather barred distinctly on its
outer web, and less so on the inner web, this marking
repeated but much less complete on the next feather
within, followed by two feathers entirely unmarked and
with the two inner feathers the most strongly barred of
all. Sometimes these two inner feathers are barred on
both outer and inner web, the black predominating on
the former, and the white on the latter (young); while
sometimes the outer web is black without marks, and
the inner web white with narrow bars of black (adult).
Of course, this instance is at variance with the rules, but
becomes intelligible when it is seen that the inner webs
of the upper tail-coverts, and sometimes the entire

EVOLUTION OF THE COLORS OF BIRDS. 198

feather, indeed, is white. A conspicuous line of white
is thus begun down the middle of the rump and car-
ried out on these two tail feathers. As the tail is flat-
tened against the tree these feathers would stand out
very distinctly and a broad line of white would be the
result, very characteristic of the bird in question. The
barring is of no utility, but a mere repetition of the
tendency to form bars on the outer tail feathers, as is
shown by the fact that the bars become obliterated with
advancing age, until in highly plumaged individuals
these inner tail feathers are almost uniformly black on
the outer and white on the inner side of the shaft.

RECOGNITION MARKS.

We may now consider the subject of color markings
from an entirely new point of view. Thus far the
question has been looked upon largely, if not entirely,
from the standpoint of the dead bird. An attempt has
been made to show that both the distribution of pig-
ment upon the feather and the formation of general
patterns of color were to a greater or less extent de-
pendant upon the laws of growth resident within the
organism. In considering this problem, however, the
ubiquitous natural selection was constantly dogging our
footsteps and demanding a hearing, but it is now time
to listen to its cause argued by an advocate for its rights
and not simply tolerate its presence as a necessary evil.

Darwin was inclined to attribute the diversity of color
in birds rather to the action of sexual selection than to
natural selection. Accordingly such instances as the
conspicuous white tail of the rabbit he found difficulty
in explaining. It is to Wallace that we must turn for
an elucidation of the real meaning of these conspicuous

colors. He has shown that a large number of color
13

194 CALIFORNIA ACADEMY OF SCIENCES.

marks, which had been previously thought to be of no
significance, were in reality of use as a means of recog-
nition by the individuals of a species among themselves,
or by the individuals of one species of other allied
forms. Mr. E. B. Poulton has elaborated and system-
atized the various theories of color marks, principally
as originally presented by Mr. Wallace, and I will use
his classification and terminology as given in his work
On the Colors of Animals, merely simplifying it so as
to exclude such classes as are not found among birds.
His table of colors classified according to their uses may
be modified for the present purpose as follows:

A. APTATIC COLORS (deceitful). Causing an animal to resemble its
environment, or to be mistaken for some other species.
I. Crypric Coors. Protective and aggressive resemblance.

1. Procryptic—Protective.
( Variable.

w. General protective tesa blancs) Goustant

b. Special protective resemblance.
2. Anticryptic—Agaressive.
f Variable.

c. General aggressive resemblance
Be | Constant.

d. Special.
II. PseuposeMaTic Coutors. False warning and signaling.
1. Pseudaposematic—Deceptively suggesting something dangerous
to an enemy.
2. Pseudepisematic (alluring colors). Deceptively suggesting some-
thing attractive to prey.

B. SEMATIC COLORS (signaling). Warning and signaling colors.
I. AposemaTic Cotors. Warning colors.
II. Episematic Cotors. Recognition marks.

lL, Diverter,
w. Recognition by distant stragglers.
b. In close flight.
c. In migrations.
2, Discriminative,
3. Sexual.
4. Socialistic.
d. In darkness of night.
e. In burrow.
J. In care for young.

C. EPIGAMIC COLORS. Colors displayed in courtship.

EVOLUTION OF THE COLORS OF BIRDS. 195

Let us now consider these various classes and see how
they may help in the elucidation of the color problem.
Omitting the more inclusive divisions as sufficiently
clear, let us commence immediately with (1) Procryptic
colors. This includes all of that large class of forms
whose colors harmonize with their environment for the
purpose of protecting them from enemies. In many
cases it is extremely difficult to say whether an apparent
adaptation to a given environment is due solely to nat-
ural selection, or solely to the direct action of the envi-
ronment, or to the co-operation of these two factors. In
many instances, such as the white of Arctic animals and
the pale color of desert forms, the latter view will prob-
ably be found to be correct; but as the influences of the
environment are to be discussed later, nothing more need
be said upon the subject at present.

These protective colors may vary with a changing
environment, as is the case with the ptarmigan (Lago-
pus), which is dark in summer and white in winter,
and possibly with some of the goldfinches such as Spinus
psaltria, which is bright yellow and olive green during
the summer when the yellow Composite, upon which
it is almost invisible, are in bloom, and buffy and olive
brown in winter when it is almost equally inconspicuous
among the dry weeds. It seems probable that this is
a case of a bird which acquired its yellow breast largely
by sexual selection, and afterwards this character was
made use of by natural selection as a variable protec-
tive color. The house-finch (Curpudacus mexicunus fron-
talis) varies with the seasons also, and its winter coat is
a protective one, but the summer dress does not happen
to be of a color which can be turned to account.

By far the greater number of general protective colors
are constant, and harmonize with the most prevalent
environment only. Nearly all of the Gallinaceous birds

196 CALIFORNIA ACADEMY OF SCIENCES.

are colored upon the upper part of the body some
mottled shade of brown or dun, which blends wonder-
fully with the ground upon which they alight. The
perfection of protection of the colors of the nighthawk
and whip-poor-will which sleep in exposed places during
a large part of the day, is a source of wonder to every
country boy. Most of the smaller flycatchers are so ob-
scurely clad and blend so completely with the branches
upon which they alight, that in walking through the
forest the observer frequently knows them simply as
roa, et prutera nihil, The horned lark as it squats upon
the plain is often nothing but a song in the air, and
defies the closest scrutiny. Most of the sparrows, in the
general style of their dress—Zonotrichia, Spizella, Mel-
ospiza, Peuceea and Passerella, are colored harmoniously
with their environment. The buff and olive green of
most of the vireos make them very difficult to detect,
even when singing close at hand. So also with the
wrens, titmice and thrashers.

It will be noticed that species habitually found near
the ground are generally colored brown, while species
found among the trees are either greenish or olive, as
with so many of the vireos, warblers, etc., or gray, the
colors thus blending either with the leaves or branches.
I would suggest that there is great probability that the
habits of birds have been more or less determined by
their colors. If, for example, the natural color of the
pigment of a group of birds was olive green or gray,
those individuals which formed the habit of living
among the trees would survive, those frequenting the
ground, being more conspicuous, would perish. Still
more probable is it that, other things being equal, the
brown birds which got up among the trees would be de-
stroyed, while those which remained near the ground
would live.

EVOLUTION OF THE COLORS OF BIRDS. 197

Iam not sure that any instance exists among North
American land birds of (b) special protective resem-
blance. The nearest approach to it is the perfect imi-
tation of the bark of the tree by the markings of the
back of the brown creeper (Certhia familiaris americana).
This is in reality only an unusually complete case of gen-
eral protective resemblance. Mr. Wm. V. Praeger has
suggested an instance among wading birds which should
be inserted here as illustrating this very unusual class.
He says:* ‘‘To the protective colors which are unusual
among the shore birds I had always considered the
neck and head marking of the genus #gialitis a strik-
ing and curious exception, till a short time ago, when
looking at an 4, semipalmata, which I had wounded,
trying to hide by crouching in a hollow in the sand; and
while admiring the perfect blending of its brown shades
with the surroundings, I saw in its white rings one of
the commonest objects of the sea shore—the empty half
of a bivalve shell. The white about the base of the bill
was the ‘‘ hinge,” the collar the outer rim, and the top
of the head the cavity of the shell, filled—as they
usually are—with sand. In the cabinet drawer the re-
sembance is not so noticeable, but such resemblances
rarely are, and it was striking among the natural sur-
roundings when I first observed it, and it is most perfect
when the bird is crouching as it does in the presence of
danger or when on its eggs.”

(2) Anticryptic colors differ from the preceding
merely in the fact that they are used by rapacious birds
in approaching their prey, instead of by harmless
species in avoiding danger. (c) General aggressive col-
ors are illustrated by the mottled plumage of most of the
owls, the burrowing owl (Speotyto cuniculariu hypogea),
being a particularly good example. These forms are all

*Auk, viii, p. 236.

198 CALIFORNIA ACADEMY OF SCIENCES.

constant in their color and I know of no species which
is marked with general variable anticryptic colors. The
snowy owl (.Vyelea nyctet), would come nearer answering
the requirements than any other species, but the change
in this species is dependent more upon age than sea-
son, and it would accordingly not apply. Of (d) special
anticryptic colors, the belted kingfisher (Ceryle aleyon),
is a partial example, and the only one with which I am
familiar. The colors of this bird are such that they
blend, not with the environment of the bird particularly,
but with the water aud sky as scen by the fish as its foe
descends upon it from above.

(II) Pseucdosematic colors are seldom met with among
birds—at least among North American species. Wallace
has suggested that many of the plumes and appendages of
birds may be used to frighten enemies, but I have been
unable to detect any instances of their use for this pur-
pose. Ordinarily, when an enemy threatens, a bird will
seek safety in flight, rather than try the doubtful experi-
ment of frightening a foe by its formidable appear-
ance. There are some cases of protective mimicry
among birds, although I know of none among North
American species. ‘‘A most extraordinary case,” says
Dr. Stejneger,* ‘is that of the Indian, so-called drongo-
cuckoo (Nurniculus dicruroides), which, as indicated by
the name, so exactly imitates the king-crow, or drongo-
shrike (Dicrwrus), inhabiting the same locality, in size,
form and color, that there is required considerable at-
tention in order not to confound them, though the ar-
rangement of the toes, of course, at a closer inspection
is alone sufficient to separate them. This imitation is
the more strange since it has even extended to the curi-
ously furcated tail, a feature elsewhere entirely unex-
ampled among the cuckoos.” Further study may bring

* Riverside Natural History, iv, p. 380.

EVOLUTION OF THE COLORS OF BIRDS. 199

to light instances of these (1) pscaduposematic colors
among North American birds.

, There is at least one excellent illustration of (2) psew-
depisenatic colors, however, among our native birds.
Dr. Stejneger quotes Mr. Charles W. Beckham on the
case in point, referring to the kingbird (Tyrannus tyran-
nus):* “ Several years ago, in May, I saw one of these
birds occupying an exposed perch on a pear tree in
bloom, about which many bees were darting. Several
times I observed that the bird caught the insects with-
out leaving his perch by quickly turning his head
and ‘grabbing’ them. My attention being thoroughly
aroused,I noticed that many of them seemed to fly directly
towards him; the majority appearing to ‘shy off’ ata
short distance and change their course, but very few
that came within reach escaped him. The question
naturally suggests itself: Did the thrifty Hymenoptera
mistake the fully displayed orange-red crown (I could
see that the crest was erected), for a flower? Once since
I have observed the same phenomenon, but not as well
as upon this occasion. Mr. C. C. Nutting, who has
spent considerable time studying the birds of Costa Rica
and Nicaragua in their native haunts, states that he has
seen Muscivora mexicana perched upon a twig, and wav-
ing its curious and brilliant fan-shaped crest after the
manner of a flower swayed by a gentle breeze and thus
attracting insects within reach.”

It is not impossible that the crest of the kinglets and
other insect eating species similarly adorned, even
though developed by sexual selection may have after-
wards been used for attracting insects. It would re-
quire a long series of careful observations to demon-
strate this point however.

* Riverside Natural History, iv, p. 469.

200 CALIFORNIA ACADEMY OF SCIENCES.

By far the largest number of useful markings are in-
cluded under the head of (B) Szematic Cotors. As a
general rule the ground color of a bird is the protective
color while many of the detailed markings. are for
recognition. Warning colors (I. Aposesiutic) are ap-
parently almost as rare in the bird world as the false
warning marks. Theonly genuine example with which
I am acquainted is to be found among the water birds.
It is the conspicuous white patch upon the head of the
tufted puffin (Lunda cirrhutu) which is very prominent
when the bird is upon its nest at the end of a burrow,
serving as a warning for intruders to avoid the sharp
and indeed dangerous beak which they are sure to
encounter.*

The (I1) £pisemutie colors however, are very con-
spicuously representedamong all classes of birds. These
are what are commonly known as recognition marks.
White, particularly in contrast to black, or some other
dark shade, is most frequently used for this purpose.
The markings of the wing and tail can doubtless be ex-
plained in this way in nearly every instance. It is
upon this class of markings that Wallace lays the greatest
stress, as accounting for a large proportion of the
diversity of markings and patterns in the bird world.
In explaining these markings he says: ‘‘If we con-
sider the habits and life-histories of those animals which
are more or less gregarious, comprising a large pro-
portion of the herbivora, some carnivora, and a con-
siderable number of all orders of birds, we shall sce
that a means of ready recognition of its own kind, at a
distance or during rapid motion, in the dusk of twilight
or in partial cover, must be of the greatest advantage
and often lead to the preservation of life. Animals of

* of, Zoe iii, pp. 161-162.
t Darwinism, p. 217.

EVOLUTION OF THE COLORS OF BIRDS. 201

this kind will not usually receive a stranger into their
midst. While they keep together they are generally
safe from attack, but a solitary straggler becomes
an easy prey to the enemy; it is, therefore, of the
highest importance that, in such a case, the wanderer
should have every facility for discovering its companions
with certainty at any distance within the range of
vision,

Some means of easy recognition must be of vital im-
portance to the young and inexperienced of each flock,
and it also enables the sexes to recognize their kind and
thus avoid the evils of infertile crosses; and I.am in-
clined to believe that its necessity has had a more wide-
spread influence in dertermining the diversities of ani-
mal coloration than any other cause whatever.”

Mr.J.E. Todd, entirely independently of Wallace, pub-
lished in 1888 a similar theory of animal colors, calling
them directive colors.* He makes the following classi-
fication of directive colors: 1. ‘‘ Marks and tints, pro-
moting recognition at a distance, to guide in straggling
flight and to bring stragglers together [A]. 2. Those
indicating the attitude of the body and its probable
movement [B] in darkness of night, or in dens; [C] in
close movement of large numbers, by day as well as by
night; [D] in intercourse of the sexes [E] in the care of
young.”

In a previous paper? I have proposed a somewhat dif-
ferent classification of recognition markings. Neither
Wallace nor Todd seem to have drawn a sharp line of-
distinction between such recognition marks as enable
an individual to find those individuals with which it is
to his advantage to consort, as for example, such marks
as enable stragglers to regain the flock; and those recog-

* American Naturalist, xxii, pp. 201-207.
t Zoe ii, pp. 210-216.

202 CALIFORNIA ACADEMY OF SCIENCES.

nition marks by which individuals are enabled to
mutually avoid one another, as, for instance, the dis-
tinctions of species by characteristic marks, by which
infertile crosses are prevented. These two classes may
be termed respectively directive and discriminative
recognition marks. <A third class includes such marks
as are of use in the intercourse of sexes, the care of
young, or in the darkness of holes or burrows. For
these the term socialistic marks may be used.

In the paper in Zoe cited above I found difficulty in
explaining these recognition marks in harmony with the
survival of the fittest, for the individuals possessing
these marks are not thereby benefited but rather
benefit others. In the introduction to this paper (see
ante p. 108), I have indicated the manner in which these
might be explained through the necessity for the sur-
vival of the family.

Directive marks may be of three sorts: those which
enable stragglers at a distance to regain the flock—espe-
cially well illustrated by a flock of moving sandpipers,
which as they wheel about now flash into sight as a flut-
tering mass of shining white and then disappear, like a
flash light which throws its rays now upon one point of
the compass and now upon another—those which enable
a bird to follow its companion or companions through
dense underbrush, as for instance some of the towhees,
(Pipilo), which are so conspicuously marked with black
and white, and which always frequent the densest tangle
‘of the bushes—and those which enable migrating birds
to follow their leaders. As migrations are so largely
conducted by night, call notes and the general manner of
flying are probably more relicd on for purposes of recog-
nition than color marks, although I am inclined to
think that the latter may at least in some instances also
be called into play. Todd has suggested a number of

EVOLUTION OF THE COLORS OF BIRDS. 208

different methods in which recognition at a distance
may be effected. Thus one means is ‘“‘ by having the
general color more or or less strikingly contrasted with
the environment,” as with crows, buzzards, bluebirds,
woodpeckers, etc. It seems hardly possible that colors
such as the complex patterns of the woodpeckers, for
example, can have been developed merely in order that
the general effect shall be in contrast with the environ-
ment. With the crows and blackbirds, however, which
habitually consort in flocks, it is quite -possible that
means of recognition has been at least one factor in the
production of the black color.

It is difficult in many instances to say definitely
whether a certain mark should be classed among those
which are useful for recognition at a distance or in close
flight. The white rump-patch and flash of scarlet serve
to distinguish the red-shafted flicker (Colaptes cafer) as
far as the bird can be seen, and the same may be said of
the superciliary stripe and wing bars of the varied robin
(Hesperocichla nevia). The markings of Audubon’s and
the yellow-rumped warblers (Dendroicu auduboni and D.
coronata) are especially good recognition features. They
are probably of more use near at hand than at a dis-
tance although they might be of service at tolerably long
range, and especially during migrations. These mark-
ings are most conspicuous just as the bird takes flight,
at which time the yellow rump patch and white mark-
ings of the tail combine in a pattern which irresistably
catches the eye. The meadow lark (Sturnella magna)
just as it settles down on the ground exposes the white
outer tail feathers, which do not appear as a rule either
when the bird is at rest or in flight. Allusion has al-
ready been made to the reddish under tail coverts of the
Californian towhee (Pipilo fuscus crissalis). As the
bird hops rapidly about among the bushes they are

204 CALIFORNIA ACADEMY OF SCIENCES.

frequently exposed, and when it alights quickly the tail
is frequently thrown forward as if the bird had lost its
equilibrium for the instant, thus showing this patch of
contrasted color. It is without doubt an instance of a
recognition character which would be of great use as a
directive mark to birds living in the dense underbrush.

The white patch on the wing of the shiny-crested fly-
catcher (Phuinopeplu nitens) is concealed when the bird
is at rest, but very conspicuous in contrast to the black
of the body when the bird is in motion. The wing
band of Townsend’s solitaire (Alyacdesles townsendii) is
concealed when the bird is at rest, but conspicuous in
motion. In many species the white outer tail feathers
are folded out of sight while the bird remains passive,
but are noticeable the moment the bird starts to fly.
Such, for instance, is the case with Junco, Anthus, etc.
The subterminal or terminal spots or bars on the tail are
always more conspicuous when the bird is in motion
than when at rest.

The grouse, pigeons and hawks, present so many in-
teresting modifications of recognition markings upon
the tail that it may be worth while to consider them in
a little more detail. In the genus Dendragapus the tail
is black, tipped with a distinct band of ash gray in the
dusky grouse (D. vbscurus), and with a narrower band in
the sooty grouse (D. obscurus fuliginoses), which is
almost entirely wanting in Richardson’s grouse (D. ob-
seurus vichurdsvni). In the Canada grouse (D. cunuden-
sis) the tail is tipped with ochraceous rufous, while in
Franklin’s grouse (D. franhlinit) it is black to the tip.
The black tail is doubtless a directive recognition mark,
but the modification in the terminal bar is in all proba-
bility a character belonging to the class of (2) diserimi-
native marks. When, from differences in climate, food,
or selective influences two forms occupying adjacent

EVOLUTION OF THE COLORS OF BIRDS. 205

territory have differed to such an extent that the sexual
organs are affected and they are no longer fertile inter
se,* any mark by which they might distinguish their
own kind from the other stock, would serve in the per-
petuation of the race by preventing infertile crosses.
These different wing bars in the genus Dendragapus
may not improbably be characters of this sort. In the
band-tailed pigeon (Colwmbe fusciatu) the terminal third
of the tail is lighter and is separated from the basal por-
tion by a broad band of blackish. It may be in this
instance that the black bar was originally developed as
a discriminative mark, and that now, when no longer
needed for this purpose on account of the isolation
of the species from allied forms, natural selection
has turned it to account as a directive mark. In
the passenger pigeon (Lectopistes migratorius) the tail
markings are highly complex. They are thus described
in Ridgway’s Manual: ‘‘ Shading from dusky on middle
feathers through gradually lighter shades of gray, to
white on outer webs of lateral pair, the inner web of
each feather (except middle pair) with a transverse
blackish spot, preceded by one of rufous.” The neces-
sity for directive recognition marks would be especially
necessary in a species moving in such large flocks, from
which individuals or small parties would constantly get
astray. It may be that the complex pattern on the tail
feathers was evolved first as a discriminative mark, for
except in size, the species might easily have been con-
fused with the mourning dove (Zenaidura macroura)
which has the tail marked with a simple irregular sub-
terminal band of dusky. The white-fronted dove (£n-
gyptila albifrons) has an especially characteristic form of
recognition marking on the tail, all but the middle pair
of feathers being black, broadly tipped with white. The

“See discussion of physiological selection ante. p. 123.

206 CALIFORNIA ACADEMY OF SCIENCES.

white-winged dove (Melopelia leucoptera) has the termi-
nal fourth of the tail white, the basal portion of the
feathers (except middle pair) being siaty.

Among the hawks the tail is often the most character-
istic mark of recognition at a distance. The marsh
hawk (Circus hudsonicus) may always be distinguished
ata distance by his white rump patch, and the black
markings on the head of the sparrow-hawk (Falco spur-
verius) serve to distinguish it, and yet both these spe-
cies, and nearly all other hawks have some char-
acteristic marks upon the tail. Thus the tail of the
marsh hawk is tipped with white, with a broad blackish
subterminal band and several narrower bands anterior
to it. Accipiter has the tail crossed by several bands of
blackish and narrowly tipped with white. The tail of
Parabuteo is black, with the base and tip white, while
in many species of Buteo it is crossed by a subterminal
band of blackish and with a white tip. There are very
many variations in the form of tail marking in the
genus Butco, and these are doubtless discriminative
marks which were originated soon after the different
forms began to diverge. Among the red-tailed hawks
(Buteo borealis) the general color of the tail is a striking
example of a directive mark by which the species may
be known at a long distance. The different races vary
in the detailed characters of the tail, and these marks
are apparently discriminative in nature, all the mem-
bers of the genus having them in some form. In the
broad-winged hawk (Buteo latissinus) the ground color
is blackish, crossed by from two to four broad bands of
light brownish gray, while in Swainson’s hawk (B.
swainsonc) there are nine or ten bands. In the white-
tailed hawk (B. albicaudutus) the ground color is white,
crossed by a broad subterminal band of black, and an-
terior to this by five bars or lines of slate or dusky. If

EVOLUTION OF THE COLORS OF BIRDS. 207

we suppose the various species of this genus to be the
descendants of a common ancestor, we can hardly in-
terpret this great diversity in the distribution of the
bars on the tail to any principle but natural selection
producing discriminative recognition marks.

The tail-markings among the Caprimulgide are pe-
culiarly characteristic. They are interesting because,
instead of affording merely specific distinctions, they
form generic characters also. In the whip-poor-wills
(Antrostomus) the three outer tail feathers are tipped
with white on the inner web in the male. In the poor-
will (Phalenoptilus) all but the inner pair are broadly
tipped with white in the male, and less broadly in the
female. The tail markings in the parauque (.Vyctidro-
mus) are very peculiar. They are thus described in
Ridgway’s Manual: ‘‘ Outer tail-feathers nearly uniform
blackish; next mostly white, the outer web edged with
dusky; four middle tail-feathers without any white,
their ground-color mottled brownish-gray, relieved by
irregular ‘ herring-bone’ blotches of dusky along the
shaft.” In the night-hawks (Chordeiles) the males have
a broad bar of white on all but middle feathers near the
tip of the tail. The variations in the tail patch of the
different forms of the genus Antrostomus presents so
many interesting features that a tabular view of them
may serve to bring the points before the mind:

208 CALIFORNIA ACADEMY OF SCIENCES.

ANTROSTOMUS.

$ terminal third of three outer tail-feathers white or buffy.
Q no white or buffy on tail.

carolinensis. .

6 four outer tail-feathers with
white tip.

cubaneusis ... 2 : White patch occupy-
@ three outer tail-feathers with ing less than termi-
bet hp nal third, and de-
3 three outer tail-feathers with creasing in extent
macromystax white tip. from exterior feather
@ three outer tail-feathers with
buff tip.

| 6 White patch, occu-

pyiug more than ter-
| minal third, and in-
° creasing in extent
from exterior feather
J 2 No white tail patch

vociferus—Tail patch more extended.

vociferus arizone—Tail patch less extended.

There are many interesting questions opened by this
survey of the tail markings of the Caprimulgidee. The
initial markings were doubtless discriminative in nature
coupled in many instances with other marks such as
the wing bars and throat patch, and were of use at the
time when each of the present four genera was a species
diverging from a common stock. As the species became
more and more distinct, and in some instances more
completely isolated geographically, these marks were no
longer of necessity for purposes of discrimination and
gradually became of service as directive marks. Then,
as the species of Antrostomus began to divide up into a
number of forms, a new set of discriminative marks be-
came necessary. The white patch occupying less than
the terminal third and decreasing in extent from the
exterior feather would present a very different general
effect from the patch occupying more than the terminal
third and increasing in extent from the exterior. If
these markings be discriminative in their nature it is
possible to understand why they would be more constant
and more completely developed in the male than in the

EVOLUTION OF THE COLORS OF BIRDS. 209

female, or how they may even be wholly lacking in the
latter. The female selects the mate and not the male;
and it would be quite possible for natural selection so to
adjust matters that the female would always prefer to
mate with a male marked in one manner. Those females
which selected for mates individuals differently marked
(27. €., belonged to a different race, and consequently
sterile when crossed), would leave no offspring and the
tendency would accordingly be for those only to survive
which selected mates of the ome pattern. In this man-
ner by a process of natural selection a form of sexual
selection having no relation whatever to esthetic
choice, but simply to the survival of the fittest would be
instituted. Wherever a marked character is present in
the male which seems to be more of a mark of recog-
nition than of beauty, and absent in the female, it may
be explained, it seems to me, according to this principle
which may be called the Law or SexuaLt ReEcoanitTion.
The crown patches of the white-crowned sparrow (Zono-
trichia leucophrys), and the golden-crowned sparrow (Z.
coronata), the black cap of the male gnatcatcher (Poli-
optila), the black throat patch of the male house-sparrow
(Passer domesticus), and many other instances, are exam-
ples of the workings of thislaw. This law of sexual recog-
nition might frequently be coupled with the law of sexual
intensification, so that both sexes would progress along
a certain line, the males in advance, until the latter had
reached the goal of specialization, after which the female
would ‘‘catch up” to the male. Markings which had
thus been sexual recognition marks would become ordi-
nary discriminative marks and finally directive marks.

These sexual recognition marks are closely connected
with the fourth and last class, viz.: socialistic markings.
These are such marks as assist in the domestic and
social relations of the family. They are particularly

14

210 CALIFORNIA ACADEMY OF SCIENCES.

well represented among mammals—the white face mark-
ings and ringed tail of the raccoon (Procyon lotor) being
an admirable example, but instances are not lacking
among birds. Here, however, it is more difficult to
distinguish them from other forms of recognition marks,
but future study with this end in view will undoubtedly
disclose many examples. In the corner of my sleep-
ing room, just opposite the foot of the bed, stands a
mounted western great horned owl (Bubo virginianus
suburcticus), On awakening one morning before dawn
a white spot caught my eye which at first I was unable
to account for. It soon occurred to me that it was the
white throat patch of the owl, and such it proved to be,
standing out with wonderful clearness as relieved by
the dark plumage surrounding it. It is easy to see how
valuable this mark would be to a family of owls in the
prosecution of their domestic duties. The white throat
patch of the night-hawks (Chordeiles) would be similarly
useful. The conspicuous white markings on the eye-
brows, chin and chest of the burrowing owl (Speotyto
cunicularia hypogea) is an admirable instance of a
socialistic mark which is useful in the darkness of the
burrow. Many of the white markings about the heads of
woodpeckers doubtless serve the same purpose, and also
the white throat of the belted kingfisher (Ceryle alcyon)
and the white breasts of the white-breasted swallow
(Tachycineta bicolor) and the violet green swallow (T. thal-
lussina). In these instances the white color is of that
peculiar purity that would be especially conspicuous in
the darkness of a burrow. The white feathers of downy
young birds are doubtless of no utilitarian significance
so far as their origin is concerned, but would neverthe-
less serve to mark the position of the bird in the nest
if in a dark burrow or hole.

EVOLUTION OF THE COLORS OF BIRDS. 211

VARIATION OF COLOR WITH SEX, AGE AND SEASON.

Before natural selection can be established as a factor
in the differentiation of color-patterns, it must be shown
that the markings are not perfectly constant. Probably
nowhere in the system of the bird is there such scope
for individual variation as in pigment distribution. In
the first place, no two feathers are ever marked exactly
alike. On comparing the pattern even on the right and
left wing of the same bird very marked individual diff-
erences are at once apparent, although the general effect
may be the same. Besides, these individual feather
differences which are universally present but which do
not alter the general appearance, variations of a more
striking nature are not uncommon. The pattern may
be more or less sharply defined or intensely colored
even in birds of the same age, sex and locality. As a
good example of variation in the markings of the tail, I
have figured the three outer tail-feathers of the downy
woodpecker (Dryobates pubescens) in very pale, normal
and in extremely darkly barred examples (Plate I, figs.
1-3). Again, the general color may be paler or darker,
differences of this sort being very striking in some
species, as the meadow lark (Sturnella magna), for ex-
ample; or, on the other hand, a species may be so con-
stant that the closest scrutiny fails to show any well
marked individual differences.

I have by no means attempted in the above survey of
recognition marks, to give all the instances under each
class, for nearly all birds have them to a greater or less ex-
tent, but merely to illustrate and establish the validity
of the different forms of recognition markings included
in the classification given. A consideration of (C)
Errcamic CoLors opens up, an entirely different field.
These are the colors displayed in courtship, and if the
conclusions arrived at with regard to sexual selection

212 CALIFORNIA ACADEMY OF SCIENCES,

have any validity (see unte, pp. 99-102) they do not so
much belong to the class of useful markings as to the
division of beautiful colors.

Darwin devotes a large portion of his work on the
Descent of Man to a consideration of sexual selection in
birds. He there tabulates the rules or classes of cases.
under which birds may be grouped according to the
constancy or variability of the style of coloration with
sex, age or season, this purpose being to show how
heredity acts with regard to the transmission of sexual
characters. His classification, which is an amplification
of Cuvier’s, is as follows:

‘“©T, When the adult male is more beautiful or con-
spicuous than the adult female, the young of both sexes.
in their first plumage closely resemble the adult female,
as with the common fowl and peacock; or, as occasion-
ally occurs, they resemble her much more closely than
they do the adult male.

Il. When the adult female is more conspicuous than
the adult male, as sometimes, though rarely, occurs, the
young of both sexes in their first plumage resemble the
adult male.

III. When the adult male resembles the adult female,
the young of both sexes have a peculiar first plumage of
their own, as with the robin.

IV. When the adult male resembles the adult female,
the young of both sexes in their first plumage resemble
the adults, as with the kingfisher, many parrots, crows,
hedge-warblers.

V. When the adults of both sexes have a distinct.
winter and summer plumage, whether or not the male:
differs from the female, the young resemble the adults.
of both sexes in their winter dress, or much more
rarely in their summer dress, or they resemble the
females alone. Or the young may have an intermediate.

EVOLUTION OF THE COLORS OF BIRDS. 213

character; or again, they may differ greatly from the
adults in both their seasonal plumages.

VI. In some few cases the young in their first plu-
mage differ from each other according to sex; the young
males resembling more or less closely the adult males,
and the young females more or less closely the adult
females.”

To these classes as given by Darwin the following may
be added:

VII. When the adult male is unlike the adult female,
the young of both sexes may differ from either sex of
the adult, as with the blue-bird (Sialie sialis).

VIII. When the adult male is unlike the adult
female, the young male sometimes has a peculiar first
plumage, while the plumage of the young female is like
that of the adult female, as in many of the woodpeckers.

These classes may be more clearly presented in a table
commencing with the most developed forms in which
the characters originating first have been transferred
first to the female and secondly to the young, and
proceeding down from this point through the classes
where the inheritance is less and less complete. Dar-
win has called attention to the fact that these various
divisions are not sharply marked, but shade into one
another, so that in many instances it is difficult to say
definitely to which class a particular species should be
assigned. It should also be remembered that while in
a large number of instances sexual selection has been the
factor which has produced the difference, it is not
always so, sexual intensification, sexual recognition and
other factors having played a part.

214 CALIFORNIA ACADEMY OF SCIENCES.

J. Young like adult (IV).
2. Young like some ancestral stage of the adult (III).
[ ( 3. Young like winter plumage
of adult.

Resemblance of | wale like female. 4. Young differ eter adults at
sexes differs with | any season, or intermediate
L

Male like female.

between summer and winter
plumage of latter.

the seasons; or
both sexes may 1 ;
meer ah He coo co 5. Young with peculiar first
seasons (V). plumage differs 1

from female. Bepeaee
Male unlike eae 6. Young like female in win-
in both plumages ter.

Female more conspicuously colored — young like

male (II).
7. Young like adult female (I).

8. Young with peculiar first
plumage (VII).

Male more conspic- | 9- Young male more or less like
uously colored. adult male; young female like
adult female (VI).

10. Young male unlike adult of
either sex; young female like
adult female (VIII).

Male unlike fe- |
male.

When the plumage of the male and female is the
same there is generally no variation with seasons, but
among some forms, especially some of the water birds
and waders, a bright plumage is assumed during the
breeding season by both sexes, corresponding to the
male plumage of species which are sexually dissimilar,
while the dull plumage of winter is analogous to the
ordinary dress of the female among forms sexually dif-
ferent in colors. The most complete stage of transmis-
sion of peculiar characters is (1) that in which the
species has but one phase of plumage, at all times of its
life, and in both sexes. The following list of species
and genera belonging to this class is based upon an ex-
amination of specimens in the National Museum. It
makes no pretense of being complete, but will serve to
indicate representative forms. An asterisk indicates

EVOLUTION OF THE COLORS OF BIRDS. 215

that there is a slight difference in the shade or intensity
of color of adult and young:

Dendroica dominica. Pica.
Icteria virens.* Cyanocitta.*
Vireo. Aphelocome.
Clivicola riparia. Perisoreus.
Stelgidopteryx serripennis. Corvus,
Spinus pinus.* Sayornis.*
Ammodramus. Contopus.
Melospiza. Eimpidonax.
Passerella. Cheetura.
Picicorvus, Geococcyx.

Cyanocephalus.

It is a noticeable fact that most of the birds in the
above list are more or less dull colored forms. The
yellow-throated warbler (Dendroica dominica) and yellow-
breasted chat (Icteria virens) are the only two in which
yellow figures at all prominently, and the genera Cyano-
citta and Aphelocoma are the only ones in which blue
occurs. Besides these there are the highly specialized
crows and magpies, and all the rest are without distinct-
ive marks, save the peculiar Geococcyx, which, as will
be shown later, is doubtless a degenerate form, so far as
color goes, at least. This lack of forms with elaborate
patterns of color in this first list is what would naturally
be expected; first, because elaborate colors are generally
assumed first by the male, and only occasionally the
female has been enabled to acquire them; and second,
because these dull colors are apt to represent old estab-
lished types which have found their protection in their
insignificance, so that there would have been ample
time for the female to have acquired what small degree
of specialization the male had attained.

