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Mem York MACMILLAN & CO., 66 FIFTH AVENUE > 1804 Flies Oak Gall University Press Oscford. Shram. Cc ewinaT West, NE Alternating Generations A Biological Study of Mak Galls and Gall Flies By Hermann Adler, M.D. Schleswig TRANSLATED.AND EDITED BY CHARLES R. STRATON E.R.C:S)_Ep., F.E:s3 WITH ILLUSTRATIONS Orford Ae He CLARENDON” BRESS 1894 Orford PRINTED AT THE CLARENDON PRESS BY HORACE HART, PRINTER TO THE UNIVERSITY RDITOR so PREEACE Wuite pursuing the study of galls as a branch of comparative pathology, I was fortunate enough to become acquainted with Dr. Adler’s monograph of Alternating generations in oak gall-flies. The originality of the matter, and the important light which it threw on some of the great biological problems of the day, induced me to ask his permission to publish it in an English dress. I have to thank him for the generous manner in which he placed his work, and the beautiful drawings which accompanied it, at my disposal. Unfortunately the stones from which the illustrations were printed had been broken up, but I venture to think that they have been very faithfully reproduced. In the Introduction, and especially in the description of Cyntps Kollar? in the Appendix, I have used freely the writings of Dr. Beyerinck and Professor Mayr. I have added an analytical table of galls, a short bibliography, and a list of the Cynipidae. I have enclosed my own notes to the text in brackets, and in these I have given the synonyms of each species, its popular name, the inquilines and parasites which v1 Editor's Preface. have been reared from it, and the species of oak on which it is recorded as having been found. My grateful acknowledgements are due to Dr. Adler, Mr. E. A. Fitch, and Mr. A. Ehrmann for their assistance in correcting the proofs, and to Professor Mayr and Baron C. R. Osten-Sacken for information on special points. Cake West Loneée, WILTON, SALISBURY. CONTENTS PAGE INTRODUCTION . : , : : : By the Editor ix DESCRIPTION OF PLATES : ‘ : ; ‘ : : . xh ALTERNATING GENERATIONS IN Oak GALL-F ies. By Dr. Adler I CHapteR I. Earty OBSERVATIONS AND METHODS OF RESEARCH . ; . ; ; : : I II, Descriptions OF CYNIPIDAE OBSERVED. TABLE oF ALTERNATING GENERATIONS . ; : 9 III. On Gatti Formation . ; : : . Og IV. THE OvIPosITOR AND THE EGG . ; ». rte V. GROUPING OF THE CYNIPIDAE ; : . 129 VI. ALTERNATING GENERATION, AND CYCLICAL PROPAGATION. - : , : ; . 49 PEDIASPIS ACERIS AND BATHYASPIS ACERIS ; ; « ‘59 Cynips KoLiari ; ; ‘ ; ‘ . By the Editor 163 SYNOPTICAL TABLE OF GALLS . . , . - 168 CLASSIFICATION OF THE CYNIPIDAE. . , ia 172 BIBLIOGRAPHY. ; , , : : : ‘5 182 INDEX . : : ; , : F : ; ‘ ‘ « SEQT Fad INTRODUCTION A GALL is an abnormal growth of plant tissue produced by animal agency acting from within. All the natural orders of plants include species which are liable to be made use of by insects in this way. Each is visited by its own special gall-maker, which need not necessarily belong to the Cynipidae, for gall-makers are also found among the Coleoptera, Lepidoptera, Diptera, Nematoda’, and in other classes. Any organ of the plant may become the seat of this hyperplasia, but the form which the gall ultimately assumes is governed by the potentialities of growth in the part attacked, and by the nature of the animal excitation present. The rose and some Compositae produce well known galls, but the oak is the favourite home of the Cynipidae. In this monograph Dr. Adler has described those oak-galls and gall-flies most commonly found in this country, with the exception of Cynips Kollari, the Devonshire marble-gall, which does not occur in Germany north of the Elbe; as it is however one of the most familiar galls on English oaks, a description of it has been added in the appendix. Before Dr. Adler ' H. Charlton Bastian, ‘Monograph of the Anguillulidae,’ Lin, Soc. Trans. vol. xxv, 1866. x Introduction. had demonstrated the existence of cyclical propagation, many curious explanations were offered, in order to account for the lengthened interval that elapses between the death of one generation and the appearance of the next. The currant-gall, for example, appears on the male catkins of the oak in May, the fly quits the gall in June, lives for a few days only, and nothing more is seen until the male catkin appears again next May. What had become of the eggs in this long interval ? Spontaneous generation of the insect, within a gall that had no external opening, had its advocates. Later it was believed that a form of metempsychosis took place, and galls were among the stepping-stones in the path of organic evolution, by which the vegetable passed into the animal soul. By some it was supposed that the eggs, found in the fly in June, reached the ground, whence they were drawn up, mingled with the sap, and arrested next spring in the leaves or flowering catkins, there to form the currant-galls again. Dr. Adler, by proving