There are a large number of forms belonging to class

216 CALIFORNIA ACADEMY OF SCIENCES.

2, where the adults are alike, but the young have a
peculiar first plumage. The following is a list of North
American species coming under this class:

Merula migratoriu. Chelidon erythrogaster.
Myucestes townsendit. Tachycinetu.
Helminthophila swuinsont. Chorndestes.
Helminthophila ruficupilla, Spizellu.

Helminthophila virginiw. Junco.

Helminthophila lucie. Amphispisu.

Dendroica nigrescens. Peucwo,

Seiurus. Pipilo,

Lanius. Tyrannus,

Ampelis. Melanerpes erythrocephatlus.
Petrochelidon lwnifrons. Malunerpes torquatus.

Among the species in this list no little variety and
specialization of color and markings is to be met with.
These are the forms in which the female has caught up
with the male, which originally led in the evolution of
color, or else species in which the colors of the two sexes
evolve part passu. It is difficult to decide in which class
to place many of the above species, although where the
colors are very elaborate and decorative, as in the wax-
wings (Ampelis), the red-headed woodpecker (Mela nerprs
erythrocephalus), and the violet-green swallow (Tachy-
cineta thalassine), it is safe to assign them to the former
category, whereas, when the colors are obviously of
use for purposes of recognition, as with the wag-tails
(Seiurus), butcher birds (Lanius), lark-fineh (Chon-
destes), and Junco, the latter alternative is a possible
one. Still, the colors of the genus Pipilo are largely re-
cognition marks, and yet the female of the common
towhee of the Eastern States (Pipilo erythrophthalimus) is
colored differently from the male, although in the very
closely related P. meculutus group the, sexes do not dif-
fer. This seems to show that the characters of the genus

EVOLUTION OF THE COLORS OF BIRDS. 217

were acquired first by the male, as sexual discriminative
marks, and afterwards perhaps transferred to the female
as general directive marks.

It is an interesting matter to determine in what man-
ner, and to what extent the young, among these species
differ from the adult. Among a large number of them
the young are spotted or streaked. When the adults
have any distinctive mark or patch this is generally
wanting in the young, while the colors of the immature
bird are apt to be paler or duller than those of the adult.
As examples of species in which the young are spotted
or streaked may be mentioned the robin (Merula migra-
toria propinguu.), Townsend’s solitaire (Myadestes town-
sendii), water wag-tails (Selurus)—spotted on the back
with rusty—waxwings (Ampelis), lark-finch (Chondestes
sgrammacus), chipping sparrows (Spizella), Junco, etc. A
instances where the young are colored similar to the
adult but without the distinctive markings of the latter
the following may be mentioned: The chestnut crown-
patch of the Nashville warbler (Helminthophila rufi-
capilla), is somewhat variable in the extent of its devel-
opment, but is wanting in the young of both sexes and
present in full-plumaged adults. Lucy’s warbler (Hel-
minthophila lucie), has, besides the crown-patch, a chest-
nut patch on the rump which is also wanting in the young.
The black-throated gray warbler (Dendroica nigrescens),
is marked with a black throat in the adult male which
may or may not be developed in the female, and
is always wanting in the young stage. The crown-
patch of the kingbirds (Tyrannus), is wanting in the
young. Perhaps one of the most complete changes from
the young to the adult plumage takes place in the red-
headed woodpecker (Melanerpes erythrocephalus). The
areas of the markings are the same for the young and
the adult, the markings of the wings and rump being

218 CALIFORNIA ACADEMY OF SCIENCES.

especially constant. In general, however, the brilliant
crimson, dark blue and white colors of the adult are re-
placed by mottled brownish and grayish hues in the
young. In all the above instances the adult colors were
probably first acquired by the male for recognition
(Dendroica nigrescens), sexual beauty (Jelanerpes erythro-
cephalus), or use in some other manner (Tyrannus).
The following are cases where the plumage of the young
is in general similar to the adult, but markedly paler or
duller. Swainson’s warbler (Helminthophila swainsoni)—
young darker than adult, sooty-colored; shrikes (Lan-
lus)—young with colors duller on breast and smoky
brown on back, wavy markings on breast pronounced;
cliff swallow (Petrochelidon lunifrons), and barn swallow
(Chelidon erythrogaster)—young paler; white breasted
swallow (Lachycineta bicolor) and violet-green swallow
(LP. thalassina)—bright colors of back of adult replaced in
the young by plain brown.

Where there are seasonal differences in plumage it is
decidedly difficult to make sharp distinctions of classes.
Frequently the sexes differ very decidedly in their breed-
ing dress, but only very slightly in winter, the male
then assuming a plumage like that of the female. In
such istances the young generally resemble the winter
plumage of the adults, but may differ in being still more
plainly colored. The following list includes these forms:

Dendroiea andubond, Plectrophenes.

Dendroies coranata, Caleurius,

Dendroica maculosa. Guiracd coruled.
Dendroica paliurum, Culamospiza melanocorys.
Cinclus imexicanus. Dolichonyax orysivorus.
Geothlypis trichus. Nu nthoeephulus.

Lowi. Ageluius,

Acanthis, Scolecophaqus.

Leucosticte. Otocuris,

Spenrus,

EVOLUTION OF THE COLORS OF BIRDS. 219

The above list stands midway between those species
in which the adult female is like the adult male, and
such as differ in the adult plumage. It includes such
forms as vary with the seasons in which the male may
or may not be different from the female in summer, but
is not markedly so in winter. This division of forms
displaying seasonal differences is perhaps the least mod-
ified type of sexual adornment, for the male simply puts
on his courting garb for a brief season, when it is
obviously assumed to attract the attention and admira-
tion of the female.

(3.) In the water-ouzel (Cinclus mexicanus) the sexes
are alike, but in winter the breast is much suffused with
white. The young plumage is like that of the adult in
winter. In this class the colors are obviously not sexual,
but the more primitive plumage appears during the
winter months. The increased vigor during the breed-
ing season has produced acceleration in the develop-
ment of pigment at that time of the year.

(4.) Asan example of this class may be mentioned
the palm warbler (Dendroica palmarum), in which species
the sexes are alike, the colors becoming broken and
obscured in the winter plumage. The young are a
plain dusky, streaked above and below.

(5.) In Audubon’s warbler (Dendroica auduboni), the
yellow-rumped warbler (D. coronatu), and the magnolia
warbler (D. maculosa) the plumage of the male in sum-
mer is similar to that of the female, but all the markings
greatly accentuated and intensified. In winter the sexes
are very much alike, while the plumage of the young
is considerably less specialized than that of the female.

The changes in plumage of the crossbills (Loxia) is
especially complex, for the male frequently passes
through three phases of plumage—first a gray phase,
then yellow, and finally red. In general, it may be said

220 CALIFORNIA ACADEMY OF SCIENCES.

that the adult male is red, the female yellowish, and the
young dusky gray, but whether the yellow plumage of
the male is simply a mark of immaturity or a seasonal
difference, I have not been able to determine.

(6.) The red-winged blackbirds (Agelaius) present
some interesting features in the changes of plumage.
Not only does the male differ from the female in summer,
but both sexes also change in winter, and the plumage
of the young resembles but is not identical with that
of the female.

The uniform black of the males is in winter broken
up by rusty terminals to the feathers, while the brown
and gray streaks of the female become duller in winter,
and the white tinge on the lower parts less marked. The
changes in the red wing-patch are shown in Plate XII.
The adult males in summer are colored with a patch of
brilliant red or crimson. In winter itis yellowish much
broken up with dusky and black mottlings. The wing
patch of the adult female Agelaius pheniceus and d.
gubernator in summer plumage is a pale reddish tinge
greatly interrupted with mottlings, while the wing patch
of the summer female of A. tricolor is a dull crimson,
mingled with bluish black mottlings. The last figure
represents the adult female in winter, or young, phase
of the shoulder marking, the color being there a plain
unmodified brown. The various stages of transition
from the unspecialized to the most highly developed
phase are thus to be found in this group by studying the
variations of age, sex and season.

The third division includes those forms in which the
adult male is, at all seasons of the year, unlike the
female. Instances where the female is more conspicu-
ously colored than the male are very rare, and I do not
think there is any example of this group among North
American land birds. The phalaropes form one of the

EVOLUTION OF THE COLORS OF BIRDS. 221

most notable illustrations of the class. When the male
is more conspicugusly colored than the female and there
is no marked seasonal change, the young may differ to
a greater or less degree from the adults. The following
is a list of species in which (7) the young are like the
adult female:

Mniotilta varia. Geothlypis philadelphia.
Protonotari« citrea. Geothlypis macgillivrayt.
Helminthophila chrysopteru, Sylvania mitrata.
Helminthophila pinus. Sylvania pusilla.
Helminthophila ruficapilla. Sylvania canadensis.
Parula americana. Vireo atricapillus.
Dendroica tigrina. Phainopepla nitens.
Dendroica westiva. Progne subis.

Dendroica ceerulescens. Coccothraustes vespertina.
Dendroica blackburnic. Pinicola enucleator.
Dendroica cerulea. Cardinalis.

Dendroica virens. Pyrrhulozia sinuata.
Dendroica townsendi. Habiu.

Dendroica occidentalis. Piranga cestiva.
Dendroica vigorsit, Icterus.

Setophaga ruticilla. Quiscalus.

Geothlypis agilis. Trochilus.

It is a noteworthy fact that every species in this list
has some mark of specialization, while nearly all are
highly modified forms. This is the class which, more
than any other illustrates sexual selection—the males
having been originally marked like the present females
and young, andthe brilliant and varied hues of the male
warblers, humming birds, cardinals, and tanagers of to-
day have doubtless been assumed principally as sexual
charms.

Of species in which the male is more conspicu-
ously colored and the young have a peculiar first plum-
age, the following are representative examples:

222 CALIFORNIA ACADEMY OF SCIENCES.

Dendroica pensylva nic, Pusserina,

Dendroica castuned, Spica americana.
Dendvoicu striate, Molothrus uter.
Piranga ludoviciana, Pyrocephulus rubineus.
Piranga rubra. Sphyrupicus varius.
Spiyus lawrence. Melanerpes carolinus.
Pipilo erythrophthalinvus Stalia.

This class, like the preceding, is made up of brightly
colored species without exception. The colors are sexual
ornaments, but in every instance have been partly
acquired by the female, which thus stands in an inter-
mediate position between the generalized plumage of
the young and the specialized plumage of the adult,
marking three stages of evolutionary progress. Thus
the colors of the female western tanager (Paranga
ludoviciuna) lack the brilliancy and purity of the hues
of the male, while the young, in their first plumage are
streaked above and below. The summer tanager (P.
rubra) is colored in a corresponding manner, as is also
the scarlet tanager (P. erythromelus). A specimen of
this species in the collection of the National Museum—
a male in full plumage—is colored a deep yellow, with
black wings, entirely different from the greenish yellow
of the female. This is proably an instance of reversion
to an earlier plumage, or may possibly be simply the
correlative of the red pigment, due to some derangement
in the bird’s system. The adult female of Lawrence’s
goldfinch (Spinus lawrence’) is without the black of the
head, and with all the colors somewhat duller, while the
young are plain dusky with a little yellow on the wings
and a decided tendency towards a streaked plumage.
In the indigo bunting (Passer(ue cyanes) the bright blue
of the male is replaced by a dull brown, while the plum-
age of the young is like that of the female, but streaked
above and below. The bluebird (Sial/u siulis) is a fa-

EVOLUTION OF THE COLORS OF BIRDS. 223

milar example of this class, the colors of the female
being much duller than those of the male, and the young
having a mottled breast.

The next class (9) includes those species in which the
male is more conspicuously colored than the female and
the young of each sex resembles its respective adult.
The following forms are representative of the class.

Habia ludovicianu. Sphyrapicus thyroideus.
Picoides, Melanerpes aurifrons.

In the three-toed woodpeckers ( Picoides) the adult male
differs from the female chiefly in the yellow crown patch,
which is lacking in the latter. The young do not
materially differ from the adults. In the golden-fronted
woodpecker (Melanerpes aurifrons) the general plumage
of the young is like that of the adult except that the
colors of the former are duller, particularly the yellow
of the belly. The adult female differs from the adult
male chiefly'in lacking the red on the head, and with
the yellow of the head duller. In the young male the
markings of the adult are indicated but very dull, while
the yellow of the young female is so pale as to be almost
buffy. Williamson’s woodpecker (Sphyrapicus thyroideus )
is interesting as coming one step nearer to class 10,
and also of note from the fact that the female is so
markedly different in color from the male. The young
female is like the adult female, having a barred plum-
age, but the yellow of the belly is decidedly paler. The
young male has the plumage marked in uniform masses
of black and white like the adult, but the bright yellow
is replaced by white and the red throat patch is lacking.

In the rose-breasted grosbeak (Habia ludoviciana) we
have approached one step nearer to class 10 showing
how these different divisions shade into one another.
In this species the young male is intermediate in plum-

224 CALIFORNIA ACADEMY OF SCIENCES.

age between the adult male and female. The general
color is brown like the adult female instead of black
hike the male, but the characteristic rose-colored patches
of the adult male on the under wing coverts are fully
developed in the young, and the patch of rose on the
breast is present, but much interrupted. The young
female is almost like the adult, but the yellow paler and
colors less pronounced. This species exhibits four stages
of evolutionary progress, and it is possible to trace the
phylogeny of the adult male from the plain brown
streaked bird with pale salmon-yellow under wing coverts
(young female) to the more pronounced pattern of the
adult, although still obscured by streaks, and with the
yellow of the wing coverts more strongly developed
(adult female), through the stage in which the yellow
patch, by the action of sexual selection in intensifying
the correlative color becomes changed into rose, and an
irregular patch of the same is developed on the breast
(young male), until finally the black, white and rose
plumage, in well defined patches, is assumed by the
adult male.

The last class (10) which, however, has already been
encroached upon, includes those species in which the
adult male is more conspicuously colored than the adult
female, and the young male unlike the adults of either
sex, but the young female generally like the adult, The
best examples of this class with which I am familiar are
to be found among the woodpeckers, especially Dryo-
bates and Xenopicus. Here the general color and
markings do not vary much with age or sex, but the
head markings are very peculiar. The red head mark-
ings of the adult male are upon the nape, those of the
young male cover the top of the head, being, in fact,
not only differently located, but more extensive. The
head of the female is unmarked with red in the adult.

EVOLUTION OF THE COLORS OF BIRDS. 225

Mr. Charles Richmond informs me that the young female
often has the top of the head marked with yellow. Spec-
imens in the National Museum collection failed to
demonstrate this, as the sex of the young is in many
instances undetermined. This peculiar head marking
of the young seems to point to but one thing—to a
different and perhaps more highly developed ancestral
plumage. At any rate, the crown patch, which once
extended over the entire head, has now become restricted
to a comparatively narrow line across the nape.

THE DIRECT INFLUENCES OF THE ENVIRONMENT.

In the preceding discussion of the factors involved in
the evolution of the colors of North American birds, the
distribution of pigment in accordance with the internal
laws of growth was first considered, then the utility of
the various colors and their patterns, and the manner
in which natural selection had been instrumental in their
development, and finally beautiful colors, which had been
evolved by sexual selection, and the manner in which the
laws of heredity had tended to reproduce, first in the fe-
male, and afterwards in the young, at successively earlier
and earlier stages, in accordance with Cope’s law of accel-
eration, the characters which had been acquired by the
male. We have yet to observe how these various mark-
ings may be modified by the direct action of the en-
vironment — by the influences of food, temperature,
moisture, ete. —in a way which is of no utility to the
species. If the results of environmental influence were
a detriment to the species, the race would either die out
or become adapted to the change, but if these direct in-
fluences were an advantage, they would be seized upon
by natural selection, and made the most of.

Eimer considers that the part played by the direct in-
fluence of the environment in the evolution of colors is

15

226 CALIFORNIA ACADEMY OF SCIENCES.

a very importantone. In his ‘‘ Organic Evolution ”’ he
discusses the changes caused by food, climate, and the
direct action of the color of the environment upon the
nervous system. This latter influence, which is so im-
portant a factor in the changes of color of the frog, for
example, probably does not affect birds to any extent
whatever, and need not be discussed here. Compara-
tively few experiments have been attempted to deter-
mine to what extent food may affect the colors of birds.
With regard to the effects of food upon color, Mr. Frank
Beddard says:* ‘‘If the nature of animal colours is
borne in mind, it seems impossible to doubt the modi-
fying action of food; those that are due to structural
peculiarities of the parts coloured (e. g. feathers of many
birds) may be altered just as much as those that are
caused by the deposition of pigment; for the ‘struc-
tural’ colours depend largely upon pigment for their
manifestation.

The mere increase in the deposition of pigment
may lead to an alteration of colour, oftenest perhaps in
the direction of melanism; and there is evidence that
various substances, when taken into the body, do influ-
ence the amount of excreted matter. Where there is
an obvious relation between waste matter and the skin
pigments, it cannot be doubted that variation in the
amount only of the food may lead to colour changes.”

These remarks of Mr. Beddard’s are along the line of
a theory of melanism and albinism, suggested to me
by Dr. Stejneger, viz.: That there is a general tendency
among birds to assume a uniform coloration—generally
black, which is prevented from asserting itself in most
instances by natural selection, etc. White, Dr. Stejn-
eger regards as a mark of degeneration, and, just as

* Animal Coloration, p. 48.

EVOLUTION OF THE COLORS OF BIRDS. 227

animals (more especially man) in old age have their hair
turn gray or white, so species or genera, when on the
wane, may become albinistic. This might be considered
an exemplification of Hyatt’s principle of geratology
(see ante, p. 78). If a species is in its prime, it would
tend to be superabundantly nourished—the adjustment
of the means of living being so perfect—and the surplus
of vitality would gradually expend itself in an increase
of pigment. If the pigment were a dark one, the more
closely the feathers were crowded with it, the blacker
the plumage would become. If a species, on the con-
trary, had passed its prime and were wearing out, it
would be imperfectly nourished, and less and less pig-
ment would be deposited until an albino form would re-
sult. Both the black and white coloration, as previously
suggested, might be of use as recognition marks, and
thus the assumption of such a plumage would be hastened
by natural selection. If, on the other hand, the black
or white, instead of being of use for recognition, was a
detriment to the species, by making it over-conspicuous,
the result would probably be the extinction of the race.

One of the few direct influences of food which have
been observed upon the colors of birds is the effect of
cayenne pepper. Dr. Sauermann has experimented
with a number of species, with some interesting results.*
These observations have been recorded by Beddard in
his Animal Coloration, together with other notes on the
effects of food upon color. Canaries have generally been
experimented upon, the unfledged birds being fed with
food mixed with the pepper. Beddard reports Dr.
Sauermann’s experiments as follows:} ‘‘Cayenne pepper,
of course, is a composite substance, from which a num-
ber of chemical substances can be extracted; the red

* Archiv, Anatomie und Physiol., 1889; Physiol., Abtheil., 543.
tlie. p. 54.

228 CALIFORNIA ACADEMY OF SCIENCES.

colour is caused by a pigment termed capsicin, which
can be separated from the pepper; and it might easily
be supposed that the change from yellow to red in the
feathers of the canary was simply caused by a trans-
ference of the pigment * * * but Dr. Sauermann
has shown that itis not so. Yellow-coloured canaries
were not in the very slightest degree affected by the pig-
ment alone; but, curiously enough, parti-coloured birds.
did react,—the brown parts of the feathers becoming
distinctly lighter in hue. It is a fatty substance
(triolein) which appears to convey the pigment, and
produce thus a changing of the colour from yellow to
red; and further experiments were made with other
birds, showing that it is not only canaries which are in-
fluenced by their food in this way. Some white fowls,
belonging to a special breed, showed traces of yellow
among the feathers after feeding with cayenne; but in
this case there were not racial but individual differences
in susceptibility, for all the specimens of the birds ex-
perimented with did not react to the stimulus.

A similar series of experiments was made with some
other colours: it was found with carmine that the yellow
colour was destroyed and the birds became white. This
unexpected effect is explained by the fact that a mixture
of violet and yellow produces white. The proof that the
fatty constituent, triolein, plays the chief part in the
colouring of the feathers may perhaps help to explain
the very singular fact that the Amazon parrots change
from green to yellow when fed upon the fat of certain
fishes.

With regard to the white fowls referred to, the ex-
periments made by Dr. Sauermann were particularly in-
teresting. The interest lies in the fact that the pigment
was not absorbed equally by all the feathers; only special
tracts were affected; the breast feathers, for instance,

we

EVOLUTION OF THE COLORS OF BIRDS. 229

became red, while the head remained white. It is there-
fore quite credible that in a state of nature partial alter-
ation of colour may be produced by a change of diet.”

It is difficult to say whether the change in color of
the caged housefinch (Carpodacus mexicanus frontalis)
from red to yellow is due principally to a change in food,
or to the confinement and general deterioration of the
system from captivity. Food, nevertheless, plays some
part in this, as well as in many changes in the color of
birds in a wild state, which, with the present lack of
experimental data, are far too complex even to be sur-
mised.

With regard to the influence of temperature, moisture
and sunlight, however, it is less difficult to arrive at
some general conclusions. Mr. Frank M. Chapman, I
believe, first directed my attention to one of the most
obvious and immediate influences of the environment
upon color. In certain species a mark is left upon each
primary indicating where the superimposed feather cov-
ered it. In the grackles (Quiscalus) each primary is a
dark brown except the tip which is not covered by the
overlying feather, this part being glossy black. So
exactly is the boundary defined that the rounded edge
of the overlying feather is clearly marked. There can
be no possible doubt that this marking is due to the
direct influence of the environment, and it seems not
improbable that sunlight has been the agent. An an-
alogous, though in a way quite different, effect is shown
on the wing feathers of the blue bird (Sialia). Here also
each primary leaves its impression upon the next feather,
but the concealed part, instead of being duller, is brighter
blue, the exposed tip being dull brownish.

It is a very important, and, at the same time, a very
difficult matter to determine just what is the effect of
sunlight upon pigmentation. By some it has been

230 CALIFORNIA ACADEMY OF SCIENCES.

argued that sunlight favors the deposition of pigment,
and has been instrumental in the production of the
brilliant colors of tropical birds. Others, on the con-
trary, maintain that the effect of sunlight is to bleach
the plumage, and that the pallor of desert forms has
been thus induced. No one, so far as I know, has had
the temerity to advocate that both these apparently an-
tagonistic views may be correct, and yet an impartial
consideration of the matter seems to force us to this
position. Mr. Beddard has discussed in a very fair and
unprejudiced manner the influence of ight upon pig-
mentation.* He has givena number of examples show-
ing that light frequently does not produce more brilliant
coloration, and that animals living in the darkness may
lose their color from causes other than absence of light,
but he has also adduced a number of cases in which the
color was undeniably due more or less completely to
light. Thus he says: ‘‘ Asa general rule, those insects
whose pupe are exposed are brighter in color than those
insects whose pupe are concealed, either in the ground
or in a dense cocoon.” He mentions the larva of a tiger
beetle (Cicindela cuinpestris), ‘‘which lives in a hole, from
which its head and thorax alone protrude; and these are
of the same green as the perfect insect, while the rest
of the body is of the usual whitish yellow of a grub”
(Andrew Murray, Disguises of Nature, p. 9), and adds:
‘‘ Here light may have been influential in distinguishing
the two halves of the body.” Perhaps the best instances
of the influence of light upon pigmentation are the ex-
periments of Cunningham upon young flounders, which
developed pigment upon the white under side of the
body when placed in an aquarium with mirrors on the
bottom, and Poulton’s instance of the pale cave-dwelling
amphibian, Proteus, which became gradually darker

*1. oc. pp. 61-70.

EVOLUTION OF THE COLORS OF BIRDS. 231

after being removed into the light. The very general
absence of color in cave-dwelling animals is itself a
very strong argument in favor of the influence of light
in developing pigment. ‘‘ What we do find,” says Bed-
dard, in commenting upon the argument that the lack
of pigment in cave-dwelling forms is due simply to the
need for color being obviated, ‘‘ is a uniform absence of
pigment, which is highly suggestive of a direct action
of the environment—and an environment obviously
different from that which has caused or permitted the
bright and varied coloration of deep-sea animals.”

Mr. 8. W. Garman published in the Proceedings of the
American Association for the Advancement of Science
in 1877, * a paper on Variation in the Colors of Animals,
in which he advocated a view which appears in direct
opposition to the evidence above adduced—viz.: that the
pale colors of animals are produced by the bleaching
power “ exerted by the confusing blending prismatic re-
flection of sands and snows,” etc. This law of the bleach-
ing power of reflected light he considers as the universal
cause of pale or white coloration, accounting for the
pallor of desert forms and the white or pale color of the
underparts of so many animals. Besides the evidence
above adduced which goes to negative this view I need
only to call attention to the white-bellied and violet-
green swallows which have such pure white breasts and
yet spend nearly all their time in the free open air far
above any influence of reflected light, and the fish crow
and raven which live along the sands of the sea shore
where the reflected light is full as bright as on the desert,
and yet show not the slightest intimation that they have
any idea of relinquishing their proverbial garb of black.

Again, it will be found that the abdomen is far more
frequently white than the breast—in fact the color gen-

*Vol. xxv, pp. 187-204.

232 CALIFORNIA ACADEMY OF SCIENCES.

erally grows paler from the breast towards the tail—but
the abdomen is more protected from the direct influence
of reflected light than the breast, because, especially
during the middle of the day when the influence would
be strongest, it would be protected by the bird’s shadow.
This is supposing that the bird is habitually upon the
ground, but if it were habitually among the trees or
bushes it would be protected by them.

I would accordingly suggest the very opposite view of
the white color of the under parts of animals. We have
seen that color is a normal product of growth, which
would at first be thrown out upon the integument in a
perfectly indiscriminate manner. The influence of
sunlight would, in the course of time, tend to attract
the greater amount of pigment to that part of the body
most exposed to the light. Thus there are many in-
stances where two pigments have apparently combined
upon the back, and perhaps the upper part of the breast,
while only one, and that one not very intense, remained
upon the belly, often fading into pure white on the
under tail coverts. This state of affairs may be fre-
quently found among the warblers, where the back is a
dark olive green, and the breast a light yellow. Plates
XVIlland XIX of the colorsof the genus Dendroica well
illustrate this point, for it will be noticed how uniformly
the abdomen and under tail coverts are pale colored and
unmarked. According to this view there would be a
general tendency for the bulk of pigment to be directed
to that part of the body which received the most sun-
light, but if it happened that some advantage, either
from recognition, protection, or sexual adornment, was
to be gained by retaining patches of color upon the
breast, or by having the breast uniformly colored some
dark shade, natural selection would counteract the influ-
ence of sunlight.

EVOLUTION OF THE COLORS OF BIRDS. 233

But how, it may be asked, can this view be reconciled
with the undoubted bleaching effect of sunlight upon
desert forms? It seems to be a general rule that dry-
ness as well as heat is necessary to produce the pallor.
Sea coast species are generally darker than inland forms,
even among such as the savanna sparrows(Ammodramus)
which live much of the time on the sands of the beach.
However, it may be said that moisture in the air means
much foggy and cloudy weather when the sunlight is
obscured, while dryness means that the sun is constantly
shining. One fact which should be borne in mind is
that the moist sea coast climate does not produce more
brilliant colors, but rather darker and duller hues than
the normal. The facts seem to show that the maximum
of sunlight and moisture together favors the develop-
ment of the most pure and brilliant coloration, sunlight
without moisture has a tendency to burn and bleach,
while moisture without sunlight produces darker and
duller colors. Nor need this be particularly difficult to
understand. The intense heat and dryness of the desert
might well leave the plumage in a state bordering on
pathological, so drying and parching the feathers that
pigment could not well be deposited. The heat and
dryness may also have an influence upon the structure
of the feather itself, leaving it in a condition analogous
to the ‘‘worn” plumage at the close of the breeding
season, which is much paler. This burnt or worn ap-
pearance of the feathers of desert birds may often be
noticed. On the other hand, moisture alone might well
have a direct influence on the pigment cells in making
them darker, or it might be a condition favorable to the
- deposition of more pigment.

A complete discussion of the relation between climate
and color demands a few preliminary remarks on the
general principles of the geographical distribution of

234 CALIFORNIA ACADEMY OF SCIENCES.

animals, particularly with regard to the land birds of
North America.

GEOGRAPHICAL DISTRIBUTION AS A FACTOR IN THE EVOLU-
TION OF COLORS.

Inasmuch as the geographical distribution of life
bears such an important relation to the evolution of
species, a knowledge of the past and present distribution
of birds must throw some light upon the subject of the
evolution of their colors. It will accordingly be neces-
sary to briefly glance at the distribution of birds in
North America, and to note the bearing of the question
upon their colors.

The land mass which now forms Europe and Asia was,
according to Wallace, the original home of the primitive
vertebrates, whence, at a slightly later period, they
migrated to North America. Spreading southward over
South America and Australia they were very early cut
off from North America, and the fauna of Australia has
never since intermingled with any other. At one or
two successive later periods communication between
North and South America was established, resulting in
a very considerable intermingling of forms. The pres-
ent fauna of North America, as Dr. C. Hart Merriam
has shown, is derived from two centers, a northern and
southern.* According to his second provisional bio-
geographic map of North America} the principle life
areas are as follows: treeless area extending across the
northern edge of Alaska, southeast to the southern
border of Hudson’s Bay, and thence across a narrow
strip of the northern coast to the Atlantic; a Boreal area
occupying nearly all of British Columbia and extending

"cf, North American Fauna, No. 3.
+ The Geographic Distribution of Life in North America, Proc. Biol.
Soc., Wash. vii.

EVOLUTION OF THE COLORS OF BIRDS. 235

south on the Sierra Nevada Mountains irregularly into
northern Lower California, upon the Rocky Mountains
irregularly into Arizona and New Mexico and in more
isolated spots of the Alleghany Mountains south to
Tennessee and North Carolina; the Upper Sonoran area
lying in the center of the continent including the Great
Basin region, the upper prairie and Mississippi Valley
district, and touching both the Atlantic and Pacific in
restricted areas; the Lower Sonoran area, comprising
the south Atlantic and Gulf States the greater part of
Mexico and Lower California; the Lower Californian area,
occupying only the Cape region; and the Tropical area
extending from Central America north along both coasts
‘of Mexico, and the southern extremity of Florida, and
including the West Indies. Between the Boreal and
Sonoran areas extends a transition region in which the
life is typical neither of the northern nor southern area,
but rather a more or less complete intermingling of the
two.

The birds of the Boreal area are similar to the birds
of the northern Palearctic region, becoming practically
identical in the circumpolar zone, and progressively dis-
tinct southward. There seems to be little doubt that
the birds of this northern region are, for the most part,
the descendents of the original land birds of this dis-
trict. Especially is this the case with such forms as are
typical of the Boreal area. The birds of the Sonoran
area, while originally descended from the same stock,
present forms which have probably come from the Neo-
tropical region since the decline of the ice age. It is
possible, indeed, that the upper and lower Sonoran areas
represent two separate invasions from the south. The
tropical fauna of North America is, in all probability,
comparatively speaking, a very recent feature.

Wallace furnishes the following list of Boreal genera

236 CALIFORNIA ACADEMY OF SCIENCES.

of birds ‘‘ which have as much right to be considered

typically Nearctic as Palearctic:” *

1. Regulus, 9. Corvus, 16. Euspiza,

2. Certhia, 10. Ampelis, 17. Plectrophanes,
8. Sitta, ll. Loxia, 18. Tetro,

4. Parus, 12. Pinicola, 19. Lagopus,

5. Lophophanes, 13. Linota, 20. Nyctala,

6. Lanius, 14. Passerella. 21. <Archibuteo,

7. Perisoreus, 15. Leucosticte, 22. Haliwetus.

8. Pica,

Mr. Wallace’s list of typical Nearctic genera of land
birds consists chiefly of Boreal forms. This is to be ac-
counted for by the fact that the Boreal area contains the
most primitive Neotropical birds and consequently the
most typical genera. The list is as follows:

1. Oroscoptes, 17. Pheenopepla, 33. Empidias,
2. Harporhynchus, 18. Xanthocephalus, 34. Sphyrapicus,
3. Sialia, 19. Scolecophagus, 35. Hylatomus,
4. Chamea, 20. Pipilo, 36. Trochilus,
5. Catherpes, 21. Juneo, 37. Atthis,
6. Salpinctes, 22. Melospiza, 38. Ectopistes,
7. Psaltriparus, 23. Spizella, 39. Centrocercus,
8. Auriparus, 24. Passerculus, 40. Pediocetes,
9. Gymuokitta, 25. Poccetes, 41. Cupidonia,
10. Picicorvus, 26. Ammodramus, 2 Ortyx,
11. Mniotilta, 27. Cyanospiza, 42. Oreortyx,
12. Oporornis, 28. Pyrrhuloxia, 43. Lophortyx,
13. Icteria, 29. Calamospiza, 44. Callipepla,
14. Helmintherus, 30. Chondestes, 45. Cyrtonyx,
15. Helminthophaga, 31. Centronys, 46. Meleagris,
16. Myiodioctes, 32. Neocorys, 47. Micrathene.

Taking these two sts as a whole, it is surprising what
a lack of brilliant and diversified color they present.
There are some notable exceptions, to be sure, but they
are very few. Regulus is adorned with a coronet of
flame or scarlet, Ampelis from the far north displays its
beautifully varied, although not gaudy, attire; Sialia
tries to appear unassuming in luxurious blue; Oporornis,

* Geographical Distribution of Animals, ii, p. 119.

EVOLUTION OF THE COLORS OF BIRDS. 237

Icteria and Sylvania (Myiodioctes) make the northern
woods and swamps gay with their parti-colored attire,
while Dendroica, which should have been included in
the list, is still more varied and resplendent in its dress;
to say nothing of the Trochilide or the beautiful part-
ridges of the west—Oreortyx, Callipepla, and the rest.
The Trochilidee may be excluded from this list as un-
questionably of Neotropical origin, leaving only two or
three of the warblers in the list of really brilliant colored
species representative of the Nearctic region. Further-
more, it is noticeable that with the exception of the
blackbirds and humming-birds, irridescent or metallic
colors are almost wanting.

It thus becomes apparent that there is some general
connection between the geographical distribution and
color of birds. The species found in the arctics are, for
the most part, white in color, those which take their
origin in the north temperate zone are to a large extent
plainly colored, while the brilliant species have appar-
ently nearly always originated in the tropics, In the
above lists of Nearctic genera another fact is noticeable,
viz.: The large number of birds marked with streaks.
Such, for example, are Harporhynchus, Certhia, Mnio-
tilta, Passerella, Leucosticte, Melospiza, Ammodramus,
Pocecetes, etc. Reasons have been given for consider-
ing this style of coloration more primitive than any
other. Assuming this to be true, it is acurlous circum-
stance that the oldest stock of birds should be the least
specialized in their colors. This does not seem so sur-
prising, however, when we consider the disadvantages
under which they have lived as compared with their
southern allies.. One of the greatest has been,an in-
auspicious climate. The cold of the glacial period in
particular must have made food very scarce and compe-
tition very keen, so that all the energy of the bird was

238 CALIFORNIA ACADEMY OF SCIENCES.

directed to making aliving and maintaining the species,
without any great regard to progress in respect to beauty.
Looking atit broadly, the genial climate and abundance
of food in the tropics has afforded a vast amount of sur-
plus energy to be expended, or favored katabolism, while
the cold climate and comparative scarcity of food in the
north has demanded a constant effort to accumulate
enough energy to preserve the vitality of the species,
so an anabolic state of life has prevailed.