the existence of cyclical propagation, has shown that the interval between the. appearance of the currant gall-fly in one spring, and its reappearance in the next, is occupied by another agamous generation. But while he showed that this rule holds good for the majority of species, he has also demonstrated that, in some at least, no sexual generation now exists. Pliny’ knew that a fly was often produced in a gall, but he did not associate it with the cause of gall-growth ; on the contrary, he thought galls grew in a night, like fungi. Many early observers, however, considered them as insect productions and were aware that a variety of insects emerged from them; but the attention which some of these authors bestowed upon this subject was Y Pliny, Vat, Atst. Xvi. 9, 105 XXIV. 5. Marcellus Malpight. xd not always due to its biological interest. Mathiolus, one of the best of the commentators of Dioscorides, and a believer in spontaneous generation, declared that weighty prognostications as to the events of the year could generally be deduced by observing whether galls contained spiders, worms, or flies. Most of the older writers describe gall-flies, which are now known to be agamous, as possessing males ; but their descriptions are often perfectly clear, and the flies can be recognized as the males of one of the Torymidae, or of some other species of parasite. There is no reason to think that any males of agamous species were actually in existence at the time when these authors wrote. The earliest systematic writer on galls was Marcellus Malpighi, Physician to Innocent XII. He _ was Professor of Medicine at Bologna, and afterwards at Messina, and his treatise * ‘ De gallis,’ published in 1686, gives an extremely accurate description of the galls then found in Italy and Sicily. Dr. Derham, Canon of Windsor, in the notes to his Boyle Lectures (1711-1712) compares Malpighi’s account with the galls then found in England, and says, ‘I find Italy and Sicily more luxuriant in such productions than England, at least than the parts about Upminster (where I live) are. For many, if not most, of the galls about us are taken notice of by him, and several others besides that I have never met with, although I have for many years as critically observed all the excrescences and other morbid tumours of vegetables as is almost possible, and do believe that few of them have escaped me®.’ Malpighi’s work does not appear to have been known either to Linnaeus or Fabricius ; they include ' Dioscorides, i. 146, Paris, 1549. * W. Derham, F.R.S., Physico-theology, iii.c. 6. Xil Introduction. inquilines and parasites under the genus Cynips, indeed St. Hilaire, Latreille, and Olivier reserve the name Cynips for certain Chalcididae, and use Diplolepis for the true gall-maker, but with few exceptions subsequent writers have applied the general name Cynips to the gall-makers. It is well, however, to bear this change of nomenclature in mind, when the males of certain species are said to have been found’. Réaumur has left excellent descriptions and drawings of many species of galls *, but the first to bring order out of the confusion in which the Cynipidae still remained, was Theodor von Hartig of Brunswick*. He greatly improved the existing classification of this family and carefully pointed out the true relationship which the various dwellers in galls bore to each other. He arranged gall inhabitants into three classes ; first, the gall-makers which he com- pared to the actual householders ; secondly, inquilines, guest-flies, cuckoo-flies or lodgers, who take up their quarters uninvited within the gall, and live on its food stores, but do not directly aim at the gall-maker’s life ; and thirdly, parasites who deposit their eggs on the larvae of their host or his lodgers, with the object of destroying them, and who are therefore murderers. Besides those living in a state of symbiosis, there are also true commensals in some galls, It is the simultaneous presence of these various classes that frequently gives rise to confusion in carrying out breeding experiments. Synergi among inquilines re- semble true gall-making species so closely, that caution ‘ Westwood, Zoological Journal, No. 13; Cameron, P., British Phyt. Hymen. vol. ili. p. 140. 2 Réaumur, Meéemortres pour servir a Whistoire des wmesectes, 1734-42. * Hartig, ‘Ueber die Familien der Gallwespen,’ Germar’s Zeitschr. f.d. Ent. Il. Heft 1, p.. 176, 1840; Ill. 322-358, 1847; 1V..3905, eas: Gall-Dwellers. XI is always necessary to see that the gall-maker, and not a Synergus, has been obtained’. Both Malpighi and Canon Derham were aware of the attacks of parasites, and actually saw galls pierced by them. The latter says, ‘I apprehend we see many vermicules, towards the outside of many oak-apples, which I guess were not what the primitive insects laid up in the gem from which the oak-apple had its rise, but from some supervenient additional insects, laid in after the apple was grown, and whilst it was tender and soft’.’ Ratzeburg, a forester like Hartig, in his beautiful ‘Forstinsekten’’ corro- borated Hartig’s division of gall-dwellers. Giraud, Schenck, Reinhard, Taschenberg, Schlechtendal, Wachtl, Forster, and Lichtenstein, have since each advanced our knowledge of the Cynipidae, and the history of galls generally has been admirably written by Lacaze-Duthiers. The entomologists of America have not been behind those of Europe; Baron C. R. Osten- Sacken, before he quitted America in 1877, had discovered eighteen new species ; Bassett, thirty species, and Walsh and Riley had each added much to our knowledge. Professor Gustav Mayr of Vienna has not only increased largely the work of previous observers, but has arranged all that is known of gall-makers and gall- dwellers in a series of admirable monographs * and has * See Walker, Ent. Mag. vii. p. 54. * Derham, Physico-theology, iii. p. 389. > Ratzeburg, Dre Forstinsekten, vol. iii, Berlin, 1844. * Mayr, G., Die muitteleuropaischen Eichengallen in Wort und Bildern, Wien, 1870-71; Die Einmuethler der mitteleuropdischen Eichengallen, Wien, 1872; Dieeuropdischen Cynipiden-Gallen mut Ausschluss der auf Eichen vorkommenden Arten, Wien, 1876; Die ewropdaischen Torymiden, Wien, 1874; Exncyrtiden, 1876; Olinx, 1877; Eurytoma, 1878; Telenomus, 1879; Die Genera der gallenbewohnenden Cynipiden, Wien, 1881 ; Die europdischen Arten der gallenbewohnenden Cynipiden, Wien, 1882. XIV Lntroduction. drawn up excellent analytical tables of species. In this country Westwood, Halliday, Walker, and Fitch have all done good work, and Mr. Peter Cameron}, in his account of the Phytophagous Hymenoptera, just completed for the Ray Society, has brought together an immense amount of valuable material, new and old. Dr. Adler began his observations on gall-flies in 1875, and added certain new species described in these pages. His important work, however, was the unfolding of their life-history ; proving that while many species were linked together in alternate agamous and sexual genera- tions, others remained wholly agamous. Among biologists theories of dimorphism and meta- genesis had been discussed in connexion with the Cynipidae and were current as early as 1865’; in 1873 Bassett announced his theory of seasonal dimorphism ’ ; and in the same year Riley established the existence of alternation of generations between Cynips operator and Cynips operatola*. The numerous publications of Thomas and Frank have cleared up many special points, and Beyerinck® in an admirably illustrated monograph, full of original work and careful reasoning, has added much both from a botanical and zoological point of view. The existence of alternating generations in living ' Peter Cameron, A Monograph of the British Phytophagous Hymenoptera, 4 vols. 1882-1893. Ray Society. 2 Reinhard, ‘ Die Hypothesen tiber die Fortpflanzungsweise bie den eingeschlechtigen Gallwespen,’ Berlin. Ent. Zeitschr. vol. 1x. 1865. * Bassett, Canadian Entomol. vol. v, pp. 91-94, May, 1873. * Riley, American Naturahst, vol. vii. p. 519, 1873. 5 Beyerinck, Dr. M. W., ‘ Beobachtungen iiber die ersten Entwick- lungsphasen einiger Cynipidengallen, Natuwurk, Verh. der Koninkl. Akademie, Deel xxii. Cyclical Propagation. XV organisms was first discovered by Chamisso’, the author of ‘ Peter Schlemihl,’ whoin 1815 accompaniedthe Chan- cellor Rumjanzow’s expedition as naturalist in a voyage round the world. He noticed that among the Salpae a solitary salpa gave rise to a generation of a different form, united in chains of twenty or more, and that each link of this ‘associated’ form again produced the ‘solitary’ form. He concluded that all solitary salpae produced chains, and on the other hand that all those in associated chains were the parents of solitary ones ; so that a salpa mother was not like its daughter or its own mother, but resembled its granddaughter and its grand- mother. At first the accuracy of Chamisso’s observations was doubted, chiefly for the reason that no similar phenomenon was then known in nature. By degrees, however, facts began to accumulate ; in hydroids and flukes similar generation-cycles were observed, and in 1842 Steenstrup? collected all that was known on the subject of alternating generations into a monograph published in Danish and subsequently translated into German and English. He described this mode of reproduction .as ‘a peculiar form of fostering the young in the lower classes of animals.’ In 1849 the late Professor Owen® suggested the existence of a residual germ-force in the cells of the apparently asexual generation, and thought that alter- nating generations were due ‘ to the retention of certain of the progeny of the primary impregnated germ-cell, or in other words to the germ-mass, with so much of the spermatic force inherited by the retained germ-cells from 1 Adalbert de Chamisso, De Animalibus, Fasc. i, De Salpa, Berlin, 1819. ay, J. S. Steenstrup, Ueber den Generationswechsel, Kopenhagen, 1842, and Ray Society, 1845. 3 R. Owen, Parthenogenests, 1849. XVI Introduction. the parent cell or germ-vesicle, as suffices to set on foot and maintain the same series of formative actions as those which constituted the individual containing them’: and this may be taken as the earliest suggestion of the continuity of the germ-plasm. Fresh instances of this class of phenomena have steadily accumulated, in which the life-cycle of the species may be represented by two or more generations, differing in form and organization, existing under different conditions, and reproducing themselves in different ways. The simplest cyclical rhythm occurs in reproduction by metagenesis, where a sexual and asexual form alternate ; this is the law of development in Medusae and Trematoda. In heterogenesis a sexual and an agamous generation alternate, and in this rhythm the agamous may be juvenile or adult ; in Cecidomyia, for example, the parthenogenetic generation reproduces itself when still immature, while in the Cynipidae, on the other hand, it does so only when perfectly developed. Another variety occurring between one hermaphrodite and one sexual form, is seen in Angiostomum nigro- venosum, the lung parasite of the frog ; and a still more perfect alternation is found in the thread worm of the snail, Leptoptera appendiculata, where two perfectly formed sexual generations are linked in a cycle. Sometimes the sexual or asexual member of the cycle may be complex. The liver-fluke of the sheep gives rise to an active ciliated aquatic embryo, which, after a time, pierces and enters a water-snail to become a passive sporocyst; from its germ-cells rediaeare formed within the sporocyst, and after several asexual genera- tions, they give rise to minute cercariae, which leave the snail and creep up the stalks of grass; here they The Larval Theory. XVI become encysted, are eaten, and grow within the sheep to become adult sexual flukes. In this series the Cercaria and fluke form members of the sexual division; all the others are members of the asexual division of the cycle. In all cyclical propagation, whether in the animal or vegetable kingdom, there is a tendency for one genera- tion to become subordinated to the other. In flowering plants the sexual is subordinated to the asexual, and even some ferns exhibit a similar tendency, a fern- plant rising vegetatively from the prothallus; while in other ferns there is a tendency to apospory, a fern- prothallus springs from the site of the spores, and the asexual becomes subordinated to the sexual. In flukes, in the same way, vediae may be budded off from the sporocyst and the species be continued without ever actually becoming sexual. In the Cynipidae it will be seen that in some, like Cynips Kollar, the sexual genera- tion has been wholly subordinated to the asexual, and in others, like Rhodites rosae, this process is still going on and males are becoming functionless and extinct. Leuckart' regarded interpolated generations as larval states, and following his teaching, Lichtenstein? looked upon the agamous as larval stages of the sexual species. He believed that the biological evolution of a gall-fly extended from the time when a female emerged from a true egg in a condition to be fecundated by the male, until another egg was reached presenting the same con- ditions. All other stages he considered as larval, al- though in them reproduction by budding was possible, and he held that in this way insects might go on re- producing themselves indefinitely without ever reaching 1 R. Leuckart, Zur Kenntniss d. Generationswechsels u. d. Parthenog. b. d. Insekten, 1858. 2 J. Lichtenstein, Les Cynipides, 1881, p. x. b XV1il LIutroduction. the perfect state. Certain Phylloxeridae reproduce themselves by subterranean budding, without ever arriv- ing at the winged and sexual forms: he regarded this as analogous to the multiplication of plants by suckers, without the intervention of seed. He compared the life-history of gall-flies with that of Phylloxera, and gave it as his opinion that, among the gall-flies, the individuals of Neuroterus lenticularis had been mistaken for females, because they possessed an ovipositor and eggs, that, properly speaking, they had no sex, but were only larval forms of Spathegaster baccarum, their so-called eggs being gemmations. It is difficult to regard species which have a perfect female form, and ovaries filled with perfect eggs, as being perpetually in the larval state, because they are wholly agamous and have no males. In Rhodites rosae, as has been said, the few males found are functionless, and are disappearing; it seems therefore more in accordance with facts to speak of the individuals of agamous species as females, the males of which have ceased to appear. Although the Aphides are scarcely comparable to the Cynipidae, since