The birds of the tropics are, as Wallace has shown, by
no means universally brilliantly colored.* Indeed, he
questions if the proportion of brightly colored species
is very much greater than in more temperate regions.
This is due to the fact that bird life is so much more
profuse in these regions and there are accordingly so
many more brightly colored species here than in other
parts of the earth, that we are accustomed to think
of them all as being of variegated plumage. Notice the
enormous family of Dendrocolaptide in South America,
however, in which no bright tints are developed. Still,
granting all this, the fact remains, that the greatest
variety and brilliancy of color occurs in the tropics while
forms characteristic of the north are seldom brightly
colored, and perhaps never arrayed in the splendor of
many tropical species.

Mr. Frank Beddard alludes to some more special in-
stances of the relation between color and locality in his
work on Animal Coloration.| He summarises Dr. L.
Camerano’s system of geographical colors,{ as follows:
‘The Palearctic region—that is, Europe and Northern
Asia—has as prevailing tints grey, white, yellow, and |
black; in Africa yellow and brown are most abundant;

*ef. Tropical Nature.
t pp. 44-47.
t Zoologische Anzeiger, 1884, p. 341.

sEVOLUTION OF THE COLORS OF BIRDS. 239

green and red are the prevailing tints of the Neotropical
region (Central and South America), yellow and red of
the Indian. Australia is to be distinguished from the
rest by the great abundance of black animals.”

Mr. Beddard adds: ‘‘A closer scrutiny of many of
the above instances and of others which seem to indicate
some connection between locality and colour, will proba-
bly show that other causes are probably responsible for
the colour changes.” He, however, gives a number of
examples in which there appears to be some definite
relation between a locality and a particular color. The
most remarkable instance is that of a forest in southern
Brazil, where ‘‘ Dr. Seitz found a perfectly circumscribed
region in which the insects were almost entirely blue; a
few miles away from this locality the insects were red,
yellow—any color but blue; but in the particular locality
blue was so characteristic a tint, that out of twenty
butterflies ten were entirely blue and the remaining ten
partially blue.’”’” Another remarkable instance which he
records is that of a butterfly found in South America,
which is almost identical in color with an entirely distinct
European form. Protective mimicry is in this case pre-
cluded by the fact that the two genera have always been
restricted to the localities in which they are at present
found: In commenting on this, Mr. Beddard says:
‘“‘We see here a particular type recurring in regions
widely separated, which may be reasonably supposed to
be due to similar environmental conditions.” Whether
or not this easy explanation is sufficient to account for
the phenomena, a parallel, but more remarkable case
can be adduced which certainly cannot be so simply ex-
plained. The meadow lark of North America (Sturnella
magna) is a very distinctively marked bird with its
bright yellow breast, black crescentic throat patch,
streaked flanks and brown streaked back, with a dark

240 CALIFORNIA ACADEMY OF SCIENCES. ,

double line down the top of the head. Equally well
marked is the South American Sturnella defilippit, in
which the yellow is replaced by rose color. While at
the National Museum, Mr. Ridgway handed me a bird
which at first glance was unhesitatingly pronounced a
meadow lark, so exactly did its colors match those of
our bird. An inspection of the bill and feet, however,
showed that it was structurally entirely unlike Sturnella.
It was an African pipit (Aucronyx croceus). Still more
remarkable is the fact that another African species
(Mucronyx amelic) is colored like the South American
Sturnella.*

Surely no one would be rash enough to attribute this
remarkable correspondence of colors and markings to
environmental influences! It would be better to say
that it was purely accidental or else confess complete
ignorance on the point. To attribute such instances as
this purely to environmental influences is to forget that,
as Dr. Schurmann has said, every modification in an
organism depends primarily upon the nature of the
organism itself, which reacts upon its environment.
Even if we granted, in this instance, that the environ-
mental conditions in America which produced the two
species of Sturnella were exactly reduplicated in the case
of the two African species of Macronyx, still the same
colors would not have been produced unless the two sets
of species were also precisely alike in their respective
constitutions—all of which is too great a tax upon our
credulity. It would be far more easy to believe that, as
Prof. Cope has suggested, a species might retain its
specific character and yet change its generic type. It
would then be merely necessary to assume that two
species were evolved and afterwards separated in their
distribution. A change in habits or environment might

* Recorded by Dr. L. Stejneger. Riverside Natural History, iv, p. 488.

EVOLUTION OF THE COLORS OF BIRDS. 241
then produce a divergence in the character of the bill
and feet of the two species without altering their colors.
Incomplete and unsatisfactory as this explanation cer-
tainly is, it is merely introduced to show that it is not
absolutely necessary to assume so improbable an hypo-
thesis as a definite complicated coloration being painted
on, so to speak, by the environment.

Mr. Ridgway, in 1873, called attention to the relation
between color and geographical distribution in North
American birds.* He showed that certain species hay-
ing a wide geographical range varied in one of two
ways, either in a tendency towards melanism ‘ which
may be either an increase in the intensity of color or in
the extent, of the black parts of the plumage,” or ina
tendency towards greater brightness or increase in the
extent (hyperchromism), of one of the three primary
colors, red, blue, or yellow. This tendency towards
melanism and hyperchromism increases towards the
equator and towards the Pacific Coast, but with red the
color increases in intensity towards the tropics and in
amount towards the Pacific. Mr. Ridgway calls especial
attention to the different races of the Arizona goldfinch
(Spinus psaltria), as illustrating the principle of a tend-
ency toward melanism as the tropics are approached.
As this is an instance of unusual interest from the com-
pleteness and uniformity in the links of progression, I
have figured three of the races (Plate VII). The typical
race of the species (S. psaltria), is the one occupying the
northern-most limit of the range, north to about latitude
40° The back is in this form plain olive green with a
cap of black confined to the top of the head, and with
the wings and tail a dark brownish. This race shades

*Am. Journ. Science and Arts, iii, Ser., Vol. iv, pp. 454-460, Vol. v,
pp. 39-44.
16

242 CALIFORNIA ACADEMY OF SCIENCES.

by insensible gradations into the one next south of it
(S. psaltria arizone), found in New Nexico, Arizona and
northern Mexico in which the black on the head is
somewhat extended, the shoulders have become black
and mottlings of black have appeared in the back. Con-
tinuing south through Mexico the amount of black on
the back steadily increases, the rump patch being the
last to relinquish the olive color; ‘‘the bird now is var.
Mexicana, and continues with nearly the same char-
acteristics south to Costa Rica and Panama, from which
latter countries we find specimens in which the black is
often appreciably more intense and lustrous than in
those from Mexico. These three forms all have white
on the tail; but in specimens from New Granada, and
occasionally in those from Panama, there is usually a
total absence of white marks on the tail, or else they are
greatly reduced in size.”’ Perhaps the most remarkable
thing about this variation towards black, which bears
such a direct and obvious relation to the changes in lat-
itude, and hence of climate, is the fact that the female
remains almost or quite the same through all the changes
of the male.

The following is a possible line of explanation of this
state of affairs: the black pigment was first present in
the system of the bird in conjunction with the yellow,
giving rise to an olive green color. It was set apart in
a limited area upon the head by the action of sexual
selection or for sexual recognition. Then, the tendency
to the production of pure black pigment having once
been originated, would be favored by katabolism, which
would increase under the more abundant food and higher
temperature of the tropics. A specimen of S. psaltria
arizone was taken hy Mr.W. Otto Emerson at Haywards,
Alameda County, some years since, while last summer I
saw in Berkeley, Alameda County, a goldfinch in com-

EVOLUTION OF THE COLORS OF BIRDS. 243

pany with a flock of S. psaltria, which had the back
almost, if not entirely, black. It is probable that these
exceptional instances are not stragglérs from the south,
for the bird is generally a resident wherever found, but
rather an unusual manifestation of the melanism which
normally only asserts itself in a warmer climate.

Mr. Ridgway also mentions the following instances of
melanism toward the south: ‘‘ Myiarchus Lawrencit,
which, starting with a grayish brown crown in the
northern examples (var. Lawrencii) gradually assumes
a blacker and blacker crown, as we trace it southward,
first through var. nigricapillus (Costa Rica and Panama),
and finally ending in var. nigriceps (Ecuador), which
has the crown deep black. Sayornis nigricans from
California and Northern Mexico, has the crissum pure
white; Mirador specimens have it clouded with dusky,
while in Costa Rica specimens (var. aquaticus), it is
entirely blackish, only the middle of the abdomen being
white. * * * The same law as regards the Pacific
province of North America is made evident by the well-
known cases of Picus villosus var. Harrisit, P. pubescens
var. Gairdneri, Sphyrapicus varius var. ruber, the North-
west coast forms of Falco peregrinus, F. Columbarius,
Bubo Virginianus, Scops asio, and numerous other simi-
larly affected species.”

Mr. Ridgway mentions Xanthoura luazuwosa and var.
guatemalensis,.and the different races of Geothlypis as
examples of intensification in yellow toward the tropics.
In Geothlypis the yellow not only becomes intensified
but extended upon the white of the belly both in the
warm moist climate of the Mississippi Valley and Gulf
States and in Mexico. Sylvania pusilla pileolata and
Helminthophila celata lutescens are illustrations of the
intensification of yellow in the Pacific provinces.

Cardinalis virginianus and Piranga cestiva become

244 CALIFORNIA ACADEMY OF SCIENCES.

much more intensely red toward Mexico and Central
America. The house finch (Carpoducus mexicanus fron-
talis) is an especially instructive instance with respect to
the spreading and intensification of the red color. To-
ward the south (C. mexicanus) the red becomes intense,
but sharply restricted in extent. In the western United
States it is less intense but more diffused, while upon
Guadalupe Island (C. amplus) it is both more intense
and more diffused, but the most deeply colored portion
is restricted to the same areas as in C. mexicanus.

The transformations in the genus Sphyrapicus are of
so much interest that I have figured the heads of the
three forms, arranging them in a regular series of in-
creasing red (Plate XV). It is to be observed that
the series is a double one—from young to adult female,
to adult male, increasing in redness, and from east to
west the same. The upper map in Plate IX. indicates
roughly the areas occupied by the three forms, A being
the distribution of the S. varius group and B of 8S. ruber.
In spite of the fact that these two forms are classed as
distinct species, there can be no doubt that they are
climatic modifications of the same stock. The young of
S. varius and S. varius nuchalis are mottled brown, and
with the colored areas of the adult male white. This is
doubtless the plumage of the primitive bird from which
the genus arose. The next stage of differentiation is
shown in fig. 2 of the adult female in winter. The
acquisition of a black plumage, not very different from
the present coloration of Dryobates, may have been due
to sexual selection, to a surplus of pigment in the sys-
tem, to direct climatic influence, or to the need of recog-
nition markings. However this may be, the brown
color of the young would seem to indicate the presence
of red pigment in the system in conjunction with the
black. This is deposited, in small quantities at first,

EVOLUTION OF THE COLORS OF BIRDS. 245

upon the white throat and upon the head, being in-
creased by sexual selection to the stage now shown by
the adult female of 8. varius in breeding plumage (fig.
3). The element of climate is next brought in con-
picuously to supplement the work of sexual selection.
The birds in the Rocky Mountain district have the red
considerably extended. Thus fig. 4 shows the female
of S. varius nuchalis, in which the red has spread very
considerably. On comparing the breeding plumage of
the adult males of the two races (figs. 5 and 6), it will
be found that a corresponding difference in the amount
of red exists in them also. The climax is reached in
the Pacific Coast form, in which the red has become
almost complete upon the head and breast.

As an instance of an increase in blue from north
to south, Mr. Ridgway cites the blue jays of the genus
Cyanocitta. The modifications of this genus in North
America are so interesting as regards the relation of
color to geographical distribution, that they must be
considered in some detail. The lower map on Plate X.
is a conventional representation of the areas occupied
by the different forms of the group. There.are but two
species, C. cristata (A) and C. stellert (B), occupying re-
spectively the eastern and western half of the North
American continent. Coming from the same stock, the
eastern bird early developed conspicuous white recogni-
tion marks on the wing and tail, which are lacking in
C. stellert in all its forms. It remained very homoge-
neous throughout its range, only varying slightly in
Florida, where the white on the wings and tail is re-
duced, probably by direct climatic influence. In the
western provinces C. steller? is as variable as C. cristata
in the east is constant. In the first place, there are two
general forms, one occupying the region upon the Paci-
fic Coast, and the other the Rocky Mountains south into

246 CALIFORNIA ACADEMY OF SCIENCES.

Central America. This latter form is distinguished by
the presence of a white spot above the eye, which is
wanting in the coast races. It is difficult to understand
how climate could have produced this marking, and it
seems more proper to regard it as a discriminative char-
acter. Toward the north the ranges of the two forms
come together, and specimens from this region are in-
termediate in character. The farther we get away from
this neutral ground the more distinctly marked do the
characters become, showing the part isolation must play
in the matter. Toward the south the white spot becomes
successively larger and more distinct, until it reaches
its maximum at the southern limit of the species. Itis
a remarkable fact that this bird is increasing the extent
of white in the very region in which Spinus psaltria is
growing melanistic. Furthermore, the eastern represent-
ative of the genus, C. frontalis, has the amount of white
reduced toward the south, as would be expected. It
seems inconsistent to maintain that the increase in tne
white spot of C. stelleri toward the south can be due to
the same climatic influence which generally tends to in-
tensify the plumage, and I think some other explana-
tion must be sought. From the map it will be seen that
the interior and coast forms intergrade only at their
northern limit, and here the distinctive white marking
probably originated before the species spread to the
south. As it extended southward, it would become suc-
cessively farther and farther separated from the coast
form, and, lke the land mollusks of the Hawaiian
Islands, would become more and more modified, with-
out regard to climatic influence. There are other
changes taking place in these two forms besides the
transformation of the white spot. The northern forms,
both coast and interior, are darker, with more of black
and brown, while the tendency toward the south is to

EVOLUTION OF THE COLORS OF BIRDS. 247

assume a completely blue plumage. This may be ac-
counted for by supposing that the rainy northwest coast
region was the original home of the species. The pig-
ment would there be darkened, but as the species spread
toward the south the tendency would be not towards
darkness so much as intensification and purity, which
would result in a complete intense blue plumage at the
southern limit of its range. The differences in color
between the various forms may be tabulated as follows:

Variety. Forehead. Head. Back, iSeeon ar ces and
Coast form. (|1. Stelleri...... Inconspicuous blue streaks.|Black !...|Deep black.|/Deep Berlin blue.
No white 2, Frontalis . ..|Conspicuous blue streaks. .| BrownishjBrownish
spot..... | slaty... slaty.|Deep azure, light-
: er than above.
(|3. Annectens..|Indistinct light blue....... Black..../Dusky ..... Very deep Berlin
Interior | 4. Macrolopha.|Bluish white.... -|Bluish blue.
form: : black ../Dark ashy .|Bright blue.
Post ocular 4 5, Diademata..|Pure white................. Bluish
black ..|Dark’r than
spot present | above,more
bluish...... Bright blue.
16. Coronata. ..]Deep blue...........-...-4. Deep blue|Deep blue.|Deep blue.

Mr. J. A. Allen has drawn particular attention to the
relation between climate and geographical races, both as
to the modifications in the bill, feet and tail, and of
color.* He calls attention to the increase in the inten-
sity of color to the southward and also to the ‘‘ increase
in the extent of dusky or black markings at the expense
of the intervening lighter or white ones; or conversely;
the reduction in size of white spots and bars.” He draws
attention to the extreme pallor of desert forms as con-
trasted with the same species in a moist climate, and
says: ‘‘ This coincidence of bright and pale tints, with
the relative humidity of the locality, is certainly sug-
gestive, if not demonstrative, of the relation of cause
and effect between these two phenomena, since the same

ef. Boston Soc. Proc., 1872, pp. 15, 212, 219.
Bull. Mus. Comp. Zool. ii, pp. 229, 247.
Am. Jour. Sci. and Arts, xii, 1866. Baird on Geographical races.

248 CALIFORNIA ACADEMY OF SCIENCES.

rule is traceable over large portions, at least, of the Old
World.”

One of the very best illustrations of the influence of
climate upon color is illustratedin Plate XIII. The song
sparrow (iJlelospicu fasciata) is a resident of all the tem-
perate portions of the North American continent. The
plate shows the general color effect of the back of each
of the various forms into which it divides in the various
portions of its range. In the Eastern States the color is
normal, being neither excessively dark uor light. In the
arid regions of the great plains and Rocky Mountains it is
pale (AZ. fusciuta montana), while in the Colorado desert
region of Arizona it is very small, and the colors pecu-
liarly light (A. fasciata fullax). Coming to the Pacific
Coast, the darkest forms are found, JJ. fasciuta samuelis
and heermanni in California, the dark rusty form JI.
fasciata guttata in Oregon and Washington, and the
darkest of all, M/. fusciuta vusina, along the northwest
coast of British America. It will be noted that these
various races coincide perfectly with the regions of
greatest and least rainfall, the palest forms being found
on the hot arid deserts and the darkest in the region of
most constant rains. It may be thought a singular con-
tradiction that in the genus Sphyrapicus the bird of the
Rocky Mountain district is darker than the eastern form,
while in the genus Melospiza, the variety inhabiting this
district is paler. This may be understood, I think, by
taking into account the difference in habits of the two
species. Sphyrapicus inhabits the forests exclusively,
and in the mountainous districts would resort to the
groves of pine, etc. Melospiza, on the contrary, is con-
fined to underbrush, and in the Rocky Mountain district
would most frequently live in the sagebrush country,
which is dry and comparatively exposed to the sun’s
rays.

EVOLUTION OF THE COLORS OF BIRDS. 249

In the Aleutian Islands a distinet species of song-
sparrow (M. cinerea) is found, which is larger and grayer
than any of the others. Mr. Ridgway thinks that if
material could be had from the intervening district it
would prove to intergrade with J/. fasciata rufina, but
whether this be so or not, it is perfectly evident that the
form is a comparatively recent off-shoot from the same
stock as the others. On comparing the extreme forms
of this species it is difficult to believe them so closely
related, yet every stage of intergradation between them
can be shown.

On Plates IX to XII, inclusive, I have represented in
a conventional manner the distribution of a number of
forms, which are of interest as showing the effect of en-
vironmental influences. As a general rule, a closely
related group, resident over the North American con-
tinent, divides into two species, and then subdivides
into several races. Mr. Seebohm, in his ‘‘ Geographical
Distribution of the Charadriide,”’ makes the isolation
incident to the successive glacial epochs an important
factor in the establishment of generic, specific and
varietal characters. The fact that in a number of in-
stances two tolerably distinct species of the same genus
are found upon this continent, one to the east and the
other to the west of the Rocky Mountains, would seem
to argue that the genus was originally one homogeneous
species found in the northern part of the continent,
which was pushed south bythe invasion of the ice, and
kept apart by an ice cap following south along the highest
mountain range of the continent. This may be the
explanation of the two species of Cyanocitta and of
Pipilo, for instance. The western country is much more
diversified in topography and climate than the eastern,
so it is but natural that the western species would be-
come modified into more races than the eastern.

250 CALIFORNIA ACADEMY OF SCIENCES.

The screech owl (JMeguscups asio) is a form of especial
interest, from the fact that it is intermediate between
such groups as the song-sparrows (Melospiza) and horned
larks (Otocoris), in which there is but one species in the
east and west, and such as the blue jays (Cyanocitta) in
which the Rocky Mountains divide the group into two
species. Inthe Megascops asio group there is but the one
species, but east of the Rocky Mountains it is dichro-
matic, and west of them monochromatic. If, either
from the action of natural selection or from some con-
stitutional weakness, the gray phase should die out in
the east, it is obvious what very different species would
be found on the two sides of the mountains. The fol-
lowing table, based on Ridgway’s Manual, shows the in-
fluence climate has had on the different forms. It is to
be used in connection with Plate X.

1. Jf. asio, normal.
2. floridanus, darker, markings more num-
Eastern (dichromatic) erous and extended.
3. maceallii, darker than 1, light mottlings
coarser and more conspicuous.
+. trichopsis, nearly pure ash gray and black.
Bars on lower parts numerous and narrow.
5. bendirei mostly grayish brown, marks
below fewer and coarser than in 4.
6. keunicottii, plumage tending toward a deep
umber brownish.

7. maawellie, ‘very light colored, white
\ largely predominating on lower parts.”

Western (monochromntic)

The towhees (Pipilo), are of peculiar interest in the
study of the evolution of color, but at present I will
only allude to two species which are influenced by
climatic agencies—P. crythrophthulinus and P. maculatus.
These two species are so very much alike in their gen-
eral colors that at first sight they hardly seem separable
as distinct species. The females, however, are unlike
in color, in the former species being black where the

EVOLUTION OF THE COLORS OF BIRDS. 251

latter are brown. In the latter species, moreover, the
wing coverts, scapulars and back are streaked with white
which is wanting in the former. In spite of these con-
stant specific distinctions, certain characters vary from
one species to the other in an unbroken succession.
Thus Plate VIII shows the outer tail feathers of three
forms, and the uniformity of the reduction in the
white from the eastern species through the two varie-
ties of the western is very noticeable. This reduction
in white is most pronounced on the northwest coast and
is undoubtedly due to the direct action of the environ-
ment, being paralleled to a certain extent by the Florida
race (P. erythrophthalmus alleni). Reference to the.
lower map on Plate IX will show how the two species
were probably marked off by the Rocky Mountain ice
cap, so that the white streaks on the back of the western
species may be due solely to the influences of geographi-
cal isolation and the indirect effects of a different en-
vironment. They would be of use as recognition marks
and so be encouraged by natural selection.

The different races of the two species have been mod-
ified principally by climatic influences, P. maculatus
urcticus (B 2) of the Great Plains being the form with .
the maximum of white markings, P. maculatus megal-
onyx (3) having the white markings much restricted,
and P. maculatus oregonus (4) of the Northwest Coast
region with the white markings lacking or much re-
stricted. The fact that the Mexican form of the genus
is so light colored is not easy to explain. The fol-
lowing hypothesis may throw some light upon it, how-
ever. The Pipilos were a northerly genus probably with
no white on the back, and simply with the white recog-
nition marks at the tip of the tail. They were crowded
south by the ice into two districts, one to the east, the
other to the west of the Rocky Mountains. Those on

252 CALIFORNIA ACADEMY OF SCIENCES.

the western side developed into a new race, the dryness
of the climate encouraging the production of white
markings. Later this new species (P. maculatus arctecus),
spread to the south and west. Toward the west it be-
came successively darker as it approached the moist
coast district, or if there was free intercrossing for a time
the coast birds would mingle with the interior race tend-
ing to make it darker. In the interior of Mexico, how-
ever, the birds probably inhabit a dry arid region and
have consequently not become conspicuously darker than
the form from which they were descended.

In the species so far considered there has been a tol-
erably uniform east and west isolation, but in many in-
stances the segregation is far more complex, there being
an east and west isolation of species and a north and
south isolation of varieties, or, conversely, a north and
south isolation of species and an east and west of varie-
ties. Both these forms are illustrated in the genus
Dryobates as shown in Plate XI. The different races of
the D. villosus group all belong to the same species but
the forms east of the Rocky Mountains have the wing
coverts and tertials conspicuously spotted with white,
these marks being largely or wholly lacking in the west-
ern and Central American races. It will be noted that
this is the converse of the relations in the genus Pipilo,
tending to show apparently that isolation, together with
various indirect effects rather than the direct influence
of the environment has been instrumental in producing
the difference. The eastern and western forms bear a
relation to one another midway between species, such as
in Pipilo, and varieties such as Sphyrapicus varius and
nuchalis, being much more distinctly differentiated than
the north and south races. Indeed, in group A the dif-
ferences appear to be in size rather than in color, with
the exception of the insular form D. villosus maynardi

EVOLUTION OF THE COLORS OF BIRDS. 253

which differs in having the lores white instead of
black—the result of isolation. The Central American
form, D. villosus jardinii, is characterized by the smoky
brown color of the under parts which replaces the white
or gray of the northern race. This is probably a cli-
matic influence corresponding to the tropical tendency
towards melanism in other species.

The lower map of Plate XII shows the distribution of
the barred members of the genus Dryobates—the ‘‘ lad-
der backs.” They are all apparently forms of Sonoran
origin, dividing into three species and a number of
races. The distinctions, both specific and sacial, it would
seem, are due to isolation rather than to any direct en-
vironmental influence. The forms thus far considered
have been such as display trifling varietal or specific
distinctions which are due wholly or in large measure
to their geographical distribution—either to the direct
influence of the environment or to the indirect influence
of environment together with segregation. There are
certain closely related species or varieties, on the other
hand, whose ranges are to a greater or less degree co-
incident, or overlap. Sometimes, as with Ammodrunvus
sandwichensis bryanti and alaudinus it is simply in the
winter distribution that they are together, but in a
number of cases it is the breeding range which co-
incides. If the forms are tolerably distinct species it is.
not impossible that they have been differentiated through
geographical segregation and afterwards come together,
but when the races are not very strongly marked, or
occupy a large territory in common this hypothesis will
hardly be a satisfactory one. The following is a list
of such species and varieties which occupy territory int
common:

Bubo virginianus and B. mexicanus.
Chordetles virginianus henryt and C. texensis.

254 CALIFORNIA ACADEMY OF SCIENCES.

Dryobates scaularis and D. stricklandi.

Dryobates scalaris bairdi and D. arizone.

Dryobates villosus leucomelus and D. pubescens.

Dryobates villosus harvisit and D. pubescens gairdnerit.

Tyrannus verticalis and T. vociferans.

Aphelocoma sumichrasti and A. sieberit.

Agelaius gubernator, A. pheniceus and A. tricolor,

Carpodacus purpureus californicus and C. cassini.

Calearius lapponicus and C. pictus.

Zonotrichia leucophrys and Z. leucophrys gambelt.

Spicella pallida and S. breweri.

Junco caniceps and J. cinereus dorsalis.

Junco caniceps and J. annectens.

Amphispiza bilineata, A. belli and A. belli nevadensis.

Pewceea mexicuna and P. cassini.

Passerella iliaca wnaluschcensis and P. iliaea megarhy-
neha.

Pipilo fuscus mesoleucus and P. aberti.

Megascops flammeolus, M. asio macceallit and A. trichopsis.

Thryothorus ludoviciunus and T. bewichii.

Plate XII shows the distribution of two of these over-
lapping groups. The different races of the great-horned
owl (Bubo virginianus) follow the usual lines of climatic
modification—normal in the eastern states, grayer in
the west, very dark on the northwest coast, and almost
white in the Arcticregions. Bubo mexicanus, a tropical
offshoot of the genus, is marked with black stripes
instead of dusky bars. It is quite evident that these two
species were developed in separate districts and after-
wards encroached to aslight degree upon each other’s ter-
ritory. It may be well to point here to an exemplification
of the inefficiency of climatic influences as the sole cause
of a modification. The ladder-backed group of Dryo-
bates are southern forms of the same stock as the striped
species. In the genus Bubo, on the other hand, the

EVOLUTION OF THE COLORS OF BIRDS. 255

northern forms are barred and the southern striped, yet
in both these instances geographical distribution must
have had a part in the differentiation.

The lower map on Plate XII shows the distribution of
the different forms of the nighthawk (Chordeiles). The
eastern variety extends northwest to the Pacific Coast, as
is the case with a number of eastern species. The west-
ern race is, as would be expected, paler, and the Florida
and Cuba bird darker—both due to direct environmental
influence. The differences between C. virginianus and the
southern species (B) C. tewensis, may be due to climatic
influence entirely, the overlapping of the ranges being
subsequent; but it seems more probable that the char-
acteristic wing markings of this latter species are dis-
criminative marks developed to distinguish two diverging
forms occupying the same district.

The difference between the hairy and downy wood-
pecker groups (Dryobates villosus and D. pubescens) cannot,
it would seem, have been causedby geographical isolation.
To determine positively which of the other forms of isol-
ation as tabulated by Gulick has been operative in any
particular instance is, in the present state of our knowl-
edge, impossible. The red-winged black birds (Agelaius)
afford another instance of closely related species which
do not owe their differentiation to climatic or geographical
influences. A. pheniceus, A. gubernator and A. tricolor, all
occur in California, the two latter species being confined
to the Pacific Coast. Plate XIV shows the principal mark
of distinction between these three species in the adult
male. These differences are hardly striking enough to
be of great use as discriminative marks, particularly
between A. pheniceus and A. gubernator, and would
seem to be the result of some form of isolation, rather
than of selection. In the case of the king birds, Tyran-
nus verticalis and T. vociferans, the differences appear to

256 CALIFORNIA ACADEMY OF SCIENCES.

be in general climatic, the southern form being darker
than the northern, but the ranges of the two species
overlap. The distinct white edge to the outer tail feather
is an excellent example of a discriminative mark for
distinguishing the two very similar races.

In the preceding discussion forms have been consi-
dered which are on the very threshold of divergence—
species which are either still forming or barely formed.
It is seen that in a large number of instances the cause
of the first divergence has been the direct action of the
environment. In some cases, however, it was impossible
to see how the environment could have produced the
change, and in these instances the new character was
ascribed to spatial segregation, or some other form of
geographical isolation. In still other species the geo-
graphical environment seems to have little or nothing
to do with the variation, since the diverging forms oc-
cupy the same territory. Here the factor of sexual isola-
tion and discriminative marks may be called into play,
or some other mode of segregation too intricate to be
unraveled may be at the bottom of it.

Before taking leave of this branch of our inquiry, one
interesting case of direct enviromental influence must
be noted. It is of special interest because the direct
climatic influence, instead of merely producing varieties,
seems to have pushed on, and been the principal, if not
the sole, factor in the establishment of species. I refer
to the three North American species of crested flycatchers
(Myiarchus), the colors of which are shown on Plate
VI. The darkest species is found in the Eastern States
(.M. ertnitus), while the palest species, which is nearly
identical with it except that the colors are almost com-
pletely bleached out (/. cinerascens), inhabits the arid
regions of the West. Exactly intermediate in color be-
tween these two species is J/. mezicunusof eastern Mexico.

EVOLUTION OF THE COLORS OF BIRDS. 257

Having now considered all the factors, which seem to
have an influence upon the evolution of the colors of
North American birds, a brief application of these prin-
ciples to the various families and genera must next be
attempted.

ORDER GALLINA. THE GALLINACEOUS
BIRDS.
FAMILY TETRAONIDA. Tue Grovuss, ParrripGes
AND QUAIL.

The colors of the North American representatives of
this family are in general protective in their nature, al-
though sexual characters are present in certain genera,
while some form of recognition markiug is almost al-
ways found. Black and white are found in every genus,
while brown and gray are the principal colors of the
group. Red and yellow in an impure form, as reddish
brown and ochraceous, occur in nearly all genera, and
blue in combination with black or gray in two of them.
There are no genera in which bright colors occur.

GENUS Cotinus. THE BoBWHITES.

(8)* Adult male more conspicuously colored than
female; young with peculiar first plumage.

Colors—Black, white, brown, rufous, ochraceous.

The colors are in the main protective, the black and
white markings of the head being characteristic of the
group, and probably directive recognition marks. They
may very probably be for sexual recognition also, the
colors of the female being much less distinctly indicated,
the black being replaced by brown, and the white by
buff.

Several forms, mostly races; the colors due largely if not

“These numbers refer to the table on p. 214.

17

258 CALIFORNIA ACADEMY OF SCIENCES.

exclusively to the direct influence of the environment,
and varying characteristically from the dark races of
Florida and Mexico (C. virginianus floridanus and C.
graysont), to the pale desert form of southern Arizona
(OC, vidgwayt).

Genus OREORTYX. Mountain ParRTRIDGES.

(2) Adult male like female; young like some ances-
tral stage of the adult.

Colors—Black, white, gray, plumbeous, chestnut,
ochraceous.

The varied and beautiful markings of this bird were
doubtless acquired by the male first, by sexual selection
(Epigamic) and afterwards transferred to the female.
They may also have been of use as recognition marks.
This is particularly the case with the distinct buffy or
ochraceous stripe on the inner web of the tertials, which
is‘characteristic not only of this genus but also of Calli-
pepla. The young are much spotted, representing an
ancestral stage of plumage which may still be protective,
blending with the lights and shadows of the forest.

Two races, darker in the Coast Range, lighter in the
Sierra Nevada Mountains, from climatic influence.

GENUS CALLIPEPLA. VALLEY QUAIL.

(8) Adult male more conspicuously colored than
female (usually); young with peculiar first plumage.

Colors—Black, white, gray, brown, chestnut, buffy.

The colors in this group are obviously sexual in-their
nature, being partly for recognition and partly for
adornment. The genus is of special interest as show-
ing different stages of differentiation of the sexes. The
sexes are alike in C. squamata, the degree of specializa-
tion of colors being slight, and the general tendency
being protective. In fact, both sexes are at about the
degree of development of the female C. californica, the

EVOLUTION OF THE COLORS OF BIRDS. 259

colors of the male of this form being sexual. Young
protectively colored.

Genus Cyrrtonyx. Massena PARTRIDGE.

(7) Adult male more conspicuously colored than
female; young similar to adult female, but colors duller.

Colors—Black, White, brown, rufous, buff, plumbe-
ous.

The colors of the female are protective, of the male
for recognition. The Mexican forms, C. ocellatus and C.
sallez, in which the flanks of the male are respectively
rich chestnut varied with black and plumbeous, and
‘‘plumbeous, barred and spotted with chestnut,” in dis-
tinction from C. montezwme, which has no chestnut on
the flanks, are probably thus marked for sexual recogni-
tion, in distinguishing the three races. The pattern of
markings on the head of the male is one of the most
unusual and complex among North American birds, and
may very probably have originated at the time when
Cyrtonyx first separated from Callipepla. It was quite
likely a recognition mark exclusively at first, but may
have afterwards been accentuated and modified by
sexual selection from the mere attractiveness of some-
thing odd and grotesque, as Weismann has suggested in
commenting on tailless cats. (See ante, pp. 96-97.)

Genus DENDRAGAPAS. THE GROUSE.

(2) Male differing only slightly from female; young
with a peculiar first plumage.

Colors—Black, white, slate-gray predominating—some
brownish, buffy, etc.

An apparent tendency towards melanism by way of
cross barrings is present in this group. Retiring in its
habits, this genus has probably had little need for pro-
tection, and has in past times had few enemies as shown
by its ‘‘stolidity or indifference to the presence of man,”

260 CALIFORNIA ACADEMY OF SCIENCES.

and the ease with which it may be killed (see Coues’ Key,
p. 578). Natural selection has consequently had com-
paratively little to do with modifying its colors, except
in supplying the recognition marks of the tail (see ante,
page 204). Nor has sexual selection had a very import-
ant part in modifying the colors, for although the colors
of the male are somewhat darker and more sharply cut
than in the female, this may be the simple result of
katabolism. The colored comb over the eye is, however,
unquestionably the result of sexual selection. The cross
barrings, however, were probably originally protective
in their nature, harmonizing with the lights in the forest
or the bark of trees, but they are fast becoming obliter-
ated by the increase in black. This increase would
naturally be most marked on the back where the greatest
amount of light falls. Young, brown, spotted—protect-
ively marked. The different races are geographically
isolated, and have become distinguished by discrimina-
tive marks on the tail.

Genus Bonasa. RuFFED GROUSE.

(2) Adult male like female; young like some ances-
tral stage of adult.

Prevailing colors, brown, reddish, gray, white, black.

Colors protective, blending with the ground. Ruff of
male sexual. Black and light bars of the tail probably
directive marks, assisting in recognition in close flight.
Young, spotted, mottled and barred, the latter an inher-
itance from the adult, the former a retention of an earlier
form of plumage. The different races are due to environ-
mental influences, being darkest in the Northwest Coast
region and lightest in the Rocky Mountain district.

Genus LaGcorus. PTARMIGANS.

(4) Both sexes change with the seasons, in summer
plumage differing slightly; young differ slightly from
summer plumage of adults.

. EVOLUTION OF THE COLORS OF BIRDS. 261

Prevailing colors, black, white, chestnut, brown.

The remarkable adaptations of this species to a sum-
mer and winter environment, brown at the former and
pure white at the latter season, are too universally known
to need mention. The winter plumage, however, may
not be exclusively intended for protection. White feath-
ers are much denser and warmer than colored ones, and,
moreover, it is claimed by some, that the intense cold
has a direct effect in causing the feathers to turn white.
Nevertheless there can be no doubt that protection is
one of the chief if not the exclusive object of the white
plumage. The different races, and even species, have
become modified by geographical isolation. L. rupestris
is the stock form inhabiting Arctic America in general;
L. rupestris reinhardti, is found only in Greenland,
northern Labrador, etc.; Z. rupestris nelsoni is restricted
to the island of Unalaska, in the Aleutian chain, and
L. rupestris atkensis to the island of Atkha in the same
group, while Z. welchi is confined to Newfoundland.
L. leucurus is found upon the Alpine summits of the
Rocky Mountains, where it also is effectually isolated,
for the birds are resident wherever found. Little differ-
ences have thus originated which are of no particular
utility, and may not be due even to the action of the
environment, but simply to geographical isolation.

Genus Tympanucuus. Prarrige HEns.

(2) Male like female; young like some ancestral stage
of the adult.

Prevailing colors, brown, dusky, buff, white.

The colors in this genus are almost entirely protective,
the brown stripe below the eye being the only recogni-
tion mark. The ruffs and inflatable sacks on sides of
neck of male are the only sexual characters, the former
having been imperfectly inherited by the female. The

262 CALIFORNIA ACADEMY OF SCIENCES.

colors of the young would seem to indicate that the
ancestral form was much more diversified in its mark-
ings. The plumage of the young is much spotted with
black and white on a ground of light brown, and with
a medium stripe of black on the head and patches of
dusky on the ear coverts. Three species, formed by
geographical isolation.

GrENus PEDIOCHTES. SHARP-TAILED GROUSE.

(3) Male like female; young like some ancestral stage
of the adult.

Prevailing colors, black, white, brown, gray.

Colors protective, the round white spots on the wings
for recognition. The sexual characters are in a very
undeveloped state, there being no very obvious differ-
ence in the colors of the sexes. The young are much
spotted, but also barred with black and streaked with
white.

GENUS CENTROCERCUS. SAGE GROUSE.

(7) Male differing slightly from female, young like
female but with markings less sharply defined.

Prevailing colors, black, white, gray, buffy, brown.

It is difficult to say whether the colors of this bird
are of any great utilitarian significance or not. The
only recognition mark is an imperfectly defined white
crescent on the throat surrounding a patch of black;
which is replaced by white in the female. The general
colors may be protective, but are not peculiarly so, and
this may be another bird which is on the road to the
assumption of a melanistic plumage. The young lack
the spotted plumage characteristic of so many of the
family.

EVOLUTION OF THE COLORS OF BIRDS. 263

FAMILY PHASIANIDA. Tue Pueasants, TURKEYS,
ETC.

This is the family upon which Darwin relied more
than any other as a demonstration of the validity of
sexual selection in the origination of the ornamental
colors of birds. The only genus indigenous to North
America, however, is not so peculiarly notable as an ex-
example of this principle, although the rich bronzy
colors and elaborate appendages of the turkey are not
to be despised because they cannot vie with the gorgeous
hues of the pheasants of Asia.

Genus Meneacris. THe TuRKEYS.

(7) Male colored like female but tints brighter; young
like adult female.

Prevailing colors, brown (metallic), black, white.

Colors largely sexual, but transferred to the female in
great measure. The black and white bars on the wing
feathers are probably recognition marks. The account
of the habits of the turkey given in Wilson’s Orni-
thology, shows many uses for recognition marks. The
birds are eminently social in their habits, and from
their large size and savory flesh are especially subject to
persecution by hawks, owls, lynxes,etc. The young are
peculiarly subject to the persecution of the male bird
and recognition marks might even be of use to enable
the female to keep her brood out of the way of her fierce
mate. How well Wilson understood the significance of
the colors in sexual selection, and how near he came to
anticipating this hypothesis of Darwin is shown in his
account of the courtship of turkeys. He says: *‘‘ Where
the turkeys are numerous, the woods from one end to
the other, sometimes for hundreds of miles, resound
with this remarkable voice of their wooing, uttered re-

* American Ornithology. Jardine’s Edition, iii, pp. 238-239.

264 CALIFORNIA ACADEMY OF SCIENCES.

sponsively from their roosting places. This is con-
tinued for about an hour; and, on the rising of the sun,
they silently descend from their perches, and the males
begin to strut, for the purpose of winning the admira-
tion of their mates.

If the call be given from the ground, the males in the
vicinity fly toward the individual, and, whether they
perceive her or not, erect and spread their tails, throw
the head backwards, distend the comb and wattles, strut
pompously, and rustle their wings and body feathers, at
the same moment ejecting a puff of air from the lungs.
Whilst thus occupied, they occasionally halt to look out
for the female, and then resume their strutting and
puffing, moving with as much rapidity as the nature of
their gait will admit. During this ceremonious ap-
proach, the males often encounter each other, and des-
perate battles ensue, when the conflict is only terminated
by the flight or death of the vanquished.

This pugnacious disposition is not to be regarded as
accidental, but as resulting from a wise and excellent
law of nature, which always studies the good of the
species, without regard to the individuals. Did not
females prefer the most perfect of their species, and
were not the favours of beauty most willingly dispensed
to the victorious, feebleness and degeneracy would soon
mark the animal creation; but, in consequence of this
general rule, the various races of animals are propagated
by those individuals who are not only most to be ad-
mired for external appearance, but most to be valued for
their intrinsic spirit and energy.”

The characters separating the different forms appear
to be discriminative marks.

EVOLUTION OF THE COLORS OF BIRDS. 265

FAMILY CRACID®. Tue Curassows anp GUANS.

Genus Ortauis. THE CHACHALAUAS.

Only one species of this genus comes within our limits,
and it has the appearance of being a form which has
deteriorated from some more brilliant phase of plumage.

ORDER COLUMB. THE PIGEONS.
FAMILY COLUMBIDZA. Tue Piceons or Doves.

The North American representatives of this family
are not so remarkable for the briliancy of the plumage
as is the case with many extralimital species, but the
tints are beautifully soft and pleasing, with metallic
tints of the less pronounced sort. All the genera, and
frequently different species of the same genus, are dis-
tinguished by characteristic white and black recognition
marks, principally on the wings or tail (see qizte p. 205),
or frequently on both—these being especially important
in this family from the fact that the birds generally as-
sociate in flocks, often of immense size. I do not think
it can be held that the other colors of the family are of
any utility whatever. In an old cosmopolitan group of
birds like this hereditary tendencies have been fixed for
indefinitely long periods, and it may well be that all the
North American genera are mere side shoots from the
main stem, colored in the prevailing hues of the type—
plumbeous, bluish gray, reddish or chestnut, with black
and white, and varied with some mild iridescent tints.
These colors may doubtless have first been acquired by
sexual selection, but what factors may have come into
play in the establishment of the colors of the genera
found within our limits is far too complex a question to
discuss without a knowledge of the entire order. It
need only be remarked here that sexual selection in
conjunction with isolation, has undoubtedly had a more

266 CALIFORNIA ACADEMY OF SCIENCES.

important role to play than any utilitarian principle.
As is generally the case with long established groups,
the characters of the male have been generally either
completely or almost entirely transferred to the female,
while even the young vary little or not at all from the
adult.

ORDER RAPTORES. BIRDS OF PREY.

The birds of prey are eminently the despots of the
feathered realm. Having no formidable rivals outside
their own ranks, they have no need of protective colors,
these being absent in the entire group. Some species
are marked with agressive (anticryptic) colors, however,
enabling them to steal upon their prey unawares, but
the majority of them trust rather to the sharpness of
their eye and the fleetness of wing for their food. What
good would agressive colors be to an eagle swooping
upon a lamb, or a Cooper’s hawk falling upon some
luckless chicken or rabbit? Nor would directive marks
be of any use, as a general rule, for these birds have no
enemies to escape from. It is a notable fact that bright
colors are wholly wanting in the order, and that there
are but few species in which an elaborate pattern of
coloration exists. A tendency towards melanism is
everywhere present, or else the color of the back is some
uniform shade of brown or gray, and the breast streaked
or barred with the same color. All this points to the
independence of these birds, so far as the color is con-
cerned, of utilitarian influences. The colors seem to
have been, for the most part, developed without the in-
terposition of natural selection, and simply in accord-
ance with the general principles of growth. The tail
is generally, and the head occasionally, colored with
recognition marks of some sort, discriminative, sexual,
socialistic, and possibly to a less extent directive. The

EVOLUTION OF THE COLORS OF BIRDS. 267

barrings on the tail, probably in accordance with the
principle of repetitive marks rather than for any prin-
ciple of utility, have in many species extended up on
the body, especially upon the flanks, but in some in-
stances over the back as well.

FAMILY CATHARTIDA. Tue American VULTURES.

(1) Male like female (except for the fleshy comb on
the cere); young but little different from adult.

Prevailing colors, black, dark brown, white.

In the three North American genera, Pseudogryphus,
Cathartes and Catharista, the colors are of no utilitarian
significance whatever, except that a black bird may be
seen at a great distance in the sky, and they might thus
assist one another in congregating at a place where food
had been discovered. Natural selection would thus en-
courage this general tendency towards the assumption of
a melanistic plumage.

FAMILY FALCONIDA. Vutturss, Faucons, Hawks,
EaGLeEs, ETC.

GENERA ELANoIDES, Exanus, IcTinta anp RostRHa-
Mus. THE KITEs.

(2) Male like female; young with peculiar first plu-
mage. .

Prevailing colors, black, white, plumbeous, bluish
gray, brown (in young).

It is not easy to see how the beautiful uniform white
and bluish gray marks, relieved by patches of black in
the kites, can be of any more utility than a similar style
of plumage is among the gulls. In both instances the
colors may be in general, of use for recognition, although
such pleasing tints would hardly be necessary for this
purpose. There is a great temptation in this instance

268 CALIFORNIA ACADEMY OF SCIENCES.

to fall back on the theory of a tendency towards a uni-
form coloration, white in this case, varied by sexual
selection which may have produced the beautiful clear-
ness and softness of effect, and by the need for discrim-
inative marks to distinguish between closely allied forms.
The plumage of the young is much less uniform, in-
clining to be mottled and streaked, and indicating the
coloration from which the species of to-day have been
evolved.

Genus Circus. Marsn Hawk.

(7) Male unlike female; young like female.

Prevailing colors (of male) bluish gray, and white;
{of female) dark brown, tawny and white.

How to explain the difference in the colors of the sexes
in this bird, except by an appeal to sexual selection, I
am unable to suggest, for the colors of the male are not
darker than those of the female, so that the law of katab-
olism could not be resorted to. The white rump-patch
and barred tail are excellent recognition marks, but of
what sort is difficult to determine. In reading the life
history of this species the use of so pronounced a recog-
nition mark is not made apparent.

Genus ACCIPITER. GOSHAWKS, ETC.

(2) Adult male similar to female; young hke some
ancestral stage of adult.

Prevailing colors, bluish gray, ashy, brown, rusty,
white.

The difficulty of ascribing the colors of the birds of
prey to any form of utilitarian cause is again exhibited
in this genus. Can it be said that the bluish gray of
the back in adult birds is due to sexual selection, trans-
ferred by inheritance to the female? If not it seems
that an explanation must be despaired of for the pres-
ent, since for either protective or agressive resemblance

EVOLUTION OF THE COLORS OF BIRDS. 269:

the brown plumage of the young would seem the best
suited. Neither can it be attributed to any known gen-
eral laws of growth, for why should these produce first a
brown plumage and afterwards transform it into a bluish
gray one?

GENus ParRABuTEO. Harris’s Hawk.

(2) Adult male like female; young with a peculiar
first plumage.

Prevailing colors, dark brown, black, white.

This is one of the genera which is comparatively free
from the workings of natural selection and is fast be-
coming melanistic. The white base and tip of the tail
in contrast to the black middle portion affords an excel-
lent recognition mark, which was perhaps originally dis-
criminative in its nature. The young are irregularly
marked as would be expected if the pigment was de-
posited without the guidance of natural selection.

GENERA ButTEo anp ARCHIBUTEO. THE BuZzZARDS.

(2) Adult male generally like female; young with a
peculiar first plumage.

Prevailing colors, brown, rusty, gray, black, white.

There is a general tendency throughout this group of
large and powerful birds, so well able to take care of
themselves, to become melanistic. Some species, in-
deed, have two phases of plumage, a lighter normal one,
and a melanistic one, which shade into one another
through every possible stage of transition. The red
color of the tail in B. borealis is a striking recognition
mark which can be seen at surprisingly long distances,
and may have been at first discriminative and afterwards.
directive or socialistic in nature. The different geo- |
graphical races of this species are largely the result of
the direct influence of the environment, together with
isolation. The same is true of the races of B. lineatus,

270 CALIFORNIA ACADEMY OF SCIENCES.

these two species being exceptional among hawks for
their susceptibility to environmental changes.

Genus Urusitinca. Mexican Brack Hawk, ETC.

(2) Adult male like female; young with a peculiar
first plumage.

Prevailing colors (adult) black, white; (young) brown,
ochraceous.

The adults in this genus have completely attained the
black plumage with the exception of the white recogni-
tion marks of the tail. The plumage of the young
probably has no reference to utility but is simply at a
less advanced stage of pigmentation. The different
species are distinguished by the distribution of white
marks on the tail, which afford an excellent illustration
of discriminative marks.

Genus AstuRinaA. MEXICAN GOSHAWES.

(2) Adult male like female; young with peculiar first
plumage.

The remarks made concerning the genus Accipiter
would apply equally to the present bird, the general
features of coloration being about the same.

GENERA AqQuiIta, THrRas@etus, Harireretus. THE
EAGLEs.

(2) Adult male like female; young with peculiar first
plumage.

Prevailing colors, black, white, gray, brown.

The markings of the eagles, perhaps, more than of
any other land birds, are to be explained almost exclu-
sive by the general laws of growth. Very possibly such
marks as the white thighs with black bars in Thraseétus,
the white tail of Haliwétus, and the white head of Z.
lewcocephalus, were developed as discriminative, or pos-
sibly to some extent as socialistic, marks. The plum-

EVOLUTION OF THE COLORS OF BIRDS. 271

age of the young is simply less pigmented than that of
the adult.

Genus Fatco. Tue Fatcons.

(2) Adult male generally like female; young with
peculiar first plumage. In some species (7) male un-
like female; young resembling the latter.

Prevailing colors, black, white, brown, rusty, bluish,
gray.

The colors in this widely dispersed genus are partly
in accordance with the general laws of growth, in nearly
all species enhanced by sexual selection (?), and with
recognition marks, sometimes as is I’. sparverius in the
form of characteristic head markings, but more often in
variations of the tail bars. The markings of F. spar-
verius are peculiarly pronounced, for, besides the head
markings which would alone serve to distinguish it, the
flanks have large circular spots of black, the wing feath-
ers are barred, and the tail markings are peculiar. The
distinction between F. sparverius and fF. sparverioides is
a peculiarly interesting one. In the females of both
the back is rufous, but in the former the male is
similarly colored, while the back of the latter is plum-
beous or dark bluish gray. No appeal to the laws of
growth would explain this, and it is evidently to be ac-
counted for either as a discriminative mark, or as due
to sexual selection. Probably both factors were instru-
mental in its development for in many instances charac-
ters which at first were only useful for sexual recognition,
in leading the female to select a mate of her own kind,
afterwards became attractive to her and were still
further developed by sexual selection. Certain species
have been directly influenced by their environment and
divided into geographical races, as for instance, F’. col-
umbarius suckleyi, the dark form of the Northwest coast
region.

272 CALIFORNIA ACADEMY OF SCIENCES.

GENUS PoLyporus. CARACARA.

(2) Adult male like female; young with peculiar first
plumage.

Prevailing colors, black, white, brown.

It is difficult to suggest any use for the striking distri-
bution of color marks in this genus. There are three
species, in general much alike. The Guadalupe Island
form (P, lutosus)is of peculiar interest from an evolu-
tionary point of view. Left stranded upon this island
without any more powerful rival, and from its habits of
feeding on carrion, without any need of agressive resem-
blance, it has been perfectly free to develop its own life
history independent of the bulk of its own species or of
natural selection (so far as its colors are concerned).
Some of the points in which the island species has varied
from the parent form may be tabulated as follows from
the description in Ridgway’s Manual:

cheriway. lutosus.

Rump white (with or without bars). Rump dull brownish buff, broad-
ly barred with dull brown.

Tail white,the narrow bars grayish. Tail brownish buff, broad bars of
grayish brown, bordered by nar-
rower zigzaggy lines of dusky.

Terminal dark band 2-in. or more Terminal dark band less than
wide. 2-in. wide.

Genus Panpion. OSPREY.

(2) Adult male similar to female; young somewhat
different from adult.

Prevailing colors, brown, gray, white.

The young differ from the adult simply in having the
dark feathers of the back tipped with white or buffy.
The coloration does not apparently serve any useful
purpose.

EVOLUTION OF THE COLORS OF BIRDS. 273

FAMILY STRIGIDH. Tue Barn Ovwts.

Genus Strix. THe Barn Owts.

(1) Male like female; young like adult.

Prevailing colors, ochraceous yellow, grayish, white.

The snowy whiteness of the face, and sometimes the
breast, of this bird I take to be a socialistic recognition
mark, which would be useful in their social intercourse
at night, in the darkness of their nesting place, ete.
What the use of the mottlings may be, or the general
color of the back, I ain unable to suggest.

FAMILY BUBONIDA®. Tue Hornep Owt.s, eve.

The color markings of this family are of a very un-
modified order of development, as might be expected in
a group of nocturnal birds. Two features are noticeable
with regard to the coloration: (1) the absence of any ten-
dency towards melanism as among the hawks, but rather
the reverse tendency towards albinism; (2) the absence
of a tendency towards a uniform color as among the
hawks, but rather an inclination towards a retention of
the mottled plumage. Glaucidium is an exception to
this latter rule, however. This tendency towards albin-
ism in a group of birds habitually living in the dark,
and towards melanism in an allied group which live in
the light and with little selective influence to restrain
the deposition of pigment, would seem to be in accord-
ance with the influence of light upon the development
of pigment. Still, other causes have doubtless been
instrumental in bringing ‘about this result, for the white-
ness becomes most pronounced towards the arctics where
it would be most needed for protection, whereas in more
southern latitudes it is a useful form of recognition
mark.

The different genera present some interesting features

18

274 CALIFORNIA ACADEMY OF SCIENCES.

of color modification. The adults are generally exactly
alike in color and the young frequently do not differ
greatly from them, but are sometimes distinctly barred
instead of mottled. Brown, gray, and white are the
usual colors, together with some buffy, tawny or ochrace-
ous. The genus Megascops is peculiarly susceptible to
the direct influence of the environment, and as has been
already mentioned under the head of geographical dis-
tribution, breaks up into a great variety of forms in dif-
ferent parts of the country. The Bubo virginianus group
is also interesting from the manner in which it becomes
pale toward the Arctics, being almost pure white at the
northern limit of its range. The general mottled ap-
pearance of most owls blends very well with the tree
trunks, and they are thus protected during the day at
which time they are so comparatively helpless when
attacked. The color of the burrowing owl (Speotyto
cwnicularia hypogea) blends very well with the ground
of the fields in which itis generally found, and its white
throat has already been mentioned as a socialistic mark-
ing which would be useful in the darkness of its bur-
row.

ORDER PSITTACI]. THE PARROTS, MA-
CAWS, PAROQUETS, Etc.

FAMILY PSITTACID/K. Parrots.

Like the pigeons, this cosmopolitan and long estab-
lished group should be considered as a whole in attempt-
ing to arrive at any conclusions with regard to the
evolution of the colors. These birds are eminently
denizens of the tropics, and their colors have all the
diversity and brilliancy which we naturally associate
with that region. The green colors are probably in the
main protective in their nature, blending with the leaves,

EVOLUTION OF THE COLORS OF BIRDS. 275

while the blues and yellows are doubtless the result of
sexual selection. The red appears to be generally a
mere intensification of the yellow, in accordance with
the principle of correlative colors. The adults are alike
and the young similar to them, but with the more special
color marks absent.

ORDER COCCYGES. THE CUCKOOS, TRO-
GONS, KINGFISHERS, etc.

FAMILY CUCULIDA. Tue Cucxoos, Anis, ETC.

The three North American genera of this family are
entirely unrelated so far as their colors are concerned,
and accordingly must be considered singly. With re-
gard to protective mimicry in this group, Dr. Stejneger
says:* ‘The cuckoo (Cuculus canorus), in different local
forms, occurring all over the Palearctic region, and
wandering south in winter, is astonishingly like, in ex-
ternal appearance, some of the smaller hawks, not only
in color, but also in its manner of flight, a resemblance
which in Europe caused the superstition that the young
cuckoo in the autumn turns intoa hawk. * * * *
Some Oriental] cuckoos belonging to the nearly allied
genus Hierococcyx carry the Accipitrine resemblance
still further, as the young birds have the dusky mark-
ings on the lower surface longitudinal, as in many hawks
and falcons, later on, hke them, changing into a plum-
age transversely barred. This similarity is not acci-
dental, but evidently a case of protective mimicry, a
supposition greatly strengthened by the fact that we
know of some small Malaccan cuckoos (Penthoceryx),
rusty brown above, and white beneath, barred with
dusky, which, in size, color, and general habits most

* Riverside Natural History, p. 374.

276 CALIFORNIA ACADEMY OF SCIENCES.

closely ape the appearance of certain diminutive shrikes
inhabiting the same country.”

Genus CroTropHaca. THE ANIS.

(1) Adult male like female; young like adult (but
colors lacking the metallic lustre).

Color entirely black, with dull bluish, greenish, or
bronzy reflections.

This is probably a very old and highly specialized
genus of birds, which has reached the limit of pigmen-
tation. Sexual selection has probably been a factor in
the development of its present plumage, and the black
may be a form of directive marking, for the birds are
gregarious.

GENus Grococcyx. THE ROADRUNNERS.

(1) Adult male like female; young like adult.

Prevailing colors brownish, buffy, white, bronze and
greenish.

There seems to be little doubt that this genus has
been in past times a brilliantly attired bird, the colors
having been developed by sexual selection. These have
been lost, either for the sake of protection in the com-
paratively open desert country in which the bird is now
found, or because the genus is degenerating, or possibly
the direct influence of the desert climate may have
affected the plumage. The principal reason for holding
that the group is a degenerate one in its coloration is
that while the exposed edges of the feathers are apt to
be dull brownish or buffy, the basal portion and interior
of the feather which is largely concealed has the most.
iridescent bronzy and green colors,

Genus Cocc¥zus. AMERICAN CUCKOOS.

(1) Adult male like female; young like adult, but
colors slightly duller, and tail markings less distinct.
Prevailing colors, brownish, grayish and white.

EVOLUTION OF THE COLORS OF BIRDS. 277

It is difficult to find any use for the general markings
of this group, but with what is known of the remark-
able tendency toward protective mimicry of the cuckoos
in all parts of the world, it is not impossible that the
plumage of this genus may be an imitation of the mark-
ings of some extinct species of hawk, some form allied
to Accepiter velox, for instance. However this may be,
the differences between C. americana and C. erythroph-
thalmus are doubtless discriminative in nature, the
yellow bill and distinct white spots on the tail of the
former serving to distinguish it very readily from the
latter. O. minor and C. maynardi have been differen-
tiated by isolation upon groups of islands, the direct
influence in the environment perhaps aiding in the
changes of their plumage.

FAMILY TROGONIDE. Tuer Trocons.
Genus Trogon. THe Trocons.

(8) Male more conspicuously colored than female;
young with a peculiar first plumage.

Prevailing colors, metallic green, black, red, yellow,
gray, white.

This tropical genus is represented by but one species
along our southern border, and the colors may be ac-
counted for by the action of sexual selection, together
with a tropical climate. The areas of color are the same
in the female as in the male, but the bright metallic
shades are replaced by brownish or grayish. The
plumage of the young is still less specialized than that
of the female, the species thus exhibiting three stages
of evolutionary progression. .

278 CALIFORNIA ACADEMY OF SCIENCES.

FAMILY ALCEDINID-.E. Tue KINGFISHERS.

GENUS CERYLE. THE KINGFISHERS.

(7) Adult male unlike female; young similar to
female.

Prevailing colors, plumbeous blue, bottle-green, ru-
fous, white.

The colors of C. aleyon are probably a form of aggres-
sive resemblance, making the bird inconspicuous from
the point of view of the fish. The white throat is doubt-
less also a socialistic recognition mark of use in the
darkness of the burrow where the nest is always placed.
The different American species are closely related in
spite of the great discrepancy in size and colors. Mr.
Ridgway, in a paper entitled ‘‘On some Remarkable
Points of Relationship between the American King-
fishers,”* has called attention to this fact. Each of the
large species is duplicated in miniature more or less by
an allied species or fratercule. There must be some
utility in the colors for them to have remained so con-
stant in spite of a reduction in size of the species in one
case toa half. It seems quite possible that the reddish
and green colors would be as invisible to a fish in muddy
water as white and blue to a fish in a clear stream, and
it is not impossible that the first differentiation in color
was dependent upon the clearness of the water in the
districts in which the different forms arose. It may also
be possible that green and blue are correlative colors
just as yellow and red are. Such birds as the green jay
(Xanthoura lucuosa), and some of the parrots would seem
to give an air of plausibility to this view. In the king-
fishers the combination of rufous and green is appar-
ently a more primitive one than of white and blue.

The following facts will prove this: C. superciliosa is

* Bull. Nutt. Orn. Club, viii, pp. 59-60.

EVOLUTION OF THE COLORS OF BIRDS. 279

the smallest species and the fratercule of C. inda the next
larger in the group. These two are green above and
rufous below, but the male has a pectoral band of white
and dark green bars. The next in size is C. cabanisi,
the fratercule of C. amazona. In these two the back is
green but the under parts of the male are white with a
pectoral band of green. In the female the rufous per-
sists posterior to the green band. In C. aleyon the
fratercule of C. torquatus the green is replaced by
blue while in the female the rufous still persists, but to
a less degree than in C. cabanisi, posterior to the blue
pectoral band.

If, as suggested, the red and green colors are aggres-
sive resemblance marks, concealing the bird from fish
in muddy water, they would naturally be the primitive
color because streams were more universally muddy
in earlier geological ages when erosion was greater.
Furthermore, the rufous color would naturally persist
the longest on the posterior parts of the body, for there
it would be invisible to the fish as the bird plunged from
above.

ORDER PICI. THE WOODPECKERS, WRY-
NECKS, Etc.

FAMILY PICIDZ. THE WoopPrecKkeErs.

This cosmopolitan group is especially interesting in
connection with the evolution of colors. Tropical species
are frequently very brilliantly colored, while all the
species found in North America show at least some
traces of a bright plumage. The different genera may
be divided into two groups—those which appear to have
lost much of the former brilliancy of their markings,
either from degeneration or the need for protection, and
those which are to-day either becoming more brilliant,

280 CALIFORNIA ACADEMY OF SCIENCES.

or are now at the climax of their development. Dryo-
bates, Xenopicus and Picoides belong to the former
class and Sphyrapicus, Melanerpes, and Colaptes to the
latter. Itis doubtful where Campephilus and Ceoph-
leeus should be placed in taisscale. This loss of brilliant
color in certain genera is demonstrated by the fact that
the young are more brightly colored than the adult—a
very unusual state of affairs. This is particularly notice-
able in Dryobates where the young male has the entire
top of the head scarlet, this color being restricted in the
adult to a narrow-patch on the nape. As this excess of
red in the young cannot be of any conceivable utility it
seems to afford a perfect demonstration of the former
plumage of the adult. Furthermore, in certain forms,
. particularly in the genus Melanerpes, an excess of red or
yellow, or both, very frequently occurs on the breast and
belly. If the amount were very slight and occasional,
it might be looked upon as an excess of pigmentation
along the line of future color development. From the
frequency with which it occurs, however, and its in-
tensity in some instances, it seems more probable that
it is a character which is being rapidly lost at the present
time. It seems probable that the American species of
Dryobates also had the belly colored red in past times,
for in certain Asiatic species which have the red head
like the young ‘of American forms, the belly is also more
or less strongly tinged with red. It would seem then
that Melanerpes had but recently passed the climax of
its most brilliant coloration, while Dryobates must have
long since donned the garb of old age. It seems a
rather curious circumstance that birds such as the wood-
peckers, which habitually cling close to the trunks of
trees, should have a bright color upon the abdomen.
May it not be possible that this plumage was assumed
before the tree climbing habit was acquired, and is now

EVOLUTION OF THE COLORS OF BIRDS. 281

being lost because it is so inconspicuous as to be of no
particular use? I fail to see that the bright markings in
this family can have been developed otherwise than by
sexual selection. In most cases the colors have been
transferred by inheritance to the female, but generally
in a more or less imperfect manner. Why the bright
colors, particularly of the crown patch, should be lost,
is even a more difficult question than how they were ac-
quired. Woodpeckers are not peculiarly exposed to the
attacks of rapacious birds or mammals, so the suppres-
sion of red could hardly have been due to the need for
protection.

The white-headed woodpecker (Xenopicus albolarvatus)
is remarkable from the fact that there are no hybrid
feathers along the edge of the white marking. This
would seem to argue that the white head is due to some
constitutional weakness, rather than to the operation of
natural selection. It is probably a step in degeneration,
which may have been pressed into service by natural
selection as a recognition mark. Picoides, on the other
hand, together with Ceophleus and Campephilus and
Sphyrapicus thyroideus are rapidly tending towards mel-
anism.

Characteristic recognition markings are very prevalent
in the family, particular attention having already been
called to a number of examples in preceding pages. The
flickers (Colaptes) afford the best possible illustration of
these marks. As the flicker flies the first thing to arrest
the attention is the white rump patch, and in connec-
tion with this the flash of yellow or scarlet from the
wings. Whether the characteristic round black spots on
‘the sides are a form of recognition mark or due to some
other cause is a very difficult matter to decide. The
black and red malar streaks in the eastern and western
races, respectively, might be held to be discriminative

282 CALIFORNIA ACADEMY OF SCIENCES.

marks were it not for the remarkable indiscriminate
breeding between the two species, apparently with no
deleterious result to the species.* The red of the western
species is apparently the correlative of the yellow of C.
vuratus, and is due to some form of pigment intensifica-
tion more than to selection. Mr. Frank M. Chapman
has published a note On the Color-Pattern of the Upper
Tail-Coverts in Colapies auratus,f illustrated with cuts
of the feather, in which he shows that the barred pattern
is the earliest form of marking, culminating in a solid
black feather, rimmed with light. It is an interesting
fact that the circular spots on the side are modifications
of the barred feather, as is shown by a study of the
successional taxology of the feathers on the lower part
of the body, the barred type being posterior to the cir-
cular. The fact that although Colaptes does not present
an obviously barred plumage the specialized markings
have all been developed from the bar, taken in connec-
tion with the frequency of barred markings throughout
the woodpecker family would seem to indicate that the
common ancestor of the group had been a strongly
barred form, these markings having continued to the
species of to-day with greater or less persistence.

Figs. + and 5 of Plate I show the typical markings of
the Dryobates villosus and D. scalauris groups, the striped
and barred forms. In this instance the difference be-
tween a streak and a stripe may be illustrated. In a
streaked plumage the pigmentation is generally along
the line of the shaft, and is the most primitive style of
marking. A striped plumage is produced in a much
bolder manner and is not necessarily a primitive form.

“ef. The North American Species of the Genus Colaptes, considered
with Special Reference to the Relationship of (. auratus and C. Cafer.
J. A. Allen, Bull. Am. Mus. Nat. Hist., iv, pp. 21-44.

t Bull. Am. Mus. Nat. Hist., iii, pp. 311-314.

EVOLUTION OF THE COLORS OF BIRDS. 283

In the feather here shown (fig. 4) the pigment is along
one side only, it being in fact a lateral hybrid, producing
a line of demarkation for a definite color patch.

The prevailing colors of the family are black, white,
brown, red and yellow. The relation of the plumage of
the sexes and young has already been discussed (see
ante, p. 224).

ORDER MACROCHIRES. THE GOATSUCK-
ERS, SWIFTS, etc.

FAMILY CAPRIMULGIDA. Tue GoatsucKErRs.

The colors of this family are remarkably homogeneous
in general style, being mottled brown, gray, ochraceous
and white. There are no marks of sexual adornment
among North American forms, the general colors being
very perfect protective marks, and the variously dis-
tributed white marks affording a beautiful illustration
of several classes of recognition marks. As these were
discussed somewhat fully in the preceding pages (see
ante, pp. 207-209) it is unnecessary to repeat what was
there stated. It need only be added that there is no
group which appears to present less difficulty in the
elucidation of color marks.

FAMILY MICROPODIDA. Tue Swirts.

(1) Adult male like female; young similar to adult.

Prevailing colors, blackish, grayish, brown, white.

Like the hawks, the colors of this family do not seem
to have been greatly influenced by natural selection.
From their crepuscular habits and swift flight, they can
can hardly have any enemies, nor do they need aggres-
sive colors for the capture of their prey. Cypseloides
and Chetura have not even a suggestion of a recogni-
tion mark, apparently, and their colors do not appear to

284 CALIFORNIA ACADEMY OF SCIENCES.

be of any significance whatever, but the white under
parts of Micropus may be a form of recognition mark,
possibly socialistic in nature. There seems to be a dis-
crepancy between the view that light favors the deposi-
tion of pigment and the fact that Cypseloides, which is
abroad only in the twilight or in cloudy weather, is al-
most black in color. There is not an inevitable contra-
diction in this, however, because although light may un-
doubtedly accelerate the production of pigment, or inten-
sify and make more brilliant the colors, nevertheless, if
pigment be a waste product, it must necessarily be thrown
off after enough has been accumulated in the system, light
orno light. Accordingly, if an animal were in a state
of increasing vigor and vitality, we should look for an
increase in pigmentation and a darkening, but not an
increasing brilliancy of color, in the absence of sunlight.
The direct action of the sunlight would, however, tend
to guide the pigment to particular parts of the body.
Iu most of the swifts it is almost evenly distributed, but
in Micropus, which is more diurnal than the rest, it is
nearly all on the back, the under parts being left white.

FAMILY TROCHILIDA. Tue Humminapirps.

(7) Adult male more conspicuously colored than
female; young similar to adult female.

Prevailing colors, white, black, metallic reds, greens
and blues.