they possess a form which is asexual, they may nevertheless be arranged, like the Cynipidae, in alternate generations, without having recourse to the larval theory. The importance of sexual reproduction has been greatly exaggerated, and there seems no good ground for assuming that a generation ought to extend from one fecundated egg to another. In Cyuips Kollari, and many others, no sexual generation is known; the flies all possess perfect female forms, but they are agamous and have the power of reproducing themselves by parthenogenetic eggs. At the same time it is difficult to believe that the agamous can be the primitive Advantages of Parthenogenests. X1X form; or that perfectly formed sexual organs could have been evolved unless the sexual had been the earlier generation. The power of producing parthenogenetic eggs is widely distributed among the arthropods, and appears to come into operation whenever it secures the existence of the species more effectually than sexual reproduction. When at one season of the year sexual, and at another agamous, reproduction is the more beneficial, then heterogeny will be found to prevail. In one group amphimixis has been wholly abandoned, but its members are enormously prolific, and their eggs have the power of resting over more than one year. By means of partial parthenogenesis, a much more rapid increase is ensured than could have been possible, in the same time, by sexual reproduction only. Every individual of the winter generation, unhindered by the require- ments of fertilization, is engaged in laying eggs; the number of the sexual individuals hatched from these eggs is consequently enormously greater than it could have been, had only half the winter generation been of the female sex, and had that half, in order to be fruitful, been dependent on the chances of fecundation. When generations alternate, there are alternating advantages to the species. The winter generations emerge from the gall at a time when pairing is not easy, and it is a distinct gain to the race when every individual has the power of reproducing itself inde- pendently. The summer generations, on the contrary, appear in halcyon days, when there is nothing to mar their nuptial flight, and then the species obtains greater variation, and those physiological advantages of amphi- mixis which parthenogenesis cannot afford. But amphimixis is in no way essential to heredity. b2 D6 .4 | Lntroduction. The oak gall-flies about to be described will be found to afford examples of heredity occurring among purely agamous species, as well as among those which alternate between an agamous and a sexual generation. The characters of each generation are inherited and passed on in two perfectly -distinct streams. The advance which has in recent times been made towards a clearer comprehension of heredity, is in great measure due to the influence of Weismann’, who, by discarding the idea that sexual reproduction is in any way funda- mental or essential to life, has led us to regard the facts of heredity wholly untrammelled by it. Amphi- mixis undoubtedly has its advantages, but descent may be continuous in the female line, or there may even be a male parthenogenesis’. To understand the mechanism by which alternation of generations is brought about, it is necessary to recall the minute structure of the sexual cells, and especially the behaviour of their nuclear contents ; and to observe the difference in the extrusion of the polar cells, as occurring in the eggs of parthenogenetic and sexual species. The changes in the ovum during nuclear division are briefly these*. After a resting stage the chromatin granules, which there is reason to believe are the material bearers of hereditary qualities, appear as a thread, apparently, spirally coiled within the nucleus. An accessory nucleus forms and divides into two; at each pole of its achromatic spindle is placed a centrosome with its surrounding attraction-sphere ; the nuclear membrane disappears; the chromatin 1 Weismann, Essays on Heredity, vol. ii. p. 86, 1893. * Ibid. vol. i. p. 253; Schenck, Handbuch der Botanik, Bd. ii. p. 219. * Weismann, Essays on Heredity, vol. ii. p. 118. Spermatogenesis. Sea granules become aggregated into rod-like chromosomes of equal length and of constant number in each species ; these are formed at the equator of the spindle, and split by longitudinal fission, so that their number becomes doubled. One star of chromosomes is drawn to the centrosome at each pole of the spindle, and thus two daughter-nuclei, which for convenience I will term oocytes, are formed, of unequal size; the smaller one being extruded as the first polar body. In the parthenogenetic egg this completes the division, but in the sexual egg a second nuclear division follows immediately on the first without a resting stage; the oocyte divides into two oozoa, and one 00zoon, contain- ing half the chromosomes, is extruded as the