This genus appears to afford one of the most striking
illustrations in the entire bird world of the operation of
sexual selection. Mr. Wallace has argued that the un-
paralleled beauty and diversity of color and ornament-
ation among these feathered gems is due solely or chiefly
to the operation of the general laws of growth. To me,
however, it seems a significant fact that the beautiful
metallic colors are as, a rule, confined to the head and

EVOLUTION OF THE COLORS OF BIRDS. 285

throat, in the form of a coronet and gorget of the most
beautiful iridescent hues conceivable. Why, it may be
asked, have not the general laws of growth placed some
of these colors on the abdomen and sides, leaving the
throat and head some plain, dull hue? If Mr. Wallace
can suggest any single law of growth which can throw
even a glimmer of light upon the reason for the produc-
tion of not merely the greatest amount of pigment, but
rather the most beautiful modulations of color, depend-
ent more on the structure of the feather than on pig-
mentation, upon the most conspicuous parts of the body,
and upon no other part—if he can indicate even a pos-
sible line of investigation of any law of growth explain-
ing this, it will be time enough to concede that it is an
open question whether or not sexual selection has been
the agent. It the meantime, it may be remarked that
recognition markings cannot be resorted to as an ex-
planation, because the metallic colors are only visible
when the bird is facing the observer, and recognition
markings could never be more useful from being invis-
ible from all but one point of view.

FAMILY TYRANNIDA. Tue Tyrant Frycatcuers.

So far as the significance of its colors is concerned,
this family may be divided into two divisions—the one in
which the red or orange crown patch is present and that
in which it is wanting. As has already been shown (see
ante, p. 199), this crown patch is unquestionably a form
of alluring color (pseudepisematic) in certain species,
and in all probability this is its function in all forms, as
it is present in both sexes, although generally slightly
restricted in the female. The genera in which it is
present are Milvulus, Tyrannus, Pitangus and Myiody-
nastes. The colors of these forms can be explained, it
seems to me, by the application of two principles al-

286 CALIFORNIA ACADEMY OF SCIENCES.

ready enunciated—the law of the assortment of pig-
ments, and the bleaching and intensifying of colors by
environmental influences. The original pigments of
the ancestral form were probably black and yellow.
These combined produced the olive green and brown so
common in this family, as in Tyrannus verticalis, for
instance. By selection the pigments were separated to
a greater or less extent producing such specialized marks
as the black cap of JMilvulus tyrannus, the black tail of
Tyrannus verticalis, the yellow crown patch of Tyrannus
tyrdunus, etc., or in more diffused form the yellow ap-
peared on the belly as in Pitangus derbianus, or even
extended over the entire breast in a much richer shade,
as in Tyrwnnus rueluncholicus couchii,and other tropical
species. The yellow has been both extended and in-
tensified by the influence of a tropical climate, while
from less determinate causes of intensification it has
been modified into its correlative red. As has been pre-
viously pointed out, the stages of intensification of the
yellow into red through orange, are shown in the crown
patch of the genus Tyrannus. A more complete transi-
tion of the yellow into red has occurred in the vermilion
flycatcher (Pyvrocephalus rubineus mevicanus), in which
species it is to be noted that the red color occurs in the
same parts of the body that have a tendency to be colored
yellow in other genera, viz., on the top of the head
and under parts.

Attention has already been called to Plate VI, as
showing the bleaching influences of an arid environment
on different species of the genus Myiarchus. In the
same manner, probably, the hght gray and white of the
genus Milvulus has been largely produced.

All the smaller flycatchers are to be regarded, it seems
to me, as degenerate descendants from some more brightly
colored race. They have been enabled to survive by

EVOLUTION OF THE COLORS OF BIRDS. 287

becoming insignificant. Thus, in the pheebe and black
pewee (Sayornis phebe and S. nigricans) the wing bands
are distinct rusty, but obscured in the adult. In Say’s
pewee (S. saya) the young has the reddish tinge much
brighter than the adult, and suffused over the back as
well as the breast. The young of the olive-sided fly-
catcher has the breast clear white, the streaks being more
restricted to the sides, while in the adult the breast is buffy
and heavily streaked. It seems from this as if the bird
of to-day had reverted from a brighter or more special-
ized color, to the primitive streaked style of marking.
In the genus Empidonax the different species were ap-
parently more specialized in former times, but through
reversion have become very closely alike in color, and
have lost even their distinctive recognition marks, ex-
cept the obscure wing bands.

FAMILY ALAUDIDA. Tue Larxs.

The members of this family are generally dull colored
birds not very highly specialized in respect to their
markings. Living almost exclusively on the ground,
when not in flight, the need for protective colors har-
monizing with the ground is particularly manifest.

GENus Otocoris. THE Hornep Larks.

(8) Male more conspicuously colored than female;
young with a peculiar first plumage.

Prevailing colors, brown, white, black, yellow, buffy,
etc.

Unlike most of the family the species of this genus
appear to be the descendants of a much more darkly
colored form. They have been modified largely by the
direct influences of the environment, probably in com-
paratively recent times, as the distinctions are in the
main only varietal and not specific. As usual, the dark-

288 CALIFORNIA ACADEMY OF SCIENCES.

est form is found in the northwest coast region and the
palest varieties in the western desert region, particularly
of Lower California.

The young are strikingly different from the adult in
colors and markings. The plumage is generally very
much darker, sometimes being almost black and the
feathers are tipped with white. From the darker and more
closely spotted head of the young it seems probable that
the ancestral bird had a black head. This has become
modified in the adult into a very unusual pattern on the
side of the head, which is doubtless a recognition mark
of some sort. It should be pointed out that the colors
of this genus hardly appear to harmonize with the law
of the assortment of pigment, for the special colors be-
ing black and yellow we should expect the generalized
colors to be olive green instead of brown tending toward
rusty or vinaceous.

FAMILY CORVID. Tue Crows, Jays, MaGpizs,
ETC.

(1) Male like female; young like adult, but colors
sometimes less distinct.

Black, white and blue are the characteristic colors of
this specialized family. Gray is also present as a gen-
eralized color, and much more prevalent than brown.
Yellow and green occur only in one tropical genus,
Xanthoura.

There is a strong melanistic tendency in this group,
due to the high specialization and the fact that from
their powerful size and gregarious habits these birds
have few enemies to contend with. The generalized
gray is due to the combination of the specialized blue
and black. Sexual selection has apparently been instru-
mental in the development of the blue pigment among
the jays, although it has become completely transferred

EVOLUTION OF THE COLORS OF BIRDS. 289

to the female and young. The influences of isolation
and environment upon the genus Cyanocitta has been
already discussed (see ante, p. 245). Isolation and en-
vironmental influences have also been responsible for
most if not all of the forms of Aphelocoma. The un-
usual number of closely allied species or varieties in-
habiting the different parts of southern North America
is to be accounted for by the fact that the bird is gen-
erally resident wherever found, and not much given to
wandering, except in a local manner when food is searce
in a particular locality.

The colors of the magpies (Pica) were probably first
developed as recognition marks and afterwards enhanced
by sexual selection. The habits of the magpies are such
that recognition marks would be of great service, par-
ticularly in their social relations. The yellow bill and
smaller size of P. nuttalli is doubtless the result of geo-
graphical isolation, and not a character to which any
utility can be ascribed. Perisoreus from the somber
forests of the north has simply never acquired a special-
ized plumage, since the labor of gaining a living must
have quite monopolized the bird’s attention, and en-
tirely driven any esthetic tendencies out of its head. The
different species and varieties are largely the result of
climatic influences. Clarke’s nutcracker (Picicorvus
columbianus) is marked with conspicuous black and
white recognition characters on the wings and tail, the
rest of the body being of the generalized gray of the
family. The pifion jay (Cyanocephalus cyanocephalus) is
without decided recognition marks, but has the ground
color a pale blue, not unlike that of Aphelocoma. The
black of the crows and ravens is simply the completion
of color specialization, but it is doubtless also useful as
a general recognition mark. There can apparently be
no possible utilitarian significance in the white basal

19

290 CALIFORNIA ACADEMY OF SCIENCES.

portion of the feathers of the neck of Corvus cryptoleucus,
and no way of accounting for it can be suggested.

FAMILY ICTERIDA. Tue Buacxsirps, ORIOLES, ETC.

Yellow, red and black are emphatically the colors of
this family, with brown as the generalized shade. White
is almost completely excluded except in the bobolink
(Dolichonyx), which, in respect to its colors, is more
closely allied to the Fringillide than to this group.
Nearly all the genera are very highly developed in their
colors, in most instances the specialized colors covering
the entire body.

GENus DoticHonyx. THE BoBoLiIngs.
(5) Male in breeding plumage differs from female;

young with peculiar first plumage, but similar to that of
adult female.

Prevailing colors, black, white, buffy, gray, brown.

Iam utterly at a loss for any explanation of the colors
of this bird either in accordance with the general laws
of growth or any principle of utility. The only resort
is to suppose the breeding plumage of the male to be
wholly the result of sexual selection, but this seems to
be a very unsatisfactory means of accounting for it.
Mr. Frank M. Chapman* has described the manner in
which the breeding plumage of the male is assumed,
partly by a moult and partly by the wearing away of the
tips of the feathers.

Genus Mototurus. THe Cowsirps.

(8) Adult male more conspicuously colored than
female; young with a peculiar first plumage.

Prevailing colors, black, brown, gray, buffy.

The young plumage with the breast lighter colored

“Auk, vii, pp. 120-124.

EVOLUTION OF THE COLORS OF BIRDS. 291

and indistinctly streaked indicates the style of coloring
of the bird from which this genus, in common with
Agelaius and others, was derived. The black color of
the male is to be regarded as an indication of specializa-
tion, accelerated by sexual selection, utility as a recog-
nition mark, and by the principle of sexual intensifica-
tion. The only explanation which occurs to me of the
brown color of the head and neck of the male is that
the specialization has progressed, in accordance with
Eimer’s law, from posterior to anterior. The bronzed
cowbird (M. ceneas), would then be a more specialized
form, the black having finally spread over the entire
body. The metallic colors are to be regarded as the re-
sult of sexual selection.

GENUS XANTHOCEPHALUS. YELLOW-HEADED BLAcK-
BIRD.

(6) Plumage of male in summer different from win-
ter plumage; female different from either; young similar
to female.

Prevailing colors, black, white, yellow, brown.

The yellow color of the head of the male can hardly
be explained except as the result of sexual selection, in-
completely transferred to the female by inheritance.
The white bars on the wing-coverts are undoubtedly
recognition marks, and the general black color the re-
sult of pigment intensification. That the ancestral bird
had a white breast (probably streaked like Molothrus), is
indicated by the white mottlings on the breast of the
female. There is a marked similarity between the
colors, and even the distribution of colors in this genus
and in the Arizona hooded oriole (Icterus cucullatus
nelsoni), which may be a mere coincidence, but may well
have some significance as pointing to an affinity in
origin. In both the head is yellow, the back black and

292 CALIFORNIA ACADEMY OF SCIENCES.

the wing bars white. Intense examples of Xantho-
cephalus have the head orange or more rarely pinkish
saffron, while the intense races of the Jcterws cucullatus
group bave the head varying from orange to red. The
principal difference in color distribution is that in the
former bird the black is on the posterior portion of the
under parts of the body, while in the ‘latter it is upon
the anterior portion.

Genus AGELAIUS. THE RED-wINGED BLACKBIRDS.

(6) Plumage of male in summer different from winter
plumage; female different from either; young similar to
female.

Prevailing colors, black, brown, buff, white, scarlet.

The adult males of this genus are highly specialized,
no generalized color being left, in fact. Like most birds
with black plumage, the members of this genus are gre-
garious. Gregarious birds, from their very habits, would
hardly be benefited by protective colors, and, accord-
ingly, instead of acquiring a protective plumage with
some distinctive markings, the black color itself be-
comes a recognition mark. Whether the red shoulder
patch is primarily a recognition mark or a character
developed by sexual selection is difficult to determine.
From the fact that it is so inconspicuous in the female,
it seems that it cannot be a directive mark, and was
probably developed primarily by sexual selection upon
the wings where it would attract the most attention.
Plate XIV shows the different phases in the development
of the wing patch. The differences between the adult
males of the three species, A. pheniceus, gubernator and
tricolor, may have been developed by sexual selection, or
else are discriminative marks as a result of physiological
selection. They certainly do not appear to have been
produced by geographical isolation. That these wing
markings are associated with the breeding habits, is

EVOLUTION OF THE COLORS OF BIRDS. 293

shown by their incomplete development in winter, when
the color is yellow and much broken. Sexual intensi-
fication has, in this instance, been an important factor
in the development of the specialized colors of the male,
the red of summer being merely an intensification of
the yellow, which still persists in winter, and the black
an intensification of the brown.

The ancestral bird was a brown streaked bird, lighter
or white on the breast.

GENUS STURNELLA. THE Merapow LarRks.

(1) Male similar to female (the latter duller); young
similar to adult, but duller..

Prevailing colors, black, white, brownish, grayish,
yellow.

The colors of this genus suggest somewhat an affinity
with the orioles (Icterus). The browns and grays of the
back are protective in nature, harmonizing with the
ground, and the yellow breast and black crescent are
probably due, in some measure at least, to sexual selec-
tion, partly transferred to the female by heredity. There
are slight seasonal as well as sexual differences, the full
breeding plumage being produced by the wearing away
of the tips of the feathers, but the change is not a very
marked one. The intensification of the yellow into its
correlative red in the tropical representative of the genus
has already been mentioned (see ante, p. 156).

Genus IctErus. THE ORIOLES.

(7) Adult males more conspicuously colored than
females; young generally similar to female; young male
(second year) intermediate between adult male and
female.

Prevailing colors, black, white, yellow, orange, red,
brown, olive green, gray.

This genus affords an especially good illustration of

294 CALIFORNIA ACADEMY OF SCIENCES.

the working of sexual selection in accordance with the
law of the assortment of pigments. Plate XVI shows
the generalized plumage of the group. It will be noticed
that the Arizona hooded oriole (J. cucullatus nelsoni) in
female plumage is a dull-colored greenish olive bird
without any special markings, except the partial band
of white on the wings. The young male is similarly
colored the first year. Upon the second year the young
male acquires a brownish black throat patch, as shown
in the plate. In these two phases of plumage, the
orchard oriole (J. spurius) is colored almost exactly the
same, although in both species there is a large amount
of individual variation.

In the adult female of Bullock’s oriole (J. bullock), the
colors of the male are indicated on a much duller key.
In the figure of the immature plumage of this species it
will be noticed that already the superciliary stripe has
appeared. The immature plumage of J. spurius seems
to have even tore yellow than later stages, and indi-
cates the relation of the colors of the adult of this
species to the other orioles which are so differently
colored. The female Baltimore oriole (J. galbula) is rap-
idly acquiring the plumage of the male, although the
colors are much duller and broken.

While the general tone of color in this plate is olive
greenish and brownish, the effect of Plate XVII, show-
ing the heads of the adult males, is black and yellow or
orange. The separation of the component pigments of
the undeveloped phases of coloration would produce the
colors of the specialized males. And what factor can
be suggested to account for this better than sexual selec-
tion, which explains at once the similarity of colors and
the difference in their distribution? The colors would
first be black and yellow, but by intensification of the
latter orange and crimson would be developed. This is

\
EVOLUTION OF THE COLORS OF BIRDS. 295

especially well illustrated in the JZ. cucullatus group
which becomes intensified from a yellow at its northern
limit to an intense flame in the tropics. In J. bullocki
the black has extended over the head, leaving a super-
ciliary stripe, however. In J. galbula the black has
spread over the sides of the head connecting the throat
patch and back. In J. parisorwm and I. spuwrius it has
extended still farther down on the breast. There is
apparently here a tendency towards the assumption of a
completely black plumage as the bird becomes more and
more specialized. The reddish brown color of J. spurius
seems very different from the rest of the group, but I
do not think it has been produced by the introduction
of anew pigment but rather by a modification of the
yellow. Itis apparent that the color of the ancestral
form is yellow, from the persistence of this color in the
young. By intensification this yellow becomes red as
in the analogous case of J. cucullatus, and it is perfectly
conceivable that by further intensification, even by the
simple increase in the amount of the pigment, the red
would become darker instead of more brilliant.

GENUS ScoLecoPpHAGUS ) THE BLACKBIRDS AND

GENUS QUISCALUS GRACKLES.

(7) Adult male more conspicuously colored than
female; young similar to adult female. In some species
a seasonal difference (6) in which the male differs from
the female in both summer and winter plumage.

Prevailing colors, black, brownish, various metallic
hues.

General pigment intensification, sexual selection, and
the advantage of conspicuous colors for gregarious birds
serve to explain the colors of these two genera.

296 CALIFORNIA ACADEMY OF SCIENCES.

FAMILY FRINGILLIDA. Tue Fincues, Spar-
ROWS, ETC,

This large family has comparatively few genera with-
in our limits which are highly specialized in coloration,
a large proportion of them being colored in a decidedly
primitive fashion. Brownish and buff colors prevail,
although red here reaches its maximum development.
The lack of more fully developed color markings is to be
explained partly by the terrestrial habits of so many of
the family, making dull protective markings a matter of
necessity, and partly by the severe climate, compara-
tively speaking, in which a large proportion of the group
reside.

Genus CoccotHraustes. THE Eveninc GROSBEAKS.

(7) Adult male more conspicuously colored than adult
female; young similar to adult female (but duller).

Prevailing colors, black, white, yellow, brown, olive,
gray.

The colors of the male are probably in the main, if
not exclusively, the result of sexual selection, incom-
pletely transferred to the female and young. The black
wings with white bars are doubtless recognition marks
which would be of especial use to this bird on account
its gregarious habit.

The ancestral form, as shown by the unspecialized
plumage of the females and young, was dull brownish
gray and streaked, probably not unlike that of Pinicola,
Carpodacus, etc.

Genus Pinicora. THE Pine GROSBEAKS.

(7) Adult male more conspicuously colored than
female; young similar to female, but colors duller.

Prevailing colors, red, gray, white, brown (abnormal).

The colors of the male have apparently been developed

EVOLUTION OF THE COLORS OF BIRDS. 297

from the dull gray plumage of the female by sexual se-
lection. White wing-bars, directive recognition marks.

Genus Carpopacus. Tue Purpie Fincues, ETC.

It will be remembered that each of the larger Ameri- '
can kingfishers (Ceryle) had a corresponding allied
smaller species (see ante, p. 278). The relation between
the colors of Pinicola and Carpodacus seems to be a
similar one, although less direct. The relative colors of
the sexes and young, as well as the absolute colors of
these two genera, are very similar, and undoubtedly had
a common origin and were developed by the same
agencies. In speaking of the scarlet rose finch (C. ery-
thrinus grebnitshit), Dr. Steineger says:* ‘‘ There can
be no doubt that males of this species breed in the gray
plumage. I found these breeding gray males almost as
common in Petropaulski as the red ones, their conduct
and song being exactly the same as that of the latter,
and dissection showed that the genital organs were well
developed and fully matured. Itseemsto be a question
whether these mature gray birds will ever assume the
red plumage, and I should be most inclined to believe
that we have here to do with a kind of dichromatism.”’
May not this tendency towards dichromatism in Carpo-
dacus be parallel to the phases of coloration in Pinicola
noticed by Mr. Ridgway in the following note in the
Manual of North American Birds:t ‘‘ Apparently adult
males are occasionally found in which the plumage is
not distinguishable from that of the female; in others
the general plumage is that of the female, except that
the olivaceous or tawny color on head, etc., is replaced
by a more reddish tint (varying from light dull orange-
red to deep madder-brown).”

* Bull. U.S. Nat. Mus. 29, p. 266.
t pp. 387-388.

298 CALIFORNIA ACADEMY OF SCIENCES.

Genus Loxt;i. THE CrossBILLs.

(8) Adult male more conspicuously colored than
female; young with a peculiar first plumage, but more
or less similar to adult female.

Prevailing colors, red, yellowish green, olivaceous,
grayish, white.

Like the preceding, these birds have descended from
a gray streaked ancestor, in fact, probably from the
same species. The colors are the result of sexual selec-
tion, apparently, the yellow correlative of the red having
been transmitted more or less to the females and young
males. It is probable that Coccothraustes, Pinicola,
Carpodacus and Loxia all have originated from the same
stock, the first genus alone retaining the yellow color in
the adult male plumage, the others acquiring the red
correlative.

The white wing bands of L. leucopteru. are apparently
discriminative recognition marks.

Genus Leucosticre. Tue LEucostricrrs.

(4) Sexes alike, changing color with the season;
young differ from adults at any season.

Prevailing colors, black, brown, gray, white, red
(generally rose).

The structural affinity and general character of the
colors of this genus ally it with Pinicola, ete., although
the sexes do not differ in plumage, and the young are
plain brown, less conspicuously streaked, thus differing
trom the genera previously considered. It is difficult to
determine what factors besides sexual selection have
produced the colors of this genus. Geographical isola-
tion has had some influence in the origination of specific
differences, although probably not the only factor.

EVOLUTION OF THE COLORS OF BIRDS. 299

Genus Acantais. THE REDPOLLS.

(8) Adult male more conspicuously colored than
adult female; young with a peculiar first plumage.

Prevailing colors, brown, white, gray, red, pink.

The colors of the young, female and adult, represent
three stages in the assumption of the red color, first the
plain streaked bird, not unlike the pine goldfinch (Spinus
pinus), next with the red cap (acquired first by the male
as a sexual charm, and afterwards transmitted to the
female by heredity), and lastly the red on the rump and
under parts. There is a tendency towards the assump-
tion of an albinistic plumage in the northern forms of
this genus. Just as the two large forms, Pinicola and
Coccothraustes, seem to have come from the same stock,
and developed, one the red and the other the yellow
color, so also even more intimately the small forms
Acanthis and Spinus appear to be related. The probable
genealogical connections of the genera thus far con-
sidered, which seem to constitute a group by themselves,
may be indicated by the following diagram:

300 ' GALIFORNIA ACADEMY OF SCIENCES.

PINICOLA
CARPODACUS

LOXIA

LEUCOSTICTE

SPINUS
COCCOTHRAUSTES
Bs
iy
ACANTHIS ao 5 8h
se wa xe =e
a ‘axe ya ae,
ae « Spee “Oy,
A
atv &
i os
=
\ x
est alee

Undeveloped yellow

Gray streaked ancestral form.
Genus Spinus. THE GOLDFINCHES.

(7) Adult male more conspicuously colored than
female; young similar to female, but duller; or (5) Male
in breeding plumage differs from female; young with a
peculiar first plumage, but similar to winter adults.

Prevailing colors, black, white, yellow, olive green,
brown.

EVOLUTION OF THE COLORS OF BIRDS. 301

As the male is uniformly more brightly colored and
has more special markings than the female, the colora-
tion has undoubtedly been acquired by sexual selection
in accordance with the law of the assortment of pig-
ments. Reference has already been made to the sup-
position that this golden color which was acquired for
adornment may have been utilized for protection since
the Composite to which they so commonly resort are
colored thus (see ante, p. 195). The pine finch (8. pinus)
retains a plumage but little modified from the bird which
was the common ancestor of the entire group.

GENUS PLECTROPHENAX. THE SNOWFLAKES.

(6) Plumage of male and female changes with the
season; male unlike female in both plumages; young
similar to female, but duller and more streaked.

Prevailing colors, white, black, gray, brown, buffy.

There is a strong tendency in this genus towards the
assumption of a completely white plumage. The ances-
tral form was a streaked bird, as shown by the present
condition of the young. Nexta black and white plumage
was assumed, which still persists to a certain degree,
although the black is becoming eliminated. Thus Mr.
E. W. Nelson writes as follows concerning an abnormal
P. nivalis:* ‘ One specimen, a female, obtained at Saint
Michaels, April, 1879, is pure white over the entire
body, with the exception only of the tips of the prim-
aries for an inch from their ends. This part of the
primaries is jet black, mainly on the inner web, with a
narrow edge of white at the tip, but the shaft and most
of the outer web are white. The tailis pure white with-
out a trace of black. The bases of the feathers are not
black, as is usually the case with this bird, but are dark

*Report upon Natural History Collections made in Alaska. Arctic
Series of Pub. Issued in Connection with Sig. Service, U. 8. Army, p. 182.

302 CALIFORNIA ACADEMY OF SCIENCES.

sooty plumbeous. Feet black, bill pale. This bird is
not an albino, as is shown by the black wing tips, which
have much the same color arrangement as is exhibited
in the gulls.” As Mr. Nelson also found other speci-
mens which were abnormally white, though not albinos,
it appears that these must be individuals which have
advanced beyond the species. ‘The utility of a white
plumage to an Arctic bird hke the snow bunting would
at first sight appear to be very manifest, yet, on further
consideration, it is found that the nest is not built in
the immediate vicinity of snow, and that accordingly a
white plumage would not be protective during the breed-
ing season. That the black is the earlier plumage and
the white a later phase, is proved by the fact that
the base of the feather is nearly always black, the tip
alone being white. This might be suggestive of a direct
influence of arctic cold in whitening the plumage, re-
gardless of utility. This view is strengthened by the
fact that it is during the winter months that the back is
white, this white edge of the feather wearing away at
the approach of spring (see Plate I, fig. 9). The back
of McKay’s snowflake (P. hyperboreus) does not become
black in the breeding season, the white feathers remain-
ing unworn (Plate J, fig. 10); and this is to be expected
from the fact that the bird is more exclusively a resident
of the Arctic regions. Another factor which may have
aided in the production of the white plumage is that
white feathers containing gas bubbles are much denser
and warmer. The wings and tail, which do not form
part of the bird’s clothing, would thus not need to change
to white, but would rather be of use as recognition mark-
ings by retaining their black hue.

EVOLUTION OF THE COLORS OF BIRDS. 3038

GENUS CALCARIUS

Tue LonGsprurs.
GEeNus RHyYNCHOPHANES

(5) Adult male in breeding plumage differs from
female; young with a peculiar first plumage.

Prevailing colors, white, black, gray, brown. chestnut,
buffy.

This genus is closely allied to the preceding in regard
to its colors, but has not resided so long or been so much
confined to the Arctic regions, and hence. has not ac-
quired the same amount of white. It is a significant
fact in this connection, I think, that in this genus al-
though the plumage does not become white in winter
the black markings are largely obscured either by white
or buffy tips to the feathers. This might well be in-
terpreted as the commencement of a climatic influence
which will culminate in a white plumage. Natural
selection would favor this change as resulting in a
warmer winter dress. The characteristic marks of the
different species are apparently directive and discrim-
inative recognition marks.

Genus Pooc@tes. THE VESPER SPARROWS.

(1) Adult male like female; young like adult, but
markings duller.

Prevailing colors, brown, rufous, gray, white.

This is one of the plainly attired ground birds in
which very little specialization of marking has been at-
tained, the colors being protective in nature. The only
recognition mark is the white outer tail feathers, unless
the rufous patch on the wing coverts be considered as
such. This latter may be a discriminative mark, al-
though it seems hardly conspicuous enough. On the
other hand it cannot well be looked upon as the result
of sexual selection, for it is not highly enough developed
to have been completely transmitted to the female.

304 CALIFORNIA ACADEMY OF SCIENCES.

GENUS AuMoDRAMUS. THE SAVANNA SPARROWS, ETC.

(1) Adult male like female; young more or less
closely resembling adult.

Prevailing colors, black, white, brown, gray, buff,
yellow.

This genus consists mainly of unspecialized streaked
species, although a greater degree of development is ex-
hibited in a few forms. Most of the species show yellow
either on the face and superciliary stripe or on the edge
of the wing, or sometimes in both places. What its
significance may be I will not attempt to suggest, for
although it may be the result of sexual selection it is
present equally in both sexes. Climatic influence has
played an important part in the differentiation of the
various races, particularly of the A. sandrichensis group.
The seaside sparrow (A. mevitimus) has assumed a uni-
form coloration above, but the young are streaked like
typical members of the group.

Genus CuonpestTes. THe Lark Sparrows.

(2) Adult male like female; young like some ances-
tral stage of the adult.

Prevailing colors, black, white, brown, chestnut,
gray.

Although not highly developed in color, the markings
of this genus are more specialized than any of the family
thus far considered. The complicated black pattern of
the head is to be looked upon as the combination of the
streakings of the primitive plumage into certain well
defined areas. These head stripes are probably recogni-
tion markings of some sort, but whether directive or
discriminative, is uncertain. The little spot of black
on the breast is of interest, being apparently the incip-
lent stage of a new character. The plumage of the
young points directly to a streaked ancestor in common
with most of the group.

EVOLUTION OF THE COLORS OF BIRDS. 305

GENus Zonotricuia. ‘Tur WHITE-CROWNED SPAR-
ROWS, ETC.

(2) Adult male like female; young like some ances-
tral stage of the adult.

Prevailing colors, black, white, brown, gray, yellow

This genus has become modified from the primitive
bird by losing the streaks on the breast, a corresponding
darkening of the back, and by the acquisition of highly
characteristic recognition marks about the head. These
marks were probably primarily of use as discriminative
characters. They would then be most necessary during
the breeding season, at which time they reach their
maximum development. Without the introduction of
any new colors, such striking differences are produced
as between Z. querula, with its black head and throat, Z.
albicollis, with its pure white sharply defined patch of
white on the throat, and Z. leucophrys, with the character-
istic black lines on the head. Still different and equally
well marked is Z. coronata in breeding plumage, although
in winter these last two birds are not so readily distin-
guished at a distance. It is a noteworthy fact that Z.
albicollts, although so differently marked, has the same
shade of yellow on the head as Z. coronata, but located
on the superciliary stripe, instead of on the median line,
as in the latter. This is a fact of some significance with
regard to the law of the assortment of pigments. It is
also worthy of note that the yellow of Z. albicollis is dis-
tributed as in Ammodramus, for this may indicate either
a genetic relationship or a common cause at work.

Genus SprzeLtA. THE TREE Sparrows, CHIPPING
SPARROWS, ETC.

(2) Adult male like female; young like some ances-
tral stage of the adult. °
Prevailing colors, black, white, brown, chestnut, gray.
20

306 CALIFORNIA ACADEMY OF SCIENCES.

The birds of this genus, like the preceding, have de-
veloped from a form streaked above and below, this style
being preserved in the young of all the living forms
except S. atrigularis, in which the streaks are but very
faintly indicated. The only specialization has been the
loss of the streaks on the breast, which has become uni-
form whitish, and the acquisition of a chestnut colored
cap in some species, and a black throat-patch in S. atri-
gularis. 8S. pallida has the head striped in brown, black
and gray, while S. brewerd seems to be without any form
of recognition marking. Recognition markings, when
present, are rather discriminative than directive in their
nature, the habits of the birds rendering the latter un-
necessary.

Genus Junco. Tue Juncos.

(2) Adult male like female; young like some ances-
tral stage of the adult.

Prevailing colors, dark slate, gray, rufous, white.

Although plainly enough deriving their origin from
the same stock as the other sparrows, the significance of
the colors of this genus seems peculiarly difficult to de-
termine. The white tail feathers form an excellent
directive mark, while the wing bands of J. atkent may
well be discriminative marks, but why some species
should have the breast white and some vinaceous pink,
or why the back should be gray in some species and
reddish brown in others, I do not feel able to give an
opinion. The general distribution of markings in the
genus may be due to the working of laws of wide appli-
catlon—especially to the effect of light in controlling
the deposition of pigment, as previously suggested (see
ante, p. 230).

EVOLUTION OF THE COLORS OF BIRDS. 307

Genus AmpHIsPIzA. BLACK-THROATED SPARROW, ETC.

(2) Adult male like female; young like some ances-
tral stage of the adult.

Prevailing colors, black, white, gray, brown.

Although the colors of this genus are decidedly plain
the markings are considerably specialized, the general
tendency of coloration being solid rather than streaked,
and the special markings being strongly developed. The
white superciliary stripe of the black-throated sparrow
(A. bilineata) as distinguished from the the supraloral
white spot in other species, is a discriminative mark
apparently. The evolution of the black throat-patch
may be traced from the sage sparrow (A. belli nevadensis)
where the ‘‘ sides of throat [are] marked with a series of
narrow dusky grayish streaks, but no continuous stripe,”
through Bell’s sparrow (A. belli) which has the ‘sides
of throat marked with a broad continuous stripe of
blackish,’ to the Mexican forms in which we find the
‘“‘upper part of throat black, the point of the.chin white;
fore-neck and lower part of throat uniform ash gray;’’*
and finally A. bilineata is reached with the throat com-
pletely black in the adult but white in the young. It is
doubtless a directive mark, although possibly discrim-
inative in function.

Genus Peucwa. Rurovus-cRowneD SPARROW, ETC.

(2) Adult male like female; young like some ances-
tral stage of the adult.

Prevailing colors, blackish, white, ashy, rufous, brown.

The plan of coloration in this genus reminds one
greatly of Spizella, particularly in tthe presence of a
rufous crown patch in some species and the substitution
of streaks on the top of the headin others. The young
are much more decidedly streaked than the adult, there

* Ridgway’s Manual, pp. 425-426.

1

308 CALIFORNIA ACADEMY OF SCIENCES.

being a tendency towards the assumption of a uniform
coloration. Climatic influences and isolation appear to
have played an important part in the origination of
specific and varietal differences.

Genus Metospiza. THE Sone SPARROWS, ETC.

(1) Adult male like female; young similar to adult.

Prevailing colors, brown, rufous, gray, ashy, white.

This genus is comparatively little modified from the
streaked ancestral plumage. The general effect of sun-
light upon pigmentation is shown by the tendency of
the breast to be white and the back dark, although the
bird is streaked both above and below. The special in-
fluence of the environment upon the colors is shown in
Plate XIII where the color of the back of each race is.
marked in the locality inhabited by the respective form.

GENUS PASSERELLA. THE Fox SPARROW, ETC.

(1) Adult male like female; young like adult.

Prevailing colors, brown, rufous, gray, white.

The transition from a streaked to a uniform mode of
coloration is shown by the backs of P. iliucw and P.
iliaca unalaschcensis, which are races of the same species
probably largely due to climatic influences. Upon the
breast the primitive streaked plumage is replaced by the
second type of marking, the spot. The markings of
this genus are surprisingly like those of the thrush
family, and may well have been produced by the same
or similar influences.

Genus EMBERNAGRA. TEXAS SPARROW, ETC.

(1) Adult male like female; young like adult (?).

Prevailing colors, olive-green, brown, gray, buffy,
white, yellow.

This genus appears as if it had been more brilliantly
colored in past time, but had lost most of its character-

EVOLUTION OF THE COLORS OF BIRDS. 309

istic marks. It is specialized beyond the streaked plum-
age, and the head has distinctive although very dull
marks. The only conspicuous character which has per-
sisted is the yellow edge of the wing, which is apparently
either a sexual or recognition marking, developed upon
this conspicuous place (see ante, p. 184).

Genus Pireito. THe TowHeEss.

(2) Adult male like female, or (8) more conspicu-
ously colored than female; young like some ancestral
stage of the adult.

Prevailing colors, black, white, brown, rufous, olive
green.

This genus is of particular interest in the study of the
evolution of color. In the first place, the relation of
the sexes in such very closely allied species as P. ery-
throphthalmus and P. maculatus, changes in regard to
the correspondence of color. Again, there are three
groups of species in the genus, and they seem at first
glance to be hopelessly different in point of color origin.
An examination discloses the interesting fact, however,
that there are more points of similarity than would at
first appear. The first group includes P. erythrophthal-
mus, P. maculatus, etc., the second P. chlorurus, and the
third P. fuscus. The differences between the first and
second are peculiarly striking, yet it will be noticed that
the color of the flanks of the first is the same as the
crown of the head of the second, and accordingly might
be explained in accordance with the law of assortment
of pigments, the color being distributed to different
parts of the body in different species. P. maculatus has
the feathers of the throat subterminally marked with
with white, showing that it formerly had a white throat,
such as P. chlorurus has at present. The only part about
which there appears to be some difficulty of explanation
is the olive green color of the back in P. chlorurus. The

310 CALIFORNIA ACADEMY OF SCIENCES.

following attempt to explain it according to the law of
pigment assortment may seem forced, but is suggested
for what it is worth. There can be no doubt that the
orchard oriole (Icterus spurius) was originated from
the same stock as the other orioles, from the fact that
the young plumage is almost identical in color with cor-
responding stages of other species. Here it seems that
an intensification of yellow has produced a reddish
brown instead of scarlet or crimson. In the same way,
it is not impossible that the rufous color of Pipilo has
been produced by the intensification of yellow, which
color is present in certain members of the genus, in-
cluding P. chlorurus. The combination of the yellow
with the black would produce the olive green of P. chlo-
rurus. As showing the relationship of P. fuscus, it is
to be noted that in this species the feathers of the belly
are white, basally, indicating that this part was formerly
as in P. maculatus,

These birds are all the descendants of a streaked an-
cestor, but what factors have produced the wide diversity
at present exhibited we cannot well determine. Complete
and long continued isolation of the original species into
three sections may have started the different branches
upon widely different courses of evolution. Sexual
selection, recognition and protection have all played a
part in this.

GENUS CARDINALIS

THE CARDINALS.
GENUS PYRRHULOXIA

(7) Adult male more conspicuously colored than
female; young similar to adult female, but duller.

Prevailing colors, vermilion, carmine, black, brown,
buff, gray.

Sexual selection alone can account for the colors of
this genus, the females having inherited a small portion
of the bright colors of the male. The fact mentioned

EVOLUTION OF THE COLORS OF BIRDS. 3L1

by Beddard (see unte, p. 228) that when food changes the
color of the plumage certain parts are more readily
affected than others may account for the color running
down the breast in Pyrrhuloxia and not including the
sides.

Genus Hapia. THE GROSBEAKS.

(10) Adult male more conspicuously colored than
female; young male (first autumn) unlike adult of either
sex; young female like adult female.

Prevailing colors, black, white, rose, yellow, brown.

The two species constituting this genus are peculiarly
interesting from the great difference in the adult males,
and the different stages of specialization from the com-
paratively unmodified young plumage. As these differ-
ent stages of transition were given previously they need
not now be repeated (see ante, p. 223). Sexual selection
has apparently been by far the most important factor in
the evolution of the colors of the adult male, but the
conspicuous white and black wing and tail markings are
doubtless recognition marks, partly directive and partly
discriminative. The striking example of correlative red
and yellow in this genus has been sufficiently dwelt upon
(see ante, p. 155).

Genus Passerina. THE VARIED BUNTING, ETC.

(7) Adult male more conspicuously colored than
female; young like female, but generally somewhat
duller.

Prevailing colors, black, white, red, purple, blue,
yellow, green, brown.

This genus belongs properly to the tropics, and the
males have all the wealth and diversity of coloring of
birds from that region. The colors appear to be princi-
pally due to sexual selection favored by a tropical en-
vironment, the females being dull brown in remarkable
contrast to the males.

312 CALIFORNIA ACADEMY OF SCIENCES.

GENuS SpoROPHILA. THE SEEDEATERS.

(7) Adult male more conspicuously colored than fe-
male; young like female, but young male generally
intermediate in plumage between female and male.

Prevailing colors, black, white, olive, brown, gray.

This genus is apparently becoming melanistic in ac-
cordance with the laws of growth and specialization.
The colors are doubtless useful for purposes of recogni-
tion.

Genus Evetuera. THE GRASSQUITS.

(7) Adult male more conspicuously colored than fe-
male; young like female, but young male generally
intermediate in plumage between female and male.

Prevailing colors, black, white, gray, yellow, olive
green, rufous.

The yellow and black of this genus are refined from
the olive green of the back. The presence of black upon
the breast instead of the back would seem to be in oppo-
sition to the rule that the excess of sunlight has caused
the greater part of pigment to seek the back, but may
perhaps be interpreted in harmony with this. Where
black is the result of an intensified brown or blue it ap-
pears to be caused simply by the amount of pigment
matter crowded into a given space. If, on the other
hand, the system of a bird gives rise to definite black
pigment, the color is not dependent quite so exclusively
upon theamount. In the present instance two pigments,
yellow and black, have apparently crowded together upon
the back, producing the olive green color, while upon
the breast they have been separated.

The tendency of the genus is towards the assumption
of a completely melanistic plumage.

GeENus Spiza. THe DIcKCISSEL, ETC.
(8) Adult male more conspicuously colored than fe-

EVOLUTION OF THE COLORS OF BIRDS. 3138

male; young with a peculiar first plumage, but similar
to that of female.

Prevailing colors, black, white, gray, yellow, rufous.

This bird shows three well marked stages of evolu-
tionary progress, from the young plumage through the
adult female to the adult male. The colors of this genus
are identical with the preceding, except the olive green,
which is wanting in the present group. The distribution
of color, however, appears to have been rather more
controlled by sexual selection than in the previous
instance.

Genus CaLamospizA. THE LarKk Buntinea.

(5) Male in breeding plumage differs from female;
young with peculiar first plumage.

Prevailing colors, black, white, brown, slaty, gray.

There is a strong tendency toward the assumption of
a melanistic plumage in this genus, although worn only
by the male during the summer. The conspicuous white
wing patch is probably a directive mark.

FAMILY TANAGRIDZ. Tue Tawacers.

This family consists of brilliant tropical genera, afew
species of which have spread over the boreal province.
The female are dull colored and the young generally
represent a still earlier phase of developmental history.
But two genera are present within our limits, Huphonia
and Piranga. The prevailing colors of the male of the
former are blue, black, and chestnut or rufous; of the
latter scarlet, yellow and black. The female of Piranga
is olive green and yellow, the colors of the male having
been produced first by the assortment of pigments into
yellow and black, and second by an intensification of
the yellow into scarlet. The relationship of Euphonia
to Piranga, so far as its colors are concerned, is shown
by the plumage of the female which is colored an olive

314 CALIFORNIA ACADEMY OF SCIENCES.

green, more yellowish below, but, unlike Piranga, with a
blue patch on the head. What this blue was derived
from is not apparent, unless it is separate pigment which
is wanting in Piranga.

FAMILY HIRUNDINID®. Tue Swattows.

All the members of this family are specialized beyond
the streaked plumage, and the tendency is for the back
to be darker than the breast, both being marked with
solid colors instead of mottlings or streaks. All this is
the result of specialization in accordance with the gen-
eral laws of pigment deposition. The colors and mark-
ings which cannot be explained by these general laws
appear to be the result of sexual selection.

Genus Progne. THE Martins.

(7) Adult male more conspicuously colored than
female; young like adult female.

Prevailing colors, black (metallic bluish), gray, white.

The male represents the culmination of specialization
in a black plumage, enhanced, by sexual selection, by
burnished blue lustre. The female has the underparts
gray, becoming white posteriorly, showing the manner
in which the black plumage was assumed, the abdomen
probably having been affected last.

Genus PerrocHeLipon. THe CLirr SwALLows.

Genus CHEenipon. THe Barn SwaLtows.

(1) Adult male like female; young like adult but
duller.

Prevailing colors, black, blue-black, white, chestnut,
cinnanon, gray, brown.

The general tendency in these two genera, is, as in
Progne, to have the back dark metallic black with
greenish or bluish reflections, and the under parts more

EVOLUTION OF THE COLORS OF BIRDS. 315

or less white or suffused with rusty more or less in-
tensely.

The different characteristic marks were probably de-
veloped as discriminative characters first and afterwards
modified by sexual selection.

GENUS TACHYCINETA. WHITE-BELLIED SWALLOW, ETC.

(8) Adult male more conspicuously colored than
female; young with a peculiar first plumage, although
similar to adults, the colors being duller.

Prevailing colors, steel blue, bronze green, purplish,
violet, brown, white.

The beautiful colors of the adults of this genus are
the result of sexual selection, the female, particularly
of the exquisitely marked violet-green swallow (7. thalus-
sina), being much duller than the male. This is an in-
stance where Mr. Wallace’s theory of the dull colors of
female birds as the result of the supression of bright
tints from need of protection appears to be utterly un-
tenable. White, in contrast to some dark color is the
most conspicuous marking in nature. The under parts
of the female of this bird are pure white asin the male,
but the violet-green colors of the male which are beauti-
ful but in no way glaring or conspicuous, are much
dulled in the female, conclusively showing that the need
for protection is not the factor in this instance.

Genus Crivicora. THe Bank SwALLow.

Genus STELGIDOPTERYX. THE ROUGH-WINGED SWAL-
Low.

(2) Adult male like female; young similar to adult,
but some of the feathers with lighter edgings.

Prevailing colors, brown, gray, white.

These are the two least specialized species in the group,
being not unlike the young of the preceding genus.

316 CALIFORNIA ACADEMY OF SCIENCES.

The colors are to be accounted for almost wholly by the
effect of environment and general laws of pigment de-
velopment. It is a remarkable thing that two forms so
much alike in respect to coloration as these genera are,
should have no discriminative marks. It may be that
the similarity of coloration is due to a retrogressive con-
vergance, this view being strengthened by the similarity
of the coloration of both genera to the young of Tachy-
cineta,

FAMILY AMPELIDAH. Tuer Waxwines, Etc.

Genus AMPELIS. THe WAXWINGS.

(2) Adult male like female; young like some ances-
tral stage of the adult.

Prevailing colors, black, white, brown, vinaceous,
cinnamon, ashy, gray, yellow.

I see no possible way of accounting for the beautiful
soft blended tints of these birds, except by sexual selec-
tion. The attainment of the goal of specialization by the
male has permitted the female to acquire the complete
plumage of the male, but the young still retains upon
the under parts the streaks of the ancestral bird. The
white wing markings so conspicuous in A. garrulus are
recognition marks, probably both directive and discrim-
inative in nature. The rufous under tail coverts of this
species as contrasted with the white of A. cedrorum, ap-
pear to be a discriminative mark. An interesting ex-
ample of correlation is to be found in the wax-like
appendages to the wing feathers, which are also occasion-
ally present, but in less pronounced degree, upon the
tail. This genus also affords an instance of red as the
correlative of yellow in the border of the tail, which is
yellow in North American species, but red in the Jap-
anese waxwing (A. juponicus).

EVOLUTION OF THE COLORS OF BIRDS. 317

GENUS PHAINOPEPLA. THE PHAINOPEPLA.

(7) Adult male more conspicuously colored than
female; young like female.

Prevailing colors, bluish black, white, brownish gray.

The male has become completely glossy blue-black,
with the exception of the white recognition mark on the
wing, while the female is a brownish gray, not having
yet acquired the specialization of pigmentation of the
male.

FAMILY LANIIDZ. Tue Surixzs.

Genus Lanius. THE SHRIKES.

(1) Adult male like female; young similar to adult.

Prevailing colors, black, white, gray, brownish.

I am unable to suggest any satisfactory explanation
of the colors of this genus, although the black line
through the sides of the head, and the black wings and
tail with the contrasted white are excellent recognition
markings. I fail to see any particular need for such
markings, as the birds are not very social in their habits,
and have no enemies to fear, in particular. Neither
could these be discriminative marks, for they are shared
in common by all the genera. Their origin must, ac-
cordingly, date back to some ancestral form in which
such characters may have been of utility.

FAMILY VIREONIDA. Tue VirRzos.

Genus VirREO. THE VIREOS.

(1) Adult male like female; young more or less
closely like adult, or (8) male slightly more conspicu-
ously colored than female; young with a plumage slight-
ly duller than female.

Prevailing colors, black, white, olive green, brown,
gray, yellow.

318 CALIFORNIA ACADEMY OF SCIENCES.

The vireos in their colors appear to be like undeveloped
warblers. The olive green, black and yellow, so char-
acteristic of that group is present here also, but has not
yet obtained the mastery over the gray tints. I believe
this genus shows us the effect of sexual selection which
has, comparatively speaking, been in operation fora
short time only. Although a number of species have
one or more geographical races, differing but little from
one another, the species themselves are for the most
very distinct, even though few of them have any par-
ticularly striking individual characters. In some genera
varieties merge into species in a gradual transition, as,
for instance, in Melospiza, Sphyrapicus, Zonotrichia,
etc., where the extreme varieties are almost or quite as
different as the most closely allied species, but here, as
a general rule, there is a considerable gap between the
varieties of one species and the next most closely related
form. This would seem to indicate that the different
species had diverged quite widely before attaining their
present distribution upon which the existing varieties de-
pend. The isolation which produced the different spe-
cies is difficult to determine, but it is a noteworthy fact
that to the south certain mainland and insular forms
pass by gradual stages from variety to species. This is
notably the case with V. crassivostris and V. crassirostris
flavescens of the Bahamas, and V. ochraceus of southern
Mexico, which is intermediate in color between the two
preceding. It would seem then that the differentiation
of species may have been due to geographical isolation
in a southern district.

FAMILY CCQREBIDA. Tuer Honey CREEPERS.

Genus CertHioLa. THE Honry CREEPERS.
(2) Adult male like female; young like some ances-
tral stage of the adult.

EVOLUTION OF THE COLORS OF BIRDS. 319

Prevailing colors, black, white, olive, brown, gray,
yellow.

The colors of this genus have been developed in ac-
cordance with the law of the assortment of pigments,
but whether sexual selection or need for recognition has
been the more important agent is not easily established.
Both factors have apparently been operative.

FAMILY MNIOTILTIDH. Tue Woop WarsBtLeERs.

This extensive family is particularly interesting from
the almost uniform degree of specialization of color
marks which it presents. They are to be explained
principally by the influence of sexual selection supple-
mented by the advantage of recognition marks. The
different genera, with some few exceptions, are closely
related so far as their colors are concerned, making this
family one of the best illustrations of the law of the
assortment of pigments. The prevailing colors of the
entire family are olive green, black and yellow. The
gray which so often appears may be due to the bleach-
ing of the olive green; the red, as in Setophaga, is ob-
viously the result of intensification of the yellow, and
the chestnut may be also a modification of the yellow,
analagous to the change in Icterus spurius. This leaves
the blue still unaccounted for, and it must be confessed
that this is the most difficult color to explain in harmony
with the law of pigment assortment. It is probably a
modification of the black pigment, however. In the
ceerulean warbler (Dendroica cerulea), the young is olive
green, and the color of the adult is apparently produced
by the loss of the yellow pigment.

The different species of a genus are as a rule sharply
separate from one another, and do not blend through
local races. Most species are not very widely spread
over the country, and accordingly comparatively few are
divisible into geographical varieties.

320 CALIFORNIA ACADEMY OF SCIENCES.

Genus Myiotirtra. Biack AND WHITE WARBLER.

(7) Male more conspicuously colored than female;
young similar to adult female.

Prevailing colors, black, white, brownish (in young).

This bird presents one of the best examples of black
and white as cognate colors. The markings are modifi-
cations of the primitive streaked plumage, the streaks
having simply been defined and accentuated, but whether
chiefly by sexual selection or by other factors, I am un-
able to surmise.

Genus PRoronoTaRIA. PROTHONOTARY WARBLER.

(7) Adult male more conspicuously colored than
female; young similar to adult female.

Prevailing colors, white, bluish gray, olive green,
yellow.

The markings of this genus are probably due to sexual
selection, which has not yet reached the end of its spe-
cializing influence. Individual variation in the head of
the male throws considerable light upon the law of the
assortment of pigments and correlative colors. In the
young the top of the head is olive green, but with the
assumption of a full plumage the tendency is for it to
become more and more pure yellow in color. In full
plumed individuals the yellow becomes an intense cad-
mium, sometimes tinted with orange.*

GENUS HELINAIA. SWAINSON’S WARBLER.

Genus HELMITHERUS. WoORM-EATING WARBLER.

(1) Adult male like female; young similar to adult.

Prevailing colors, black, white, brown, buff, yellowish.

These two genera may represent forms which have
degenerated in respect to their color markings, but per-
haps are species which have not yet attained the degree

* Ridgway’s Manual, p. 484.

EVOLUTION OF THE COLORS OF BIRDS. 321

of specialization of the rest of the family. The fact
which looks toward degeneration is that neither the
adult nor the young show any tendency toward a streaked
plumage (unless the specialized head stripes of Helmi-
therus be taken into account) which we would expect to
find if the birds still retained a primitive coloration.
The general aspect of both these species is that of forms
which had been brightly colored and lostall but a vestige
of their former splendor. They might be looked upon
then as geratologous forms (see ante, p. 78).

GENUS HELMINTHOPHILA. GOLDEN-WINGED WARBLER,
ETC.

(7) Adult male usually more conspicuously colored
than female [(2) adults sometimes alike]; young usually
similar to female, but sometimes (8) with a peculiar first
plumage.

Prevailing colors, black, white, ash gray, brown,
chestnut, olive green, yellow.

Sexual selection, aided by isolation, has doubtless
been the chief factor in the origination of the markings
of this genus. The colors may all be explained as sug-
gested in speaking of the family in harmony with the
law of the assortment of pigments. Geographical isol-
ation has been an important, though probably not the
only form of segregation. The different stages through
which the genus has progressed are well shown by the
forms of ‘to-day. The young of the Tennessee warbler
(H. peregrina) which is plain olive green, paler on the
breast, represents the primitive plumage of the genus.
Through pigment assortment by sexual selection a yellow
crown patch was developed. ‘Then the species was isol-
ated in two districts, east and west of the Rocky Moun-
tains, by the ice age, apparently. The eastern species
developed white recognition markings upon the tail

21

322 CALIFORNIA ACADEMY OF SCIENCES.

which have persisted through all its modifications, this
character being lacking in western forms. Sexual selec-
tion, aided by further isolation of some sort, next pro-
duced one species with a yellow and one with a black
throat. The latter by further isolation became subdi-
vided into one species with the yellow crown persisting
and another in which it is replaced by black, except on
the forehead.

In the west the color of the crown patch became
changed from the yellow to a chestnut brown apparently
by a process of intensification. The orange-crowned
warbler (H. celata) represents the stage intermediate be-
tween the yellow and the rufous crown. The general
olive green color next became bleached out into an ash,
the head being first affected, and at the same time the
yellow of the under parts became bleached into white.
Sexual selection, in conjunction with isolation, finally
produced two species from this last one, one with the
rump yellow, the other with the rump chestnut, corre-
sponding to the changes of the crown patch. Two
eastern species, H. lawrencet and H. pinus, seem to be
dichromatic, having the normal olive green and yellow
colors, and in the same locality being found with this
color replaced by gray and white. The change here is
similar to that from H. ruficapilla to H. lucic, only the
two forms have not become established as distinct species
in the former instance. Mr. Ridgway, in a foot note in
the Manual of North American Birds* suggests another
explanation, as follows: ‘‘In a large series of specimens,
every possible intermediate condition of plumage be-
tween typical H. pinus and H. leucobronchialis is seen,
just as in the case of H. chrysopteraand H.lawrencei. If
we assume, therefore, that these four forms represent

*p. 486.

EVOLUTION OF THE COLORS OF BIRDS. 3823

merely two dichroic species, in one of which (H. pinus)
the Xanthochroic (yellow) phase, and in the other
(H. chrysoptera) the leucochroic (white) phase represents
the normal plumage—and admitting that these two
species, in their various conditions, hybridize (which
seems to be an incontrovertible fact)—we have an easy
and altogether plausible explanation of the origin of the
almost interminably variable series of specimens which
have found their way into the ‘ waste-basket’ labelled
‘H. leucobronchialis.’ ”

Whatever view may prove to be the correct one, the
important point to note in this connection is that olive
green and yellow seem to be very closely related to gray
and white, and that the latter has the appearance of
being a bleached out phase of the former, due perhaps
to the failure to deposit the pigment in sufficient quanti-
ties. The accompanying table shows the probable gene-
alogy of the genus. I am largely indebted to Coues’
Key for the specific characters, which are there stated in
a particularly terse and comprehensive table.

324 CALIFORNIA ACADEMY OF SCIENCES.

Below white, Below yellowish,
rump chestnut. rump yellow.
LUCI4 VIRGINIZ’

Lower parts white axe
NX y Roe
Diver. back ash gray. oe
RUFICAPILLA LEUCOBRONCHIALIS oe
Color ashy. ft er
. f ae)
BEE ! Lower parts yellow, R.
RUPICAPILLA . backrolive: green. CHE YSORTERA
i i PINUS

GUTTURALIS Lower parts, yellow,.

back olive green,
Color olive green,ashy-on head, LAWRENCET
crown chestnut.

Crown yellow, lores black.

Duller. F ‘uat
rown blac
abe agi BACHMANT Crown yellow.
Brighter.
CELATA
LUTESCENS
Throat yellow.
Crown orange brown, Throat black.

color olive,green,

\

No'white spots on tail, White spots on tail.

Crown yellow,
WESTERN EASTERN

Plain olive green,
-PEREGRINA-

GENUS CompsoTHLyPIsS. THE PARULA WARBLER, ETC.

(8) Adult male more conspicuously colored than
female; young with a peculiar first plumage.

Prevailing colors, white, bluish gray, olive green, yel-
low, orange brown.

EVOLUTION OF THE COLORS OF BIRDS. 325

Sexual selection appears to have been the chief factor
in the origination of the color markings of this genus.
The orange brown of the breast is an intensification of
yellow analogous to that of the crown of Helminthophila
celata.

The young plumage shows the coloration of the primi-
tive bird—dull olive and gray.

Genus DenprRoica. Tue Woop WARBLERS.

(7) Adult male more conspicuously colored than
female; young similar to female, or (8) with a peculiar
first plumage; or, more rarely, (5) male differs from
female in breeding plumage only; young with peculiar
first plumage.

Prevailing colors, black, white, olive green, yellow,
orange, bluish, chestnut.

There seems to be little doubt that the varied color
markings of this extensive genus have been produced
for the most part by the action of sexual selection. As
this is a peculiarly favorable genus for the study of the
effects of sexual selection ] have prepared tables of the
_colors of the entire genus, one for the female or imma-
ture plumage and the other of the adult males (Plates |
XVIII and XIX). In order to get the colors of any one
species read the colors in a line from right to left, while
to compare the colors of the same part of the body in
different species read the columns up and down. These
two tables illustrate sexual selection, pigment assort-
ment, environmental influences, and many less universal
points. It will be noticed that although the colors in
Plate XVIII are of dull monotonous tints, nearly all the
specialized colors are suggested. The general effect of this
plate is olive green on the left half and white or yellow
on the right. The only conspicuous exceptions to the
olive color are the three species in which the rump is

326 CALIFORNIA ACADEMY OF SCIENCES.

yellow, and those in which the upper tail coverts are
bluish gray. The white and yellow on the right hand
side is considerably interrupted by streaks, but in gen-
eral is fairly constant except in D. estiva and its allies.
From this it seems undoubted that the ancestor of the
genus was an olive green bird, probably streaked both
above and below. The olive green is a combination of
black and yellow, which were apparently the original
pigments of the genus. They were thrown off indis-
criminately producing the olive green effect, and the
bulk of the pigment following the shaft of the feather
would give a streaked appearance to the bird. Excess of
sunlight upon the back would cause the greater part of
the pigment to lodge there, and the result would bea
bird not unlike a composite of the females here repre-
sented.

Then sexual selection would step in as a factor in pro-
ducing change. Occasionally a single feather or a par-
ticular spot on the bird would receive a trifle more of
either the yellow or the black pigment than usual, from
a cause .which, for lack of a better name, we may call
fortuitous, and this bird would be conspicuous among
his fellows, and more easily gain a mate and leave off-
spring. In the course of time this character would be-
come exaggerated, from the continual selection, into a
specific character. Olive green, black and yellow would
then be the three colors of the genus, the back being
darker than the breast, and the black and yellow occur-
ring upon the most conspicuous parts of the body. It
will be noticed from the diagram that these colors occur
most frequently upon the top of the head, rump, upper
tail coverts, ear coverts, throat and breast, where they
would be most noticeable. An intensification of the
yellow would produce orange, as in D. blackburnicw, while
a darkening of this, perhaps by reintroducing black,

EVOLUTION OF THE COLORS OF BIRDS. 327

would result in the chestnut of D. bryanti, etc. This
leaves oniy the blue as a doubtful color, although it ap-
pears to be related to the black through gray.

The following facts, shown by these plates, seem to be
inexplicable in any way except by the law of the assort-
ment of pigments, or else this law seems to account for
them most satisfactorily: (1) the specialized colors of the
males are generally the colors which, when combined,
would produce the generalized colors of the females; (2)
the specialized colors of the males when combined gen-
erally produce the generalized colors of the males; (3)
exactly the same color appears in widely separated parts
of the body of one species; (4) exactly the same color
often appears on the same or on different parts of the
body of different species; (5) in general, the same colors
run through the entire genus, however differently they
may be combined or modified in quantity or distribution.

The following details concerning the proportion and
distribution of the different colors of the genus in the
adult male plumage may be of interest in this connec-
tion: Black is present in nineteen out of the twenty-
four species, being confined exclusively or mainly to the
upper parts of the body in four, and to the lower parts
in one species. The top of the head is solid black in
four species, the back mainly or entirely so in two, the
ear coverts in ten, throat in six, breast in three, and
sides in one. Yellow appears in eighteen species, being
confined to the upper parts of the body in two species
and to the lower parts in eight. The top of the head is
yellow in five species, the back never so. The rump»
is yellow in four, ear coverts in four, throat in nine, and
breast in eleven. Olive or olive green occurs in thir-
teen species, being confined to the upper parts of the
body in all of them. Blue is present in eight species,
in all but one of them (D. cerulea) being excluded from

328 CALIFORNIA ACADEMY OF SCIENCES.

the lower parts, while orange or chestnnt are also repre-
sented in eight species. The following correlations of
color may also be noted: When white appears on the
throat or ear coverts it is the color of the breast and
abdomen; when the rump is yellow the throat is yellow
or white; when the top of the head is chestnut or orange,
the throat is generally the same (one exception); when
the top of the head is yellow the throat is generally
yellow or white; when the back is streaked the sides are
generally streaked.

Genus Serurnus. THE Water THRUSHES, ETC.

(2) Adult male lke female; young with a slightly
less developed plumage.

Prevailing colors, black, white, brown, olive, orange,
rufous.

The resemblance of Passerella to some of the thrushes
has already been mentioned, but the present genus ex-
hibits even. more perfect similarity to the thrushes, both
in color and markings. The orange crown of S. auro-
capillus and the greenish olive color of the back in this
species are of course exceptions. It would seem that
the markings are to be explained largely by the general
laws of pigment deposition, the greater amount of pig-
ment going to the back has made it uniform in color,
while the breast has retained the primitive spotted
plumage, the markings having been defined and accent-
uated by sexual selection, perhaps. The superciliary
stripe is a conspicuous recognition mark, and may have
originally been discriminative in character, distinguish-
ing the ancestral form of S. noveboracensis from the
ancestral form of S. aurocapillus with a white orbital
ring. The colors of this latter species are similar to the
typical colors of the family—black, yellow (orange) and
olive green, but the other species are all brown.

EVOLUTION OF THE COLORS OF BIRDS. 329

Genus GroraiyPis. THE Kentucky WARBLER, ETC.

(7) Adult male similar to female (but more con-
spicuously colored); young similar.to female, or (8) with
a more simple plumage than the adult.

Prevailing colors, black, white, ashy, brown, olive
green, yellow.

The characteristic colors of the family, black, yellow
and olive green, are conspicuously present in this genus
—the result of pigment assortment. This genus is a
particularly good example of the combined action of iso-
lation, sexual selection, and climatic influences in the
production of different species. Geothlypis has more
geographical races than is usual in the family, and these
are clearly the result of environmental influences. This
is conspicuously the case with the different varieties of
G. trichas. Such species as G. melanops and G. beldingr
are merely climatic races in which the modification has
become extreme enough to establish them as distinct
from G. trichas. The Mexican yellow throats are dis-
tinguished from the Guatemalan species by the eyelid
being black in the latter and white in the former. This
cannot be solely the result of climatic influence, but is
due either to isolation alone or to isolation and selec-
tion. The specific distinctions between G. formosa, G.
agilis, G. philadelphia and G. macgillivrayt are due to
isolation and sexual selection. G. macgillivrayt appears
to be a retarded form of G. philadelphia.

Genus Icteria. THe Cuats.

Genus Sytvanta. THE Hoop~ep WaRBLER, ETC.

(8) Adult male more conspicuously colored than
female; young with a peculiar first plumage.

Prevailing colors, black, white, gray, olive green, yel-
low.

The colors of these two genera are typical of the

330 CALIFORNIA ACADEMY OF SCIENCES.

family, and are to be explained by pigment assortment
and sexual selection.

GENUS SETOPHAGA. THE REDSTARTS.

(7) Adult male more conspicuously colored than
female; young similar to female, but duller, or (2) adult
male like female, young with a peculiar first plumage.

Prevailing colors, black, white, yellow, red, chestnut,
brown.

The colors in this genus are the result of sexual selec-
tion. By pigment assortment the olive green has
become entirely replaced by its component black and
yellow. The yellow has become intensified into the cor-
relative orange or red, and by sexual selection has been
located under the wings, where it will be most conspic-
uous when the bird is in motion. The female has in-
herited the yellow color in place of the intensified red,
and the black is replaced by olive gray. As in the orioles
where chestnut occurs apparently intimately connected
with yellow and intensified red, so here also a species is
found (8. minzata) with a crown-patch of chestnut, ap-
parently intensified from yellow. In this species the
red of the under parts is on the breast, extending down
on the belly, instead of on the sides, and is much more
intense than in S. rutzcillu. The different species have
doubtless been originated by sexual selection and _ isola-
tion ina tropical climate. The fact that in some of the
most brilliant species the female is colored like the male
is an evidence of a high degree of specialization.

GENUS CARDELLINA. RED-FACED WARBLER.

Genus Ereaticus. Rep WaRBLER.

(2) Adult male like female; young with a peculiar
first plumage.

Prevailing colors, black, white, gray, brown, red,
vermilion.

EVOLUTION OF THE COLORS OF BIRDS. 331

Like Setophaga, these two genera of brilliant red
warblers are tropical in their distribution, and the colors
have been produced by sexual selection in a hot climate.

Genus BasILEuTERUS. BRASHER’S WARBLER, ETC.

(1) Adult male like female; young like the adult (?).

Prevailing colors, black, yellow, olive green, chestnut,
orange-rufous.

The colors in this genus are the conventional ones of
the family, and are to be accounted for as in the pre-
ceding instances. The different species appear to be
largely due to climatic influences.

FAMILY MOTACILLIDA. Tue Waerarts anp
Pipits.

(5) Male in breeding plumage differs from female;
young with peculiar first plumage (Motacilla and
Budytes), or (4) both sexes change with season; young
differ from adults at any season (Anthus).

Prevailing colors, black, white, gray, bluish, brown,
yellow, olive green.

In this family a certain similarity to the colors of the
genus Dendroica is to be noted. The bluish ground
color is present in some of the species of Motacilla,
while in M. melanope and in the genus Budytes the
ground color is olive green, thus presenting the two
colors most often occurring in the generalized markings
of Dendroica. In this family also black and yellow
form the principal specialized marks. Sexual selection
has evidently been the factor which has produced the
markings in this group, although far less completely
carried out than in Dendroica. The terrestrial habits
of the members of the family may have had a tendency
to suppress the bright colors, Anthus in particular being
protectively colored. The gregarious disposition of these

3382 CALIFORNIA ACADEMY OF SCIENCES.

birds has rendered necessary the white recognition
marks.of the wing and tail.

FAMILY CINCLID®. Tue Drepers.

Genus Cincitus. THe DIpPERs.

(1) Adult male like female; young like adult (young
and winter plumage slightly different).

Prevailing colors, grayish, brownish, white.

The colors are protective, harmonizing with the rocks
along the mountain streams frequented by these birds.
The white color of the edges of the feathers on the
lower parts of the body in young and winter specimens
points to an affinity to southern forms, in which the
under parts are white. Thus in C. leuconotus of Colom-
bia and Ecuador the entire under parts, head and
middle of back are white. The adult of C. ardesiacus
of Costa Rica is very similar to C. mexicanus, but the
breast of the young is almost pure white. Why there
should be an increase in pigment toward the north in-
stead of the reverse, as is the rule, I cannot suggest.

FAMILY TROGLODYTIDAH. Tue Wrens, TuHRasu-
ERS, ETC.

(1) Adult male like female; young like adult, or (2)
young with a peculiar first plumage.

Prevailing colors, black, white, slaty, gray, brown,
rufous, chestnut.

These birds are, as arule, not highly specialized, so
far as their colors are concerned, reminding one of the
general plan of color markings among many of the
sparrows. Asarule, the back is some shade of brown
or rufous, with the breast white, and either the breast
alone, or sometimes both back and breast, profusely
streaked or spotted. Sexual selection Has had, it would
seem, but a subordinate part in producing this plan of

EVOLUTION OF THE COLORS OF BIRDS. 333

marking.. Allusion has already been made to the use
of the chestnut color of the under tail coverts of “Galeo-
scoptes as a recognition marking (see ante, p. 203), while
the black cap of this genus may serve a similar purpose.
The white wing and tail characters are either directive
or discriminative marks, and are very generally present.
When the adult are specialized beyond the streaked
plumage, either by the assumption of a uniform dark
color or of a dark back and light under parts, the young
show, to a more or less marked degree, the streaked or
spotted plumage of the ancestral form. I am unable to
suggest any explanation of the barred markings so com-
mon on the posterior part of the body among the wrens.

FAMILY CERTHIIDA. Tue Creepers.

Genus CertHIA. THE CREEPERS.

(1) Adult male like female; young like adult.

Prevailing colors, brown, grayish, white.

The colors of this genus are protective in nature, the
back harmonizing perfectly with the trunks of the trees
to which the bird clings. The breast is white in accord-
ance with the general laws of growth.

FAMILY PARID2. Tue NurnatcHes anp Tits.

(1) Adult male like female; young like adult (except.
in Auriparus).

Prevailing colors, black, white, brown, bluish, gray,
yellow.

The birds of this family are highly specialized, al-
though never brilliantly colored. The primitive streaked
pattern is almost or completely wanting even in the
young plumage, showing that the specialization has been
carried very far. Auriparus is the only genus which
displays any bright colors, and in this instance the yel-
low head is obviously the result of sexual selection,

334 CALIFORNIA ACADEMY OF SCIENCES.

being much duller in the female and wanting in the
young. How this color can be accounted for in accord-
ance with the law of the assortment of pigments I do
not see at present, for it would seem that some species in
the family should be colored olive green. The different
species of the genus Sitta are very distinctively marked.
As the sexes are alike it is difficult to decide how im-
portant sexual selection may have been in originating
these markings and the same is true of Parus. The
latter genus has been especially susceptible to the influ-
ences of climate, a large number of the species breaking
up into geographical races. Psaltriparus is equally in-
fluenced by climate. Little inconspicuous marks are
generally the most difficult to explain, and this is no-
tably the case with Psaltriparus. Of what possible
utility can be the brown head as contrasted with the
gray back? It is present in both sexes so could hardly be
looked upon as the result of sexual selection, nor is it
conspicuous enough to be of use as a recognition mark.
If, however, some other bird inhabiting the same ter-
ritory looked very much like Psaltriparus but without
the brown cap, it might serve at close range as a dis-
criminative mark.

FAMILY SYLVIIDAE. THr Warsuers, KINGLETs,
AND GNATCATCHERS.