second polar body. The oocyte which forms the first polar body is observed to split into two oozoa after extrusion. In this way three of the oozoa which have arisen from the division of the primitive germ-cell have become polar bodies, while the remaining oozoon, containing one half the number of chromosomes which the primitive germ-cell contained, is left in the nucleus, functional or capable of development. Within the spermatic tubes of the male, corresponding changes take place in the primitive sperm-cell. Its chromosomes are doubled and it divides into two sper- matocytes (spermatogens), each of which again divides without a resting stage into two spermatides (spermato- blasts). The four spermatides undergo various changes during which they become elongated, the nucleus, containing the chromatin elements, becomes the head of a spermatozoon and ends in a motile barb. The protoplasm of the cell-body is drawn out into a sheath through which the filament passes, and is continued beyond as the vibratile tail. XXIV lutroduction. affect the number of men particularly under him, and may alter their original course. In cases of alternation of generations, and in those cases in which the two generations do not differ from each other in the full-grown condition, but in which the eggs develop differently, Weismann holds that there must be two kinds of germ-plasm, each containing the determinants of one form, and that these two must be passed separately along the germ-tracks, from one generation to another, so that each must always contain the other, stored away in an inactive condition’. In the case of gall-flies there is often a well-marked difference of structure between the females of the two alternating generations. The wing determinants in the winged 7rigonaspis crustalis (Plate II, fig. 18 a) must have been modified in the wingless Lzorhiza renum, (Plate II, fig. 18), but they appear again in 77igonaspis crustais. It is improbable that lost determinants would be redeveloped in this way, and therefore there must be two alternating sets of determinants. These, Weismann holds, are contained within the same germ- plasm, but never become active at the same time’. I believe that in the function of the polar bodies we possess the simplest explanation of the problem of alternating determinants in the Cynipidae. According to Weismann’s earlier views’, the first polar body removed ovogenetic nuclear substance which, after the maturation of the egg, had become superfluous and injurious; while the second polar body removed as many different kinds of idioplasm as were afterwards introduced by the sperm-nucleus. He has abandoned this view of the function of the * Weismann, Germ-plasm, p. 176. 3 Ibid. p.278! 3 Essays on Heredity, vol. i. p. 365, Oxford, 1891. Function of Polar Bodies. XXV first polar body, and, according to his later views, the extrusion of both polar bodies is a provision for with- drawing promiscuously certain ids of germ-plasm for the purpose of securing variation. He says: ‘I consider this remarkable and apparently useless process of the doubling and two subsequent halvings of the idants, as a method of still further increasing the number of possible combinations of tdants in the germ-cell of one and the same individual'’; the three quarters of the mass of germ- plasm which passes into the polar bodies, he regards as being ‘again lost ’,’ while what is left in the ovum, after being increased by one spermatozoon, is halved by the first nuclear division ; one half going to direct ontogeny, and the other half going to form the primary sexual cells: but as these last divide and behave at first like other somatic cells, they must, he thinks, possess active idioplasm as well as unalterable germ-plasm, and they must therefore contain more ids in their nuclear matter than do somatic cells *. I incline to the view that these cells in the Cynipidae remain somatic, and simply form the rudiments of the sexual organs, but that the function of the polar bodies is to provide the primitive reproductive cells. These bodies contain the inactive germ-plasm, historically prepared for this very purpose, and it is scarcely possible to imagine that nature would create this phyloge- netic epitome of the species to be extruded as func- tionless, or to be broken up and again lost*. It is undeniable that they are of great value in promoting variation. The first division of the nucleus into two oocytes is not a reducing division, but results, so far as the germ-plasm is concerned, in two equivalent cells. 1 Weismann, Germ-plasm, p. 247. ? Ibid. p. 248. 