(7) Adult male generally more conspicuously colored
than female; young like adult female, or (8) with a
peculiar first plumage.

Prevailing colors, black, white, olive green, yellow,
orange, red, bluish gray.

The colors of this specialized family may be explained
by sexual selection and the assortment of pigments.
The similarity to the Mniotiltide and Motacillide in
point of coloration is quite striking, the olive green,

EVOLUTION OF THE COLORS OF BIRDS. 335

black, and yellow being present and also the bluish
gray. Phyllopseustes has not progressed beyond the
generalized stage of the warblers, no conspicuous sexual
ornaments haying been added, but the crown patch of
Regulus is a highly developed character. In R. calen-
dula the yellow has been completely intensified into
scarlet, but in R. satrapa the intensification is less com-
plete. This species exhibits the pure yellow and its in-
tensification into orange in different parts of the crown.
The bluish or black crown patch of Polioptila is prob-
ably the result of sexual selection, as it is confined to
the male. A remarkably fine illustration of discrimi-
native marks is shown in the outer tail feathers of P.
plumbea and P. californica, the outer web being white
in the former and black in the latter species.

FAMILY TURDIDA. Tue Turusues, SoviTairEs,
STONECHATS, BLUEBIRDS, ETC.

(8) Adult male usually more conspicuously colored
than female; young with a peculiar first plumage, or (2)
male like female; young like some ancestral stage of
adult.

Prevailing colors, black, white, brown, rufous, plum-
beous, bluish gray, blue.

The ancestral form from which this family arose was
a brown spotted or mottled bird, as shown by the young
of to-day. The different genera have diverged very
widely in point of color, however, but may all be ex-
plained in accordance with the law of the assortment
of pigments. If the primitive pigments were blue and
reddish brown their combination would produce Mya-
destes, the brown alone would serve for Turdus, while
the blue deepened would produce the color of the back
of Merula and Hesperocichla, in combination with
brown, the back of Cyanecula and Saxicola, and inten-

336 CALIFORNIA ACADEMY OF SCIENCES.

sified, the back of Sialia. Deepened still further it
would become the black of Merula and other species.
In the same manner variations of the brown would ac-
count for the color of the under parts of the body.

Sexual selection with isolation has doubtless been an
important, though by no means the only agent in the
production of generic and specific characters. Recog-
nition markings, both directive and discriminative have
been developed by natural selection, and direct environ-
mental influences have done their share in modifying
species, as with Merulu confinis.

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342 CALIFORNIA ACADEMY OF SCIENCES.

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BIBLIOGRAPHY. 343

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EXPLANATION OF PLATES.

PLATE I.

Figs. 1, 2,3. Three outer tail feathers of Dryobates pubescens, showing
individual variation in color marks.

Figs. 4,5. Feathers from the back of Dryobates villosus and D. scalaris,
respectively, illustrating the striped and barred pattern.

Fig. 6. Frontal feather of Geothlypis trichas occidentalis in winter plum-
age, illustrating acraptosis. The dark edge wears off in spring, leaving
the yellow base exposed. Four times natural size.

Fig. 7. Feather from back of head of Dendroica occidentalis in winter
plumage. The dark edge wears off in spring, exposing the yellow beneath.
Three times natural size.

Fig. 8. Feather from breast of Scolecophagus carolinus in winter plum-
age. The light rusty edge wears off in spring, leaving the black exposed.
Natural size.

Fig. 9. Feather from the back of Plectrophenaz nivalis in spring plum-
age, the white having worn down and left the black base remaining.

Fig. 10. Feather from the back of Plectrophenax hyperboreus in spring
plumage, the white tip remaining unworn.

Fig. 11. Feather from the throat of Jcterus cucullatus nelsoni, changing
from yellow to black by the accession of pigment without moult. Twice
natural size.

Fig. 12. Feather from the back of the head of Jcterus bullocki, showing
the change from yellow to black by the accession of black pigment. Twice
natural size.

PLATE II.

Feathers illustrating the stages of transition to a completely black mode
of pigmentation. Natural size.

Figs. 1-6, inclusive. Changes from a streaked plumage. From the
under parts of Melanerpes formicivorus bairdi.

Figs. 7-13, inclusive. Changes from a streaked to a barred, and from a
barred to a completely black feather. From the under parts of Sphyra-
picus varius.

Figs. 14-17, inclusive. Changes from a barred to a black stage. From
the edge of the throat-patch of the female Sphyrapicus thyroideus.

346 CALIFORNIA ACADEMY OF SCIENCES.

PLATE III.

Feathers of the wing of Fulco sparverius, showing transition of pattern
according to successional taxology. Natural size.

PLATE IV.

Heads of various North American birds, showing types of black mark-
ings, with their combinations and modifications.

Fig. 1. Polioptila plumbea; a, Leucosticte griseonucha; b, Melospiza
georyiana; ¢c, Petrochelidon lunifrons; d, Coccothraustes vespertinus.

Fig. 2. Zonotrichia coronata; a, Cyanocephalus cyanocephalus; b, Spi-
zelld socialis; c, Ceophleus pileatus 6 ; d, Cyanocitta cristata.

Fig. 3. Sazicola enanthe; a, Geothlypis trichas; b, Dendroica dominica;
c, Dendroica castanea; d, Dendroica pensylvanica; e, Hesperocichla nevia.

Fig. 4. Colaptes auratus; a, Passerina cyanea; b, Icteria virens;
u, Ceophleus pileatus 2; d, Sphyrapicus varius.

Fig. 5. Dendroica virens; a, Helminthophila bachmani; b, Xanthocephalus
xanthocephalus; c, Guiracacerulea; d, Cardinalis virginianus; e, Cardinalis
cardinalis igneus. ;

Fig. 6. Dendroica tigrina; a, Zonotrichia leucophrys; b, Zonotrichia
leucophrys intermedia; ¢, Campephilus principalisé ; d, Dendroica cerules-
cens.

Fig. 7. Dendroica blackburnie; a, Sitta canadensis; b, Dryobates pubes-
cens; c, Vireo atricapillus; d, Dendroica maculosa; e, Spinus psaltria
mexicand.

Fig. 8. Dendroica striata; a, Dryobates borealis; b, Dendroica chryso-
paria; ¢, Campephilus principalis? ; d, Calcarius lapponicus.

Fig. 9. Parus atricapillus; a, Spinus lawrencei; b, Motacilla alba;
c, Sylvania mitrata; d, Calcarius ornatus; e, Calcarius pictus.

Fig. 10. Ampelis garrulus; a, Parus gambeli; b, Melanerpes formici-
vorus bairdig ; ¢, Melanerpes formicivorus bairdié ; d, Xanthoura luxuosa;
e, Otocoris alpestris.

Fig. 11. Helminthophila chrysoptera; a, Mniotilta varia; b, Picus villosus;
ce, Colinus virginianus texanus; a, Callipepla californica; e, Cyrtonyx monte-
RUME.

PLATE V.

Examples of yellow and red as correlative colors. [The names /clerus
cucullatus and I. cucullatus nelsoni should be transposed.]

PLATE VI.

The upper figure shows the effect of environment in bleaching color in
the genus Myiarchus.

The lower figure shows a tail of an immature male IJcterus cucullatus
nelsoni changing from yellow to black by the accession of pigment without
moult. Natural size.

EXPLANATION OF PLATES. 347

PLATE VII.

Three races of Spinus psaltria, showing the change from a greenish
olive to a black plumage. Natural size.

PLATE VIII.

Three outer tail feathers of Pipilo, showing decrease in size of white
markings toward the west and northwest.

PLATE IX.

Geographical distribution of Sphyrapicus. A.—1, S. varius; 2, 8. varius
nuchalis. B.—S. ruber.

Geographical distribution of Pipilo. A.—1, Pipilo erythrophthalmus;
2, P. erythrophthalmus allent. B.—1, P. maculatus; 2, P. maculatus arc-
ticus; 3, P. maculatus megalonyx; 4, P. maculatus oregonus.

PLATE X.

Geographical distribution of the races of Megascops asio. 1, asio; 2, flor-
idanus; 3, maccallii; 4, trichopsis; 5, bendirei; 6, kennicottii; 7, maxwellie.

Geographical distribution of Cyanocitta. A.—1, C. cristata; 2, C. cristata
florincola. B.—1, C. stelleri; 2, C. stelleri frontalis; 3, C. stellert annectens;
4, C. stelleri macrolopha; 5, C. stelleri diademata; 6, C. stelleri coronata.

PLATE XI.

Geographical distribution of the races of Dryobates villosus. A. 1, leu-
comelas; 2, villosus; 3, audubonii; 4, maynardi. B.—1, harrisii; 2, jardinii.
Geographical distribution of the ladder-backed forms of Dryobates.
A.—l, D. scalaris; 2, D. scalaris parvus: 3, D.scalaris bairdi; 4, D. scalaris
lucasanus; 5, D. scalaris sinaloensis; 6, D. scalaris graysoni. B.—D. nut-
tallii. C.—D. borealis.
PLATE XII.

Geographical distribution of Bubo. A.—1, B. virginianus; 2, B. virgini-
anus saturatus; 3, B. virginianus arcticus; 4, B. virginianus subarcticus.
B.—B. mexicanus.

Geographical distribution of Chordeiles. A.—1, C. virginianus; 2, C. vir-
ginianus minor; 3, C. virginianus henryi. B.—C. texensis.

PLATE XIII.

Map of North America, showing the influence of climate upon the color
of Melospiza fasciata. The breeding range of each race is indicated, the
territory occupied by each variety being colored as nearly as possible in
harmony with the markings of the back.

348 CALIFORNIA ACADEMY OF SCIENCES.

PLATE XIV.

The shoulders of the different species of Agelaius, showing the transi-
tion from the unmodified brown of the female in winter to the adult male
plumage insummer. Natural size.

PLATE XV.

Heads of the different species of Sphyrapicus, showing the transition
from the brown plumage of the young, through successive stages of in-
creasing red, to the adult male of S. ruber.

PLATE XVI.

Heads of the female and young of the genus Icterus.

PLATE XVII.

Heads of the adult males of the genus Icterus.

PLATE XVIII.

Color chart of the undifferentiated plumage, female or young, of the
genus Dendroica. For description of a single species, read the lines from
right to left; for comparison of the same part of the body, read the
columns.

PLATE XIX.

Color chart of the specialized plumage, adult male in spring, of the
genus Dendroica.

( Nore.—In one or two instances the colors in the above charts were
supplied from descriptions; otherwise, from specimens. ]

INDEX.

ACANEIB. by an ceremuies dieisisai isan ve 153, 218, 299

Acceleration, Cope on, 73; instance of
Cope’s law of. 169, 219

Accipiter............658
VOOR eess sid osvcnemes ow ssi aeRO 217

Acquired characters, ou the inheritance

of, 2-50; Galton on inheritance of,
5; Ryder on inheritance of, 19; con-
sequences of dispensing with in-
heritance of, shown by Cunning-
ham,27; inheritance of by protozoa
admitted, 28; Morgan on inherit-
ance of, 29; Elliot on inheritance
of, 38; three evidences of inherit-
ance of, as given by Spencer, 43;
inheritance of, Ist and 3d evi-
dences in proof of, as given by
Spencer, criticized by Romanes
and’ Ball, 43-44; transmission of
diecussed, 29: transmission of, di-
vided into two classes, 34; Japanese
goldfish experimented with by Dr.
Wahle, 34.

Acquired habits; inheritance of, be-
lieved in by Darwin, 40; inherit-
ance of, criticized by Ball, 40; in-
heritance of, supported by Rom-
anes, 41-42.

Acraptosis, definition of, 135; instances
of acraptotic feathers...... 135, 136, 137

Acroptosis, definition of... -135, 137

Megialitis ......... ety va. 197
semipalmata...... . 197

#sthetic taste in birds, explanation of,
93-95; Poulton’s views on, 93-95;
opposed by Morgan, 95-96; Rom-
anes on, 97-98; Allen on, 100; Mor-
gan on, 101,

Agelaius......... 143, 146, 218, 220, 255, 292
gubernator....... 178, 220, 254, 255, 292
phoeniceus..
tricolor..

Alcedinide... ...-....... -» 278
Allen, Grant, on esthetics..... . 100
Allen, Harrison, on pigment., « 182

Allen, J, A.,on geographical distribu-
tion as factorin evolution of colors 247

Ammodramus............. 0.05 216, 233, 304
maritimus.,..
MAQVERCONS oir: sere esisiiiete ganslars eideiBin
sand wichensis. .
alaudinus...
bryanti....
savannarum passerinus,...... 184
AmpeliG esac wiasciariosrag dopaiedews ewes 316
AM Pella sis sisi iesinenmavasy 216, 217, 236, 316
COUTOTUM wesc cedes va doves 188, 190, 316
garrulus.. ».--190, 316
japonicus..
Amphispiza.

belli.....
_nevadensis .

lerd.arevecee 20:
Anticryptic colors, defined.
Antrostomus...........6.
carolinensis............-+..e00-
cubanensis
MACTOMYStAX... eee ee eee r cence eee
vociferus GUE Medians es
arizon® ...
Aphelocoma...
californica.
sieberii..

SuMIChTaAS ti. eevee we oe aaewe
Aposematic colors, defined
Aptatic colors, defined................ 194
Aptosochromatism, definition of., .135, 137
AULD sacaaissiarasd dae nieias <isieie’oieves a meitenvariies 270
Archibuteo............++ 236, 269

lagopus sanctijohannis.. 147
Aristotle, originator of aaelcioer “of

OVOIUTION asian prmensmesaees. Gare es 2
ABEUTIDNA... cece eee eee wieasne-sexeas. 270

350

ATthI Sis css cearcnnminria slay text anaes oe 236

Attidz, habits of,as proof of sexual
BOLCCHOM os ccjece-oe nie esele e Smears ve. 88

AUriparus. ccuicarevavecie'. decawrec eee 236

Ball, criticism of Darwin’s instances
of acquired habits, 40; criticism of
Spencer’s evidences of inheritance
of acquired characters, 43-44.

Basileuterus........... ceeeee cece eee 331
Bathmism, definition of............... 65
Beards: sas sas oascaciene'y Oe anedinnd bi ernes 157

Beckham, on pseudepisematic colors.. 199

Beddard, opposed to sexual selection,
88; on sexual coloration, 89-90; on
color in birds, 138; on effects of
food on color, 226; on influence of
Ught on pigmentation, 230; sum-
mary of Camerano’s system of geo-
graphical colors, 238; on protective
mimicry, 239,

Belt, suggestions on natural selection

as originator of species ..,........ 114
Bernicla magellanica, instance of high
coloration in female............... 90

yellow-headed .............e00e
Bluebirds......... +203, 213, 222, 229, 335

AL CUT C ina ti oiaeishi sera sere iets Gee) gittinndss oss 157
Bobolink. .149, 152, 185, 290
Bobwhite, seve e185, 257
BODAG Aig. pistes ay speeds sae Boneay 6 + 260
Boreal genera, Wallace’s list of 236

Brooks, modification of doctrine of
pangenesis, 5; secondary laws of
variation, 62; laws of variation crit-
icized by Gulick

Brown birds, colors of, protective

Brown creeper, protective coloration
Of 24 tue xemeeortannes 28 weceirannien: 190

Bubonid@............

Bubo mexicanus..

virginianus..
ALCHCUB:: dic ea mcrae aawiwd 94 sa
subarcticus......... cece ee eens 210
Barta. Weise een ceer ere 162, 179, 313
WNAIBO sees cis eoaseurenes tet es, 157, 222
MATIC, aesiicantaiais ¢ aleisnaspioya aecsed attcaieisno volgen 311
Butcher-birds .......... ces ee cece eee 216
Buteo ..... a ci patton dasha nas cars sexe RE Nese oto" b 269
abbreviatus ......eccseeeee eee ence LAT
albicaudatus ..... (eslead Howe RAB. 206
Dorealis,...¢ ocawes se getdee. 3 Z 150, 206, 269

COTULUB ge oieiii5 cwsdiers ba ote ican 19

CALIFORNIA ACADEMY OF SCIENCES.

Buteo fuliginosus ...........0....2 ee 147
Harla nic saicis sa veyinereaivietrae 9 2-6 . 147
Jatissima,..... cece cece eee ees 206
TNCatUS ci .c.i0s Laces, Geese Taees Se 269
BWAINSODI......... cece eee 147, 150, 206

CalamospiZa.....ccessecenceteeeeeee 236, 313
melanocorys............146, 152, 179, 218

Calearius., cocccccse: sceeaes eeueeees 218, 303
TAPPONICUS .. 2. ere eee cee ne eee eees 254
OTMACUS.......c cece ence cece en eeeeee 149
PICEUG. sis erie siein a rnin gb ydince ae, 94 254

Callipepp la: -ccacey shasta tiissisee Se Sieve 236, 258
californica ........ 22... scene vee. 258
SQUAMaAta....cessssces cossceeesess 258

Camerano, system of geographical col-
ors summarized by Beddard..,.238, 239

Oam pe PHiNUs: Kcnceennd acowe gs, wewswews 280
PYIDCIPALIS: acies.oiriscseieic neering ad Dates 146
Canaries, color of, changed by food.... 228
Caprimulgid@ ......... 006-20. eee 207, 283
CATACAT A: cois% weeateiacisrairsie’s greratsnein cee wai 272
Oardinalisies: 5 angccss a9 6 dasies «153, 221, 310
VITgiNIANUSs soci cess sate we 157, 243
Cardinells.... 0 j.08 seu telgiaes saea.e weee 330
Carpodacus,, wiaciaisicnsin masies ois nun tuioes 158, 297
CASSTDE. srcuesitdieieecerw cee o4 Secie’s Berea 254
erythrinus grebnitakii............. 297
MORICUNUS 24 wisiaes Mc cee Coae ase wae 244

frontalis..136, 156, 184, 195, 229, 244

purpureus californicus....
Catbird
Catharista trata.
Cathartide.......
Cathartes aura......
Catherpes............. .
Cellular continuity, upheld by Morgan,

19-20; upheld by Eimer, 20.
Centrocercus..........0. cee eeene 236, 262

Centronyx we. 236
Ceopbleus . . 280
pileatus S: See AfaArs S 147
COrthid ewes ssc ce seteaniones 236, 333
familiaris americana. 197
Certhiida . 333
Certhiola
bahamensis
Ceres sctece cies eiinrey seed sede sameeren 4
BICTOD was cowiare’s
AMMA ZONA. wcinceis stpay raat tre mee citge
CADRMIGL ociccs.e -o. dra tealer cei Gesamte
TDG nh niece volcnisgiee Bad eH tine FCN
SUPOCTCUIOBAL sists cacicisine series wemiors

torquatus...

Chachalacas ..... 265
CHB EEA 2 iccsspeasiece eAUINY Gheadoeamecd 215, 283
Chamwa..........68 sha ali ince: thivecese sare 236

INDEX. 351

Chateis severe qescarenss oe. 329 Color in birds, general principles of,
yellow-breasted .. . 215 137; Beddard on, 138; theory of,

Chelidon............. . 314 138-139,
erythrogaster..... seeeeae 216, 218 Colors in North American Genera, pro-

Chondestes .. ............ 180, 216, 236, 304 : portion and distribution of, 143-
grammacus...... seca atefalits Picts seams QUT 144; black contrasting with white

Ohordeiles...... 0.0.0... ce eee eee 207, 210 breasts, 148; black with yellow or
TOKONBIS ss, 62.55 Loreen day oeas 253, 255 browu nnder parts, 148; black with
WATGINIANUS 6 se sicsisci. seraisesewiaa aad 255 rose patch on breast, 148; upper

HERE H oss oscesaeciees scanners 185, 253 parts black, under parts light col-

Chuckwill’s widow. +» 186 ored, 148; lower parts and top of

Cinclide. = ve. 332 head, black, 149; on ways of assum-

Cinclus......... «+ 382 ing a black plumage, 150, 151.
ATdeSiacUs.... .6seecee cree eee +++. 332 | Color, Garman on variation in colors
leuconotus..........4 ath Rkitaleeincete 332 of animals, 231
mexicanus............. «...218, 219, 332 Color, variation of, with sex, age or

CICUBs canawanannnend ns eens oak ev 268 season, 211; Darwin on, 212, 213.
Hhudsonicus........ 2.0... cece eee eee 206 215; effects of food on, 226, 227, 228.

Clivicola..... ce esc cece eee cee eee ees 815 COMMDR oasis Sridceais sa eveeernes canes 265
riparia .... .». 215 | Columba fasciata. re .. 205

Coccothraustes......... eee eeee eee eeees 296 | Compsothlypis........-..:.ee cece neces 324
vespertinus.. 157, 221 Concentration, law of, 64; definition of,

OQOCCYBEB.. .wiscews dene cmeswrssae sees vee 276 11.

Coceyzus + =+276, 277 | Gontopus.......- weiskonemeneancsem aes 215
AMET CANB.. 6756 -ascadengens esas 277 | Gope on retardation, 25-26; definition
erythrophthalmus esaueuUayae 6p Bestar leseeee' a 277 of Bathmism, 65; on law of extent
maynardi and density, 66; on laws of acceler-
MULTI OM focrsus seiapasalasaversicsceetvaas: se aaestcand woos QU

Cosrebide ..... aialeiejeioretts aie qugioieiwe guaraucuas 318
Coues’ Key, table of characters of Hel-
minthophila........... seee. eeeeee S24

Cognate colors, definition of. sues ED)
Colaptesiscisccadcetied vnmatay.ueealned 280, 281
auratus. «+++. .282, 155, 156, 182
cafer.. -165, 156, 173, 176, 203
Colinuss os cic ccins vaeovee ates seswionteee 257
GTAYSOUI. 03. .cuiedasaaise sist ts 2% 258
THUG WAY] 1... cee ee cee cence 186
VirgimiaDUs ......... cece eee eeeeee 185
CUbaANENSIS.... ccc cece eee rene 146
floridanus............+ iexenas 258

Colors, classification of, by Poulton,
194; definition of aptatic, cryptic,
procryptic, anticryptic, pseudose-
matic, puesdaposematic, pseudep-
isematic, sematic, aposematic,
episematic, directive, discrimina-
tive, sexual, socialistic, epigamic,
194-210,

Colors, geographical distribution as
factor in evolution of, 234: Beddard
summarizes Camerano’s system of
geographical, 238-239; J. A. Allen
on, 247,

ation and retardation, 73; on four
laws of structure, 77-78; instance
of law of retardation, 169.
Correlation of growth, definitionof.... 19
Correlative colors, definition of 154;
instances of, 155-157; exceptions

to, 157,
CORVIAB sees ccessccnassaeee Raga six HERS 288
COrVUs., ... 0... ee eee ee wiih eae 146, 215, 236
cryptoleucus............eee eee 152, 290
Cowbirds . 290
Cracid®...... 66. cece eee e cere ee . 265
Creepers. . 833
brown. ao . 197
honey. ncas depearels BLE!
Crossbills., 166 298, 219
Crotophaga........ceeeeeeeee nee erence 276
BU vin eons var yh ee en tea wonre denlnermnayarais 146
Sulcirostris.........e cece eer erence
CROWS ies ctasas Saveaess eed
Cryptic colors defined
Cuckoo ........----+
American...
Cuculida, ........ :
Cuculus CAMOruUs.....-- se eeee eee etsieiseers 276

352

Cunningham, criticism of pammixis,
23; consequences of dispensing
with inheritance of acquired char-
acters, 27; on natural selection, 28;
criticism of Weismann’s observa-
tion on feet of Chinese ladies, 31;
environment discussed by, 37; ex-
periments on effeci of light on pig-
mentation, 230.

CUpId Oni ah ssi da seeresitoaeecas leant 236

Cusp growth, Osborn on, as evidence
of the inheritance of acquired
characters, 47,

Cyanecula....... é
Cyanocephalus................ cece eee
cyanocephalus ... ...........- 168, 289
Cyanocittacccss seven cece ee 158, 215, 245, 250
OPIS tat Aisa dence sosaeacws Hed 182, 191, 245
SteblePineds<s sauee ye udeen oes as 245, 246
CPOMIANG: pA Sean shigcicieieisveaine da Se 246
table of specific and varietal char-
ACTERB: Of nc uaescersie nase ae ae 247
CYANOSDI Ziad tiers 'e anvenceed Sa et ae meme 236
OY PselOides ice seins a scia ga va weddeseg 283, 284
CYPtOD YS s.cncleiccstzt esttiew saiew watennies 236, 259
MONLOAUMG saves ay eagse cae Gas ie 259
OCOMAEIG sacs tess cps sein sas AS Ra 259
BEN LOST aii raacs aoe naa otasny eideayleaerara tei eiecend 259

Darwin, acceptance of principles of
use and disuse and environmental
influences, 3; hypothesis of pan-
genesis, 3; on disuse, 25; on ac-
quired habits, 40; instances of in-
heritance of acquired habits criti-
cised by Ball, 40-41; acquired habits
supported by Romaues, 41; origin-
ator of theory of sexual selection,
80; description of preferential
mating, 81-82; theory of sexual
selection, 82: Wallace opposed to
his theory of sexual selection, 82-
83; on physiological selection, 112;
on color variations, with sex, age
or season, 212.

Death, Weismann’s explanation of, 8-9;
Morgan’s criticism of Weismann’s
explanation, 9.

Dendragapus ................4.

canadensis

franklinii .

ODS CUPUB yee St leased dain abaine
LULPINOBUG 04.4.4.sus8 0a Ws sees He 204
richardsonif, 025.0. 000 access ss 204

DONALOICA scissiersienie todesceadtiaiea eines 232, 325

GStlVls vas cy Hsees saeewawes 157, 221, 326

AUGU VOD siesa nts daa vs sauce 203, 218, 219

CALIFORNIA ACADEMY OF SCIENCES.

Dendroica blackburnia. ..148, 188, 221, 326

Dryanti.... ccc. ce cece eee eee eee 327
CMLULEDs wosescitseaeges 158, 221, 319, 327
CHrUulesCeDS......eeeee ee cere eeee 221
CASTADER. .... cee eee reece cennee 185, 222
coronata.158, 182, 184, 187, 203, 218, 219
dominica........... cece ee wooo 182, 215
MaculOSa.... weceesseeeeee 148, 218, 219
nigresceDS...... .....++6 216, 217, 218
occidentalis. .--136, 188, 221
olivacea. .. - 187
palmarum,.......5. cece e ee eeee 218, 219
penusylvanica...........eeee eee eee 222
striata... . 187, 188, 222
tigrina. ... . 188, 222
townsendi... 221
vigorsii.. = 221
virens.. 188, 221
Dickcissel.. 312
Dicrurue . saa 198
DIPPPs savas vaews 04 caewas doen te ees 332
Directive colors, definition of, 194, 201;
classification of by Todd, 201; new
classification of, 202.
Discriminative colors, defined..... 204, 205
DOMCHODY Rss villains rerieetee Sees cite eons, 290
oryzivorus...142, 152, 181, 182, 185, 218
Doves, Mourning.......... cece eee eens 205
white-fronted .........0. cs ee eee eee 205
White- wing edscicinin sewisins s comesves 205
DONO CUCKOO s:hisis. sssrases os Saran s oeaveiace 198
Dryobates.......... .. 179, 224, 244, 252, 280
ATIZO Nas caer seins te Sees et tele Ss 264
pubescens.......... 175, 181, 211, 254
BAITADETI ss os Lasers arceere 7 254
BCAIATIBcccnid arcniieas wes ended 254, 282
WALA Ts sais asisuogecs s aie ge siesa seat 254
BtriCklanal. os ocecins ovsvis vs ose cees 254
ViNLOSUS wire seneiaaes-vh Sams. ae 262, 282
BATES UL soca: a cescansets Setnciestiste Bathe ose 254
PALOIO: cide seis wom dodae odes: 253
leucomelas...........655 pavathe tee 254

moaynardi...

Dyer, objections to physiological selec-

tion, 123; objections to the term
segregation of the fit, 123.

Eagles........ 270
Ectopistes....... «+s. 236
MICTALOTIVG wosicvssoes sexu codes gees 205

Eimer, on continuity of body cells, 20;
instances of observed cases of
transmission of mutilations, 32; on
pigmentation, 162; on general pat-
terns of bird colors, 179; on en-
vironment in evolution of colors,
226.

Elanoides..........
forficatus.

leucurus....
Elliot, address cn inheritance of ac-
quired characters, 38-40.

Embernagra........cccee seve cccoeeeees 308

Embryogenesis, definition of by Weis-
DSM iojit ss Be Besant Ge disealt vaws 16

Empidias .......... a Apeaiainee gaa oe ++ 236

EMpPionaxscieasisaseasisaciccewsceata sa 215

Engyptila albifrons.........-......006+ 205

Environmental influences, first recog-
nized by Hilaire, 2. Morgan’s in-
stances of, 35; discussed by Cun-
ningham, 37; direct influences of
on colors of birds and insects, 238
et seq.

Environment in evolution of colors,
direct influence of, 225; Eimer on,
226.

Epigamic colors defined............ 194, 211

Episematic colors defined .......... 194, 200

ERG AI OUB igsccesitesacsiowrayaseie iets avers lane nrareereraiene 330
ruber ..... 153, 157

HUCthels s3s cr evaxssecswtsmmacs aes 312
bicolor....... 146, 149

Euphonia elegantissima................ 168

HUSPIZAs 5 ciadecnwes: <deas caaeeaes sales 236

Evolution, history of the doobutns of,

2; laws governing, 64-65; relation
of to species, 106.

EL CO's Gioia atoupriyasaes Raeeadieicle disintad aeieieisnweiets 271

ColuMMbAariUs..... 6. csc eee e eee ees 243

suckleyi 147, 271
islandus ... 152
peregrinus ... ~» 243
Bparverioides.. ......2..seeeee eee 271
sparverius....

FalecOnide,, cicccceceseuseecccveccsesce 267

Bal CODB scsseiaseelsw tin ciicein’s veces iow axeas . 271

Fecundity, segregate, definition of .... 131

Finch, pine..............65 ..- 145, 301
purple...... -» 297
scarlet rose. ~ 297

FLICKeCrs 2. ... cece eee e eee cn ceeeeeeeee 155
red-shafted... ..178, 176, 203

Flute bird....... oo cistahayletesnesicsans aiproseeers 162

Flycatchers, crested............- socens 256
fork-tailed ..... Sister ssiethe . 165
scissor-tailed........... eee eee eeee 155
shiny-crested.............. 152,196, 204
TY TODY sc.s7ia i ciaeierese rived aera Veuacene: alarsiace 285
Vermilion...i cies cedscasaes vee 157, 177

Fringillid®.............00 wees 296

Galeoscoptes carolinensis....... ..182, 187

23

853
Galline . arate dears . 257
Galton, on n the continuity of the. gated
plasm and the inheritance of ac-
quired characters, 6; on physiolog-
ical selection, 115.
Garman, on the influence of light upon
the colors of animals ......,...... 231
Geddes and Thomson on katabolism
and anabolism...... Hosier kati Saas ¥ 68
Genealogical table of Spinus and ‘its
allies, 300; of Helminthophila, 324.
GeOCOCOY Xs 5915 tee as aecaie’s vivginavencee 216, 276
Goothl y pisses: veda scses ve vesciesene 243, 329
BUDA Go. sicpsstecie tt Bivwinen eRe egeseave’s 221, 329
beldingi . 329
formosa .... reen 3829)
macgillivrayi. 221, 329
melanops.. wee. 329
philadelphia,...........0..000. 221, 329
trichas........ , 329
occidentalis. .......... 6. sce eee 135
Geratology, definition of, 78; example
of, 227.

Germ-plasm, continuity of, criticized
Vines, 13-14; Weismann’s reply to
Vines, 15; Ryder on, 18.

Gnatcatchers..
blue-gray . %

Goateuckers ........0c cece eee ee eee e wees

Gold-finches.................4.

Goose, Upland
Goshawks..
Mexican.
Grackles.
Grassquit
Greenjay..
Grosbeaks.. .... ele
Dlackheaded............0--.eeeeee
cardinal

» 222

259

oe. 204

. 204

. 204

Richardson's Biscwene orice taeeens cane 204
FUME warcucwwess saccuweas Stieoescee 260
BEG arescis'aiaigis sidiovsinyerSie sass aa siare ne ee 262
sharp-tailed................0e0ee . 262
sooty........ wees 204
Guiraca ce@ruled ......6.-- eee nee ee 158, 218

Gulick, criticism of Brooks’ laws of
variation, 72; on isolation, 125-128.

354

Gyminokitea .6 ees coewe- 23 sees vs xaos 236
Fa Dia 3 )es ani esate Seliecic Sone ee 221, 311
ludovicians.,...... 148, 154, 155, 176, 223
melanocephala .......... cscs eseeee 155

Haeckel, explanation of views of La-
marck and Hilaire, 2-3; plastidule
theory, 6; laws of heredity, 69, 79.

Halim@etis: ssesc sce. isc ckeaaseoas 236, 270
Harporhynchus ... 3 236
Hartog, difficulty in Welemann’: s the-
ory Of Variation.......+..c.ereees 56, 57
Hawk, broad-winged... .........,..05 206
Harris’ Geesaiiaes ee coiecmue gues ve se 269
MIATSD asics ceueaee ee ee ame, AGaT 268

Mexican black..
red-tailed. ..
Sparrow ....

.165, 206

Swainson’s........, .. 206
western red-tailed.,. : .. 179
white-tailed. ........ cee eee ee eee ee 206
Helinaia..... Sa ade Reheat Retna ate aimee 320
Helmintherus ..............: eee eeeeee 236
Helmintbophaga 236
Helmintbophila.. 321
celata......... 322
lutescens. 243
chrysoptera... 188, 221, 322
lawrencei....... 322
leucobronchialis.................0 322
WCB i sieiies asda ne bee tides Rens 217, 322
PCTOPTIN A rece soma sewer ns Hapanacas 321
PUDUBS 52a eaves ia ee sistiigrinns Eegigrciaties 221, 322
ruficapilla. +221, 322
swainsoni. - 218
Helmitherus,............ 00. eeeee . 320
Heredity, influence of, 26; laws of. 79
Hesperocichla. .... eee. ees cece cece eee 335
nevia.. ‘ee . -182, 186, 203
Hilaire, originator of the “doctriae of
environmental influences......... 2
Hirundinide ........--....... eee eee 314
Homology, explanation of by Cope.... 177
House-finch..... .-.86, 136, 166, 195, 229, 244
Humming-birds...........0. gistersieeie 221, 284
Hyatt on geratology.... 78
Hybrids, asymmetrical. 174
lateral. ......... 174
BYMMEtrCaliec si ve aereee sie sactiecaaees 174
Hybrid feathers, use of, 172; classifica-
tion of, 174.
Hylatomus....... see seesee ss -» 236
Hyperchromism, from anaimntiantal
influences............ atcha onenesie 241
LOLOP1 8: iiss Bs eemiensee Sana sanisvee 236, 329
virens. « 216

Leterid suis ccon. eesaenetes ceveeiacs eeeaeees 290

CALIFORNIA ACADEMY OF SCIENCES.

Teterus «2ccaeasnaee friction’ ecraeerie ay ac 157, 221
pullocki.............0006- veils 149, 294
cucullatus ............ 156, 292, 293, 295

UQMCUB. eeeececee cree es cere eee 156

nelsoni.. .133, 156, 291, 294
galbula....
parisorum.
spurius....

Ictinia.....

Idioplasma, distinctions concerning
nade by Weismann........ ....--- 16

Immortality of protozoa, explanation
of by Weismann, 8-9; admitted by
Vines, 11.

Isolation, Gulickon..... Bulede twee 125-128

Jackrabbit, black

Jay, blue
california. .

Johnson, on plumage changes in Ic-
POPUB ies. Snnpewes eines We Vee aeeEaieY 133

Junco........ 180, 204, 216, 217, 191, 236, 306
ANNECCEDS.... 6.6 cee cee eee eee eee 254
CAMICE PS! a-a.5 srerare-s oie siaieceisie sie Sia sigyaieiaye 254
cinereus dorsalis............ 0.4. 254
hyemalis.............. 147,148, 177, 191

Katabolism, definition of, 68; as factor
in pigmentation, 163.

Kingbird............. 143, 165, 199, 217, 255
LEY sea teataisicteead tea a a lneteilied tebe 2d Sea ee 155
thic k= Billed sass csi os wariweies paca oe 155

King-crow......... eidiawad 6a Paine seatesretere 198

Kingfishers) <5 siaian ciexioncas ood 212, 278, 279
Welt CO sje cus saisisieisierecete ss 175, 192, 198, 210

BAN Bl C8 ss isec9 ssiesiwes Saarastie va Sacsiseneie’d 9 334, 199
golden-crowned, ..............00 0

ruby-crowned...
Kitegs siecsiewsrys 3
Ladder-backs..
Lagopus
lagopus alleni.
Leueurds i625 weaiestcns a aieigry 0
rupestris....
ALKONSI Gist cine sears vice sve
nelsoni
reinhardti
welchi,
Lamarck, originator of the principle of
use and disuse....... elevaisaseninterd sifu 2
Lanide........0...068 Rigo air gels 34 24 317
OTIS: ys Hous aera oases ...187, 216, 218, 236
Lark-An hy: sa:iie ss diinae opena deeewe 216, 217
MABE she 22s savsiese’ cigsa avieadsiav d-c:ainueens “a Sunversiois 287
HONG}. ese vevaeeere as. ..196, 250, 287
Leucosticte............. -153, 218, 236, 298
ATPATA ess ae weissegnnede suns neeereee Ses 147
Lig Otisevss sccaaaa veswwnwanwies gornces<e 236

INDEX. 355

Longspurs ..........2ce ese c eens -esees. 303 |* Metabolism, explanation of, 67-68; as
chestnut-colored ............00e008 149 explanation of pigmentation, 163,

Lophophanes....................e0 0008 236 Micrathene.. ........ Vis graye shes cobjeis 236

TOONS. oxarenavvariiwkeku vaunnwas 236 Micropodid® ..........02 ccc eee eee sees 283

LOX ae s59 wisres -218, 219, 236, 298 MICrO DUB iasmincornish vient cai aimeeries eee 284
leucoptera.. Geiawe readies 298 Milvulus................ Senge aye m ates 285

Lunda cirrhata,. . 200 forficatus,.

Macrochires........ . 283 tyrannus....

Macronyx amelie .. . 240 Mimicry, Stejneger on, 198; Beddard
croceus......... sf anenaeteovise + 240 on protective, 239,

Magpies............... ‘lt ..152, 215, 288 | Mimus polyglottos...................- 183

Melanerpes torquatus................. 216 | Mniotilta.

Markings, classification of patterns of, 151, 152, 221, 188
159; Eimer on patterns of bird col- Moniotiltidm. .........0.. 0. cece ee ee 157, 319
ors, 179, Mockingbird .. .......... 20 ne bra @utvisieyts, 180

(MARTI gc.siorncinisrs cs dcaieperesGitearsioc> Gade 314 MOIGEHT US: 5.,.yasiasrsieienis-eiitasereins een teene, 290
PRO sy rane aviosccuayecnaaawod 149 146, 291

Meadowlark. ,156, 175, 1/8, 203, 211, 240, 293
Megoscops asio, 250; table of races of,
showing relation of color, 250.

asio maccallii 254
flammeolus..... 254
PCH OP SIG vieies:psisniw win seieeinriiaciesinarn 254
Melanerpesssssiisie scarcer sanisiguianecapaveiw . 280
GUPITODS 5 ci veer cea simesaccvaies + 223
CATOlINUS see scis exededeeies eae pee ete 222

erythrocephalus..145, 153, 216, 217, 218
formicivorus bairdi..143, 170, 173,
175, 181,
Melanistic plumage, methods of, as-
suming, 151; theory of, suggested
by Stejneger, 226; Spinus psaltria
agan instance of the assumption
of, 241; instances of noted by
Ridgway, 243.
Meldola, criticism of Romanes theory
of physiological selection, 115; Ro-
manes’ reply to, 115-116,

191

Meleagris - 236, 263, 264
Melopelia leucoptera......... evant eeeaticets 206
Melospiza ....... 196, 215, 225, 236, 250, 308
GUDOLIA ieiciaivnesa steccrd as a sisiain ea sierarascinveseters 249
FASCIA «cists dieiainfeees eaicisiese aw sremyene 248
LBMAX camo wiscinnaenunanaerreas 248
heermanni ............. 00.000 248
MONTANE, ... 2... cece ccs cece ence 248
rufina...... 248, 249
SAMUCLIS .... ce ceeeeceees evens 248
Merriam, on plumage changes, 134; on
geographical distribution and
North American faunal areas, 234,
235.
MOP lisse Sisvpn gags darecscontemcammaauainn 335, 336
CONANIS sensewwsersese jess eet wees % 336
migratoria propinqua.............+ 217

oi , 222

Morgan, criticism of hypothesis of
pangenesis, 5-6; criticism of Weis-
mann’s explanation of death, 9;
Weismann’s theory of heredity op-
posed by, 17; on cellular continu-
ity, 19-20; criticism of panmixis,
22; on inheritance of acquired
characters, 29-30; transmission of
mutilations discussed by, 30; in-
stances of environmental influ-
ences, 35; suggests modification of
Spencer’s theory of physiological
units, 58; explanation of preferen-
tial mating, 81; opposed to Poul-
ton’s views of the esthetic tastes
of birds, 95-96.

Motacilla melanope.............06 icvten ts

Motacillide

331

Mutilations, transmission of discussed
by Morgan, 29-30; transmission of
discussed by Weismann, 30-31 ;
Eimer on observed instances of, 32.

Myadeste sis. ccssscisccictes eeremicwicieioas cman 335
FOWNSONAE o5 ican; svagraegeices 204, 217
MYyiarcbus scecmsrimceisecintie wexetese 256
CIMNETASCEDS ..... eee eee eee eee 256
CTINIMUIStdcacneraxnteaiaguanasiaid: 256
lawrenceii. » 243
mexicanus .. wee 256
nigricapillus. « 243
Myiodioctes.... 2. ..ccceeece cere ewan 236
Myiodynastes .ci09 teenage vecoeesiawe cs 285

Naegeli, on the existence of a nucleo-
plasm first developing into body
cells, then becoming simplified
into reproductive cells, 10.

356

Natural selection, discussion of by
Cunningham, 27-28; argued against
as not creative, by Schurmann, 651;
conditioned by variation, 62-53;
further discussion of, 106-107, 110;
Romanes on, as originator of spe-
cies, 113-114; suggestions on as
originator of species, by Belts,
114; as originator of species up-
held by Meldola, 115.

Nelson, E. W., on Plectrophenax ni-

valis... 301
NGOCOLYS: esi. gacriginwic dvidariran oa aie ecient 236
Nighthawk, western...185,196, 207, 219, 254

Nuthatches

Nyctala...........
Nyctea nyctea. 152, 198
Nyctidromus.. 207
Oporornis... 236
PFOTMOSD ix acerca scwteaene aes eeaains tae 187
Oreortyx..... 236, 258
OLIGO Es cors yd! coinngih adap ay oo engeee 293
Arizona hooded. .133, 156
fiery.... 156
hooded.. 156
Oroscoptes........ ... 236
Ontalisisccies chicane 5 ioiwmanucied Acces aw 265
ORty Risin seers Hocalven Taaiacse rine G55 236
Osborn, on laws of cusp growth, 47;
criticised by Poulton, 17; reply to
Poulton’s criticisms, 48.
OB PREY tc Assi oe ineseneate BN ae aes RES 272
Otocoris....... 218, 250
OW WTI ccs cysts sane e ceoesds deiales Kia nee 273
DBLLO WING s se-cee ve ccielscwrens Horas 197, 210
RPGat HOmMedecspspiuerages ayes oes 254
horned iis Aeslewea WeEweR aA ENS OAS 273
BCTOCCH eseseidranes dextinws ve eke nes 250
BLOW Vesjeieaianus sabaude: opninteingays death Seiko 198
western borned.................64% 210
Pammixis, Weismann’s explanation of,
22; criticism of by Morgan, 22;
criticism of by Cunningham, 23;
Romanes’ explanation of, 23; Ro-
manes’ theories on, 24-25; Cope
views in regard to, 26,
PANGIOM sccirs seicnun se Sakae AE cE. mae s 272
Pangenesis, Darwin’s doctrine of, 3;
explanation of by Romanes, 3-5;
modification of by Brooks, Galton.
Herdman, etc., 5; criticism of by
Morgan, 5-6.
Parabuteo......... 2. 269
unicinctus harrisi...... 147
Parallelism, Cope on laws of.. 718
PAV BU GUNG sccrtsuciscoare aititisid oaks Se ener 207

CALIFORNIA ACADEMY OF SCIENCES.

Parrots...
PATtTIdPeB isi cewersees or cenraawa voee
Partridge, massena.............5 ceeee

mountain,........ eee eee eee nee

atricaplllis) ss sncmis acne smears
hudsonicus .
Passerculus...
Passer domesticus
Passerella
PRCA ce es. case cos ae
megarhyncha ..
unalaschcensis. .
Passerina.......--.+
cyanea.....
VOrsicoloP cscs ticsda se 28 HG THERE
Peckham, Prof. and Mrs., description
of sexual selection among the At-

Pediocetes ..
Perisoreus ..
Petrochelidon.
lunifrons..
Peucea ..
cassini..
mexicana..
Phenopepla..
Phainopepla.
nitens
Phalenoptilus ..

12. 236,
215, 236,

.. 180, 196, 216, :

237

PHAIATO POS ais’ <pencicatiaved s arcove Galera Tedeeuneteed-s y

Phasia nde sess tases ee tedee tee

Physiological selection, 110; Romanes
on, 110-112; Darwin on, 112; sug-
gestions on by Galton, 115; objec-
tions by Wallace, 116-117; objec-
tions by Wallace answered by Ro-
manes, 118-119; further discussion
of, 120; Seebohm’s criticism of,
120-123; summary of, 124-126.

Picus pubescens gairdneri.. K
villosus harrisii...................

INDEX.

Pigmentation, definition of, 139; Eimer
on, 162; Beddard on influence of
light on,.280; influence of light
on, experiments by Cunningham,
280; experiments by Poulton on
effect of light on, 280, 231.

Pigment, definition of, 139; law of as-
sortment of, 139-142, 145; law of
growth force in regard to distribu-
tion of, 159-160; Eimer on distri-
bution of, 162; Dr. Harrison Allen
on, 182; metabolism as factor in
distribution of, 163.

PLQCON Sits ss ses deetnhavneadae admire ~204, 265
band-tailed... -.. 205
PABBON GOP i a bicies oc oa sense ees biases 205

Pinicola............. Crees .. 153, 236, 296
ONUCL AOR: cece raves vie wn Zed 154, 221

PAPO anc xcs Gases 180, 216, 236, 309
chlorurus........ .... alan Nevaeh fe 309

erythrophthalmus.....147, 148, 216
222, 250, 309

MNVEN banca asnionisaidweeie ee ves 251
aberti.. ~ 254
fuscus...... -+. 809
crissalis.. wis ~-.182, 203
maculatus ........147, 148, 216, 260, 309
arctiCUS.......6sc005 aisle ees 261, 252
megalomyk ........ 00. eee eee 251
MeSOlCUCUB......... cece cece eee 254
OTEZONUS.........- eee ene ee es 251
PADI tiene vccans si 240, 331
African.. . 240
Piranga @stiva........e.cc cence eens 221, 243
erythromelas. 148, 160, 153, 180, 222
hepatica
ludoviciana........... 180, 222, 150, 157
TUDO jo. eens on sa ensaee ora 163, 222
Pitangus..:..........00 ipemioiaiecs wuarietec cae 285
GOL DIADUS is. « ssicis servis seaenmares 188
Platypsaris, exceptions to correlative
COlOTEION weas vesan mavencins exvesee 157
Plectrophenax.... 218, 301
hyperboreus.. ..- 162, 302
nivalis ..... 136, 152, 301
Plectrophanes...........ccc cece eee eens 236
Plover, GOldOD. is - case cs os cusiece sevens 133

Plumage changes, explained by Yar-
rell, 132, 136; Merriam on, without
moult, 134;,in breeding season,
136; classification of, 137.

Pol Optila sss sna ie ovinssy snnae vaecawe ds 209
cerulea . 158
californica . at w.. 353
plumbea 335

Polyborus............. eigisaiaveseie-a ais Ky Vases.
cheriway........6+ oi
lutosus..

Poocetes..

POOLS a: cas wican sedans ace aia amas 236

Poulton, criticism of Osborn’s evi-
dence of the inheritance of ac-
quired characters, 47; Osborn’s re-
ply to Poulton, 48; criticism of
Wallace on sexual selection, 84; on
esthetic taste in birds, 93-95; crit-
icized by Morgan, 95-96; on gen-
eral principles of color, 137-138:
classification of color marks, 194;
on effect of light on pigmentation,
230-231.

Praeger, instance of procryptic colors. 197

Prairie hens...... 6. cece ewes eee cee 261

Preferential mating, Darwinon...... 81, 82

Procryptic colors defined, 194, 196;
Praeger’s example of, 197.

Procyon lotor........ aisandvavs ae aya’, “sr ahs 210
ProghO sist ws nsanawserecs true 314
BUDE cc niayipeinie's sb sisiegetactaaicese 221
Protonotaria 320
citrea... - 221
Psaltriparus . , 334
MOlANGtIS: 6:5 aisiesiermore sinsees o seeews 187
Pseudaposematic colors..,........- 194, 199

Pseudepisematic colors, 194-199; Beck-
ham on, 199.

Pseudogryphus californianus.......... 146

Pseudohybrids, definition of,.. 2 ATT

Pseudosematic colors. ..... a , 198

saga 188

Pseudozoorubin ..
Psitacide...... ne esas ae 274:
Ptarmigan... re ...134, 195, 260
Putin, tufted ssc osc sysicse weeded d dew 200
Pyrocephalus rubineus ............153, 222
mexicanus.......... 141, 157, 177, 286
Pyrrhuloxiasiecss's eccsaeae emaganainyes 236, 310
BINUAIG: sii se eededeaees 164, 181 221
QUAI], VAC Ys 6:6 scarsinsissre tees Gs Se deiner 258
Quiscalus, .146, 221, 229, 295
RAPlOres sdaiaecanindies saxdectceee ae ote 266
Raven, white-necked............--...- 152

Recognition marks, 193; classification
of, 194; Wallace on, 200-201.

RGAPOMB:. jcc nse eee ste eae Se BS SS 299
TOA BUALE si siccesreystsye cr SBAn cick O vaetakager se 330
AMETICAD oi 610 6. ciecinsce tee, nate oe 147
Regulus....... «- 236
calendula... - 153 335
satrapa..... .. 182, 187, 335
OlIVACES sccasc seees cesses sheers 143

358

Repetitive marks..........2-.- 0. ee eee
Reproductive cells, immortality of
among matazoa, 10; simplicity of
confirmed by Spencer, 17.
Retardation, explanation of by Cope,
25-26; definition of, 73; instance
of Cope’s law of, 169,
Rhynchophanes ......... cc. ce eee eee
Ridgway on relation between color
and geographical distribution in

North American birds, 241; in-
stances of melanism 243,
ROMATUD NSLS nc. 55 neste ds Suse HEIR 28 38 276
Robin varied ..........-.-. 0.00.00. 203, 217
Rock-thrush .......... 0... eceeecee eens 81

Romanes, explanation of Darwin’s ho-
pothesis of pangenesis, 3-5; on
cessation of selection, 23: theories
on pammixis, 24-25; on inheritence
of acquired characters, 28; exam-
ples of instinct due to transmis-
sion of acquired characters, 41;
supports Darwin’s theory of ac-
quired habits, 41; criticises Spen-
cer’s Ist and 3d form in evidence
of proof of acquired characters,
43-44, supports Darwin’s theory of
sexual selection, 88; criticises Wal-
lace’s ideas on sexual selection,
92-93; on esthetic taste of animals,
97-98; on species, 106-107; on phys-
natural iological selection, 110-112;
opposes selection as originator of
species, 113-115; reply to Meldola,
115-116; reply to Galton, 115; phys-
iological selection opposed by
Wallace, 116-117; reply to Wallace's
criticism on physiological selec-

tion, 118-119; controversy with
Dyer, 123-124,
Rostrbamus ...... Sissel 34 DPE ets 267
Ryder, theory of heredity opposed to
Weismann, 17; on inheritance of ac-
quired characters, 19; experiments
on Japanese goldfish recorded by,
34,
SAN PTHACLOSs oieine cpciceseaud wes eseintaiRs aastoene 236
SADA PLPC TG isc sisesjeres a neawdees ah caer 202
Sapsucker .............. a Shaitsitore a 160, 165
yellow-breasted ................505 154
Sauermann, experiments on the influ-
ence of food on color ...........+ 227
Samleo lasicniiarien senecs cares 335
enanthe. -. 187
Sayornis........ z . 215
AQCUAICUE ccxceses coed 2 ose oe te 243
nigricans .......-.... 147, 148, 162, 243

CALIFORNIA ACADEMY OF SCIENCES.

Schurmann, natural selection not cre-
ative, 51; discussion of origin of
variations, 63.

Scolecophagus..........-- 146, 218, 236,
carolinus, acraptotic feathers of...

Scotiaptex cinereum lappopicum......

ScopB ASi0..... 1... eee ee cee e eee eens =

Seebohm, criticism of physiological
selection, 120-123; glacial epoch as
a factor in isolation, 249.

Seedeatears

Segregation, definition of environal,
industrial, sustentational, defen-
sive, nidificational, chronal, cycli-
cal, seasonal, spatial, geographi-
cal, local, migrational, transporta-
tional, geological, fertilizational,
artifical, reflexive, conjugational,
social, sexual, germinal, floral, im-
pregnational, 128-130; institution-
al, intensive, 131.

Segregation of the fit, 116,120; objec-
tions to, by Dyer, 123.

Segregation, potentixl and prepoten-

tional 2.6.6... ce cee ee eee ee eee
SOUPS ees 0 es enwraiwiniarers ae 216 217,

aurocapillus..........se0-.- 187, «

Doveboracensis..... 0... sees eeneere
Sematic colors, definition of....... 194,
Setophaga ......6.-.eeeeee a inilcnie sine

picta, distribution of black on. 147,
ruticilla, distribution of black on,

147-148; instance of correlative
colors, 156, 221.
Sexual Coloration, Beddard on...... 89,

Sexual intensification, explanation of

Sexual recognition, law of..... .......

Sexual selection, Darwin on, 80; criti-
cized by Wallace, 82-83; discussed
by Wallace, 85-86; Stolzmann's
view of, 91; Peckham on, 88; Bed-
dard opposed to, 88; Romanes sup-
ports theory of, 88; Wallace’s objec-
tions criticised by Romanes, 92-93;
Weismann on, 97; summary of 101,
102.

SHEPR 6S wssa'ne chen hy vishenekrs Ueanemares 218,

Sialia....... 222, 229, 236,
ATCHICA:. oo: Gidisis tae Ys te semi NewON
sinlis.. 213,

Bithaisscaaaceas s - 236,
canadensis ... ae
carolinensis

Siskin, pine...............-5
Size, segregate, definition of...........

312

148

90

69
209

317
336
158
222
334
188

- 187

INDEX. 359
Snow-bunting......... wees eee e eee ee 136 SYIZAs sircany evennininrein:
Snowflake «.. 301 americana.
MoR ay’ sis scistainiaee sree asians wee 302 Spizella.......
Socialistic colors, definition of ....... 209 artigularis....,....ce. essen ee esses 306
BOltair es oe. sigur vesnaink Mase EERE cies Vs 335 BLOW OPL..dnicis se visistigeie sattiesiaweis 254, 306
Townsend's......... cece eee eee 204, 217 Pala sasde sscins sanvone nd sas +». 254, 306
Somatoplasm, explanation of,....11, 12, 13 | Sporophila................ atecsie pla mreetiaty ~. 812
Sparrow, Bells iiicus ssscieees.sasaiwen see morelletti..... wae RUle MITES ieee glee 3 146
black-throated . Squirrel, black headed ground......... 68
chipping..... 217. 306 Stejneger on mimicry, 198; on albinism
dusky seaside. . 161 and melanism, 226.
POM a iasece nee «+. 308 Stelgidopteryx.
Gambel’s. oe. 184 serripennis...
golden-crowned .176, 209 Stolzmann, views on sexual selection.. 91
g@rasshopper............ee0ee wvevce 184 Stone on plumage changes without
Harris’s...... aise: thee tttaleteiBjcrsiees Vi. vase 183 moult...... fissis Sat iat eis sieiotajars wate cere 134
HOuse........68 veeee niniraavale ee sina 209 Stomechats..........cccceeeee erence caer 335
Late sinentisure ccomandaiose tasiiaw’ aeeee® 304 Strigidm............ bi tatare areiaimvateniete;s sees 273
rufous-crowned.... 307 BELEK civnarers -ateae Hoes swteraproamintan ahperarecste 273
BAG Git vireres eayndetes ee aeins sa eein Tae 807 Structure, law of, 64; definition of, 77;
savanna. - 233, 304 by Cope, 77; segregate, definition
seaside.. - 304 of, 130.
song.... “24s, ‘049, 250, 308 Sturnella........... aca cwiale-elaia’sreieuesedeva atesee 293
TOXA8 guccsssasaenciu aie edaerealeta da 308 defilippii.. 156, 240
EROOM:ceiays svisiensidla ciatamstiecsgier ais seeeee 305 magna. 156, 157, 116, 178, 182, 203, 211, 239
vesper,...... Oates PY SGlMEaS dela eee 303 Successional relation, definition of by
white-crowned..... saree aa she 209, 305 COPS ein cise 4 Satine Ta eaes es Coe TT
white-throated... . 183 Surniculus dicruroides.............. 198
western savanna 184 Swallow

Specialized colors, classification of,
148-145 ; with or without loss of pro-
tection, 145,

Species, nature of, 103-106; Romanes
on, 106-107.

Spencer, theory of physiological units,
6; on simplicity of reproductive
cells, 17; three forms of evidence
in proof of inheritance of acquired
characters, 42; first and third form
in proof of inheritance of acquired
characters criticised by Ball and
Romanes, 43-44; discussion of use
and disuse, 44-45,

Speotyto cunicularia hypogea.197, 210, 274

Sphyrapicus............+ -236, 244, 248, 280
TUDOE score cco sins ae ceisinde be cipieleias 243, 244
thyroideus........ 146, 150, 165, 223, 281
VariUs.oee. csaeaes 164, 192, 222, 244, 252

NUCHALICn os oscars eos ees 244, 245

Spinus ...........6-- 157, 218, 300
lawrencei.......... cece e ee eee 177, 222
PINUS... ...cceeemeeeeee 146, 180, 216, 299
psaltria..... . 150, 177, 195, 241, 246

ATIZODR.... 622s ee eee ees 150, 151, 242
MEXICANA .... eee eee cee ee 146, 242

tristis, specialization of without
loss of protection.............+5 146

bank 315
barn. 314
CHifhias sacrcis sie ces 314

rough-winged,... . 815

- violet-green , 218
white-bellied ............. 0.00 eee 315
white-breasted....... octane ene 210, 218

Swifts......... . veseancass 289

By Lv mia cicaiscccg cesses ccaigie sretegie Sieraiargierera wis 329
canadensis 221
Mitrata... cc. secs cescereecereesesee 222
pusilla.... 157, 221

pileolata.. .....cccsseee reece eee 243

Sylviide.

Tachycineta ...........cesseacerees 216, 315
DICOLOE seccesese os ne lees soe ve 210, 218
tha lassina,..........+- 210, 216. 218, 315

Tanager ...........ceee eee 150, 313
Louisiana, . 50, 180
scarlet ..
summer
western...

Tanagride ........ee eee aeee cor eeeene

Taxology, successional.......... seeeee 164

Teleology, explanation of by Cope.... 178

Terpsiphone cristata, as instance of
plumage change from dark to light
without moult........... iad Satptolees 134

360

Thomson, explanation of anabolism
and katabolism, 68; with Geddes
places sexual intensification upon
firm basis, 73,

Thrasmetus....8..... cece eseen ee eeee ee 270
THUAN OTS, cis aceser vee crnqares tpates 196, 332
Thrushes;; Waters coescues sesccaee ke 328
Thrush, W00d @....06. 00 sexes 13 ease 185
WALLED 2 ciate cists yentioo: srjanete Sataoaaed 186
Thryothorus bewickii..............--- 254
ludovicianus’s:iiciescie ican ines as 254
TIP ICG acpi: cantina Penmaes aaleai eee 196
TUS « vecageuiesiding.og gu er as genni Roe 333
Todd, J. E., on directive colors ....... 201
TOWHE 6a. 5 a5 Purisa Heke SARE eS 212, 230, 309
California... ..cceceececescessecs 182, 203
Transmission, abridged or simplified,
definition of....-.........0 eee 80

interrupted or latent, definitionof. 179
mutual and amphigonous, defini-

HON Ofseescauerer Sele ca osees? 79
“sexual, definition of............... 79
uninterrupted or continuous, defi-
DITION OF. 00. woe se eccee ongee 79
ProchiNawyjcise vaearsed a saesi vs seras vee 284
TrOCHIUB: 24: cce8 os seen as owe esis 221, 236
TroglodytidwWs..: sscecorse aves veya ins 332
TPIOBON a. sasegenilnias tte anteeye Ss 277
Trogonida 277
RUG ose geaye biavecs araiesonans aiaaied teeta ares 335
Turdus mustelinus.. we 185
Turkeys siscaaccs se aa . . 263, 264
Tympanuchus. 261
DYLAN HIND iiss sanicensoiiend ws 09 ch ena 285
TYTADDUB... piccweve soadasas 216, 217, 218, 285
crassirostris, instance of correla-
E1VG COLOPS <i :ce sien 25 ac daietee veveens © 1565
dominicensis, instance of correla-
tivecolors...........

melancholicus couchil
tyrannus.. ...155, 182, 199, 286
verticalis, . .153, 254, 255, 286

vociferans. . 254, 255
Wir iti ga aac ddones bees Gunes beaakee 5 270
ANCHTACIDR ices ssewereaca ce saaanser 146

Use and disuse, principle first sug-
gested by Lamarck, 2; principleac-
cepted by Darwin, 3; explanation
of, by Darwin, 25; discussion of, by
Spencer, 44-45; criticized by Poul-
ton, 47.

CALIFORNIA ACADEMY OF SCIENCES.

Vines, an examination of some points
in Prof. Weismann’s theory of
heredity, 11; admits the immortal-
ity of protoza, 11; questions ex-
planations of evolution of immor-
tal protoza into mortal metazoa,
11; criticism of Weismann’s theory
of heredity, 12; criticism of Weis-
mann's theory of continuity of
germ-plasm, 13, Weismann’s reply
to, 15.

Vireonide.......ccceee cece eee

Vigor, segregate definition of

Vultures, American..............60055

atricapillas)..« 26 scies sesaseiw da eens
CYASSLTOSTTIS....ccccewe cee Ge eens
FlAVESCENG... 000. erect reese es eeeee
flavifroms......-..006 cee eee eee eee
flavoviridis, ....... 0.6. cece eee eee
ochraceus

Variations, Weismann’s theory of, 55,
56; Weismann’s theory of, argued
against by Hartog, 56-57; explana-
tion of, 59-60; explanation of con-
servative variations, 61; explana-
tion of progressive, 61-62; second-
ary laws of, Brooks, 62; Schur-
mann’s discussion of the origin of,
63; further discussion of, 71; Gu-
lick’s criticism of Brooks’ theory
of, 72.

Wag-tail...........0000-
water.,

Wallace, criticized on account of argu-
ment on variations, 52-53; opposed
to Darwin’s theory of sexual selec-
tion, 82-83; criticism of sexual
selection opposed by Poulton, 84;
on sexual selection, 85-86; Ro-
manes’ objections to his views on
sexual selection, 92-93; objections
to physiological selection, 116-117;
Romanes’ reply to criticism, 118-
119; on recognition markings, 200-
201; on geographical distribution,
234.

Wax-wit@Sicces scstienacaiewees 216, 217,
Bohemian..

DAP ANCES C'seisia.siercsa.s esasasisie ows Reise cu
Warbler, Audubon’s
bay-breasted..... 00... ..e cece eee

190

9; against existence of. neucleo-
plasm as advocated by Naegeli, 10;
reply tothe objections of Vines, 18;
on embryogenesis, 16; theory of
heredity opposed by Ryder, 17;
summary of arguments between
Weismann and opponents, 21; ex-
planation of pammisis,22, on trans-
mission of mutilations, 30; experi-
ments on tails of white mice, 31;
criticism of observations on feet of
Chinese ladies by Cunningham, 31:
inheritance of artificially induced
epilepsy in guinea pigs, 33-34;
theory of variation, 55.56; theory
of variation criticized by Hartog,
56-57 ; on sexual selection, 97.

INDEX.

Warbler, DlaCWsncasscas ca ansdciercimdgas 161 Whip-poor-wills. ...........-.-0005 196,
black and white..................5. 320 Woodpecker 203, 210, 213, 279,
Blackburnian 148 Californian... 143, 163, 170, 173, 178,
Brasher’s...... 331 GOW DY asinsiccaawnsmiatensss sie, .. 175, 211,
cerulean.... .. 319 golden-fronted.
golden-winged.. DULY viiwncoum see seitns sap waster eens

red-headed. 145

three-toed.............. 155,

Williamson’s .......... masse

WONG $i oe cancisanustiesassrnains aces 196,

WHY DECKG, se ssicaayersgcieaiasiic: Mgreyoints 279,

MOB NO lai: cctyy cdease eeieaa erent 118, 219 Xanthocephalus..............- 218, 236
Nashville ...... AUR aeiceera dihe lense 217 xanthocephalus, instance of corre-
PAM axssseses cee s eaniundecbuane aa 219 lation between red and yellow, 155,
PAL U Bis cceas cavsse. are erent Aeaiieuiaemenaes 824 Xanthoura luxuosa....... ....... 158,
prothonotary ..........2.0 cee sence 320 guatemalensis. .....-.......--
MOG wens Anca scacnsatis eacemey oe 157, 330 ROENOPICUS 4sineneawcrvaarsces seal 224,
Ted-faceGuasceres sa¢saets awe ccas 330 albolarvatus...........02ee eee 146,
MS WAINGO MISS osicccissass aceutivercie dearer 218, 320 Yarrell, on changes of plumage. .132,
ONCE s ccectarcdansadwme nd 20aw 136 133,
SHIGE Oss sdaigndea aetna savin ventraerariens 151 Zenaidura macroura......... 06. eee eee
W000: cineca ace eames tet 819, 325 FZoOnotrvichianesscicccows thease 180, 196,
WOEM=CATNG) os ciserpeisweet ana aaraieats 320 ALDICOLNIS: is Sci wegecnagacewes 183,
yellow-rumped........ 158, 182, 208, 219 coronata.......... cee 187, 176, 209,
yellow-throated. ........ 0.2.2.0. 215 leuGOPhrys: esse caeeaeane’ 209, 254,

Water-OUzel vis cecicviciccciee se asics ccaieae 219 PATS seis) psjsce ass. <1 andiansio 184,

Weismann, value of his work, 8; on QUEDUL Bs cccinaiecccedarniareeeenyns 183,
death, 8; immortality of protozoa, AZQOVUDIDG csc siememioneses suman wanes

361

207

PLATE |.

del. C.A.K.

PLATE II.

Sphyrapicus
thyroides

Melanerpes
formicivorus bairdi

Sphyrapicus
varius
deal. CA

l
Xanthocephalus xanthocephalus

ee

Habia melanocephala

Colaptes auratus

Pirangéa ludoviciana

PLATE V.

Agelaius

oe ~

Setophaga ruticilla $

=!

Sphyrapicus varius

Agelaius phoeniceus juv.6

del. C.A.K.

LIT

6b

Seto phaga ru ticilla g

Sphyra picus varius

Agelaius phoeniceus ad.

Pyene aan |

PLATE Vt.

ete | ;

Myiarchus crinitus

Icterus cucullatus nelsoni

PLATE VII.

Spinus psaltria

Spinus psaltria arizonae

LITH BRITTON & REY S.F

PLATE VIII.

Snuogeio snyeinoew ojtdig

xkuojesow snyeynoew

ojidid

snujeuydosydsa ojidig
fj

del. C.A.K.

PLATE Ix.

PIPILO.

PLATE X.

CYANOCITTA.

PLATE Xl.

DRYOBATES SCALARIS NUTTALI & BOREALIS.

PLATE XII.

wt ae, SS

Lee
fos

BUBO VIRGINIANUS.

06 OF

a >
ih RE

CHORDEILES.

| Melospiza Fasciata

2 ” cinerea
3 fasciata rufina

4 guttata
5 “ - Samuelis
6 - » heermanni
7 » fallax

8 montana

del.C.AK.

LITH.@ ri TO! ary, SF

PLATE XIlll.

VA,

PLATE XIV.

. , ?
Agelaius phoeniceus & Agelaius gubernator J

Abelaiug: tricolor # = Agelaius phoeniceus 3 Agelaius Piterhetae

winter summer

Agelaius tricolor? summer Agelaius subernator ¢ winter

del. C.A.K,
LITH BRITTON 2 REY SF

Noof wn

| Sphyrapicus

PLATE XV.

varius Juv.
«2% ad. winter
» Pad. breeding
» nuchalis? ad.
» Sad.
» nuchalis od ad.
ruber dad.

PLATE XVI.

Icterus cucullatus nelsoni Icterus cucullatus nelsoni

Icterus bullocki

PLATE XVII.

Icterus cucullatus } Icterus cucullatus nelsoni

Teterus bullocki " Teterus galbula

Icterus parisorum Icterus spurius

peatioien

Top of Head

Back

UpperTail Coverts

Ear

tigrina

olivacea

zestiva

bryanti

czerulescens

ccronata

audubont

maculosa

ceerulea

pensylvanica

castanea

striata

blackburnize

dominica

Sracia

nigrescens

chrysoparia

Whi ye
Pot ky een

virens

townsend!

occidentalis

kirtlandi

vigorsil

palmarum

discolor

PLATE XVIII.

Coverts Throat Breast Sides Abdomen | UnderTail Coverts
Lam ’
tt, O yu *" f ] ia
\ Wath h ‘ ie
an i by, hy et)
I Be \s

a Fi
: a
ft 7?

Dendroica Top of Head Back Rump UpperilCoverts| Ear

ee

IE
tigrina |
olivacea
eestiva
bryanti
Ceerulescens |
coronata

auduboni

maculosa l

carulea | f
pensylvanica lj

castanea I oX

striata

- blackburnias
dominica
eracia
nigrescens
chrysoparia
virens
‘townsendi
_ occidentalis
kirtlandi

vigorsl

palmarum |

discolor

PLATE XIX.

rCoverts Throat Breast Sides Abdomen | UnderTail Coverts

wipe i NON aeae {t ian ity) =