3 “Ibid: p: 192. * Ibid. p. 248. XXVI1 Introduction. In the sexual egg, however, the second division follows immediately upon the first, and the extrusion of one oozoon reduces the chromosomes by half their number, which is again made good by those of one spermatozoon. While, therefore, in the parthenogenetic egg those ‘material bearers of ancestral qualities’ remain the same from generation to generation, they are changed in the sexual generation by each cross fertilization, and in this exists the advantage of amphimixis. For the evolution of species, individual variation is essential, and those individuals who derive half their germ-plasm from one parent, and half from another, each bringing in different lines of ancestors, must present greater variation than those who derive their germ-plasm from one parent and one ancestral line. Sexual reproduction is thus of advantage to the species in so far as it helps to increase inherent tendency to variation, and enables evolution to take place by the steady accumulation of slight beneficial differences’. —The random withdrawal of the ids of ancestors by a species of lot, as laid down by Weismann, must be considered simply as a provisional hypothesis ; and it seems, according to Prof. Hartog, that mathematically its effect, if it took place at all, would be the opposite of what Weismann expected ”. Although the polar bodies were first noticed by Miller in 1848, their importance has only been recently appreciated. Balfour® considered their extrusion as the removal of part of the constituents of the ger- minal vesicle which are necessary for its functions as a complete and independent nucleus to make room for equivalent parts of the spermatic nucleus ; and he con- * Darwin, Origin of Species, p. 132. 2 Prof. Hartog, Nature, vols. xliv and xlv, 1893. ° F. M. Balfour, Comparative Embryology, vol. i. p. 63, 1880. Embryonic Germ-Cells. XXVIl sidered that the function of forming polar cells had been acquired by the ovum for the express purpose of preventing parthenogenesis. Van Beneden’ and Minot® regard the polar bodies as male elements removed from a hermaphrodite cell. Biitschli, Hertwig *, and Boveri incline to regard them as abortive ova and an atavistic reminiscence of primitive parthenogenesis. Geddes and Thomson‘ consider their extrusion due to the tendency of every cell to divide at the limit of growth, but favour the view that the process may be an extrusion of male elements. They consider, however, that these cells have no history, though they occasionally linger on the outskirts of the ovum, are seen to divide, and be penetrated by ‘misguided’ spermatozoa °. The homologous character of oozoa and spermatozoa has been pointed out, and these so far retain the habits of their protozoan ancestors as to require conjugation: in the parthenogenetic form, the two oozoa conjugate in each oocyte, in the sexual form one oozoon and one spermatozoon unite to form the fertilized ovum or oosperm, but I believe the three oozoa which form the polar bodies, unite together or conjugate with three spermatozoa, previous to that repeated division by which those bodies are prepared to be received as the earliest embryonic germ or sperm-cells into the rudimentary germ-tracks’°. 1 E. Van Beneden, ‘Recherches sur la fécondation,’ Arch. de Biologie, iv, 1883. C. S. Minot, ‘Theorie der Genoblasten,’ Biol. Centralbl. ii. p. 365. O. Hertwig, Die Zelle und die Gewebe, Jena, 1892. Geddes and Thomson, Evolution of Sex, p. 107, 1892. Ibid. p. 105. H. Henking, Zettschr. f. wissensch, Zoologie, liv. 89; V. Graber, ‘Keimstreif der Insecten,’ Denkschrift d. K. K. Akad., Math.-Nat. Cl, Wien, lvii, t890. a on kf wo Fig. I. Diagram of the formation of ova in Ascaris megalocephala, var. bivalens. A. The primitive germ-cell. 4. Fully developed germ- mother-cell, with chromosomes doubled. C. The first nuclear division. LD). One daughter-cell (oocyte) extruded as the first polarbody. Z. The second or reducing division; the retained, and the extruded oocyte, dividing, and forming four grand-daughter cells (oozoa). /#. The ripe egg-cell, the functional oozoon; the other oozoa, 2, 3, and 4, being the polar bodies. Conjugation of Polar Bodtes. XX1X second polar body, and one oozoon is left containing the remaining two chromosomes. Assuming that the egg of one of the parthenogenetic species of Cynipidae had the same number of chro- mosomes as the species of Ascaris just described, four chromosomes would be extruded as the first polar body and four would remain in the nucleus. In that of a sexual species, on the contrary, six chromosomes would be found in the polar bodies and only two in the nucleus ; but when fertilization took place, two chromosomes would enter the ovum with one spermatozoon, and six chromosomes would be added to the polar bodies when three other spermatozoa united with them. Thus the polar bodies of one egg of the parthenogenetic generation would contain four chromosomes available for the supply of germ-plasm to the next generation ; while the polar bodies of a sexual egg would afford twelve chromosomes for the germ-plasm of the generation that follows. It is clear that if the polar bodies contain the germ-plasm of the next generation they must be fertilized by three spermatozoa, since in the sexual generation male and female characters are equally transmitted. If this be the case, we ought to find the eggs of the parthenogenetic generation much more numerous than those of the sexual, because the parthenogenetic germ-tracks have received the produce of twelve, while the germ-tracks of the sexual generation have only received that of four chromosomes; and it is to be presumed that the ova produced from them would bear something like this proportion to each other, if this theory held good. What are the facts? ‘The summer sexual generations, whose germ-plasm has been received from the parthenogenetic polar body, produce from 200 to 400 eggs; while the winter agamous generation, which receives its germ- XXX Introduction. plasm from the fertilized polar bodies of the sexual generation, produces from I,000 to 1,200 eggs; or in an approximate proportion to the number of chromosomes postulated as present. The impression which long prevailed that one sperma- tozoon was equal potentially to one ovum, led observers to regard the presence of more than one spermato- zoon as an act of ‘polyspermy,’ as abhorrent to nature, and a cause of monstrosity’. But Kupffer, Benecke and others record the fact that spermatozoa do enter the ‘peculiar protoplasmic protuberances,’ many appearing to form pronuclei after gaining access to the ovum, so that in numerous species polyspermy appears to be the rule. A great deal has recently been done in working out the development of the reproductive rudiment in insects, and it seems that the cells which become the ova, and which I believe to be chiefly those primarily set aside in the polar bodies with or without corresponding spermatozoa, can be traced to the rudiment of the germ and sperm-tracks of the embryo. These germinal track- cells, formed by the first segmentation, surround the primitive germ-cells to form with them the rudimentary reproductive organs. In the germogen one germ-cell and a nutritive circle of germinal epithelium cells form a cluster, and these grow forward together, the ger- minal epithelium growing in between each batch. In the upper part of the developing egg-tube, each batch is very small, but as it advances it increases steadily in size. In the egg-tube of the Cynipidae the egg-stalk curves round the next egg, and the clubbed end of the stalk gives the appearance of a large and small egg alternating. In some asexual species of Hemiptera the oocyte, * Selenka, E., Befruchtung des Eves, 1871. Alternating Streams. ex which would become in an agamous species the first polar body, is not actually extruded but may be seen as a smaller cell lying close to the nucleus ; and in some sexual eggs, both polar bodies are similarly retained. It will be apparent how simply this view of the functions of the polar bodies in Cynipidae accords with the facts of alternating generations. The fly which emerges from the gall of Spathegaster baccarum in June is sexual, and lays an egg which extrudes two polar bodies. The germ-plasm in these polar bodies after being united with that of three sperma- tozoa, is received by the embryo germ-tracks of this egg, and when from that egg the agamous WVeuroterus lenticularis emerges in April, it is this germ-plasm which forms the nuclei of the ova contained in its tubes, consequently these ova can only reproduce the sexual flies of Spathegaster baccarum. Again, the agamous Weuroterus lenticularis lays an egg which extrudes one polar body. The germ-plasm in this polar body is received by the germ-tracks of the embryo, and, when from that egg the sexual Spathegaster baccarum fly emerges, it is this germ-plasm which forms the nuclear matter of the ova and spermatozoa contained in its tubes, and consequently these ova and spermatozoa can only reproduce the agamous fly of Meuroterus lenticularts. The two streams of germ-plasm are thus going on independently, and are each capable of acquiring and accumulating beneficial variations, so that the general dictum of Weismann is correct: ‘the basis of the alterna- tion of generations as regards the idioplasm, must, in all cases, consist of a germ-plasm composed of ids of at least two different kinds, which ultimately take over the control of the organism to which they give rise.’ In XXXII Iutroduction. this way we have a means not only of securing variation, but of maintaining the fixity of species, which is equally important. Alternating generations disappear as we ascend higher in the animal and vegetable scale, or as life lengthens beyond the period when seasonal alternation could be of advantage; then the purpose in view seems to be the approximation and assimilation of consecutive genera- tions, and the continuous uniformity of the species. It is not quite clear how this result is attained, but the polar bodies cease to monopolize the transmission of the unalterable germ-plasm; probably another nuclear division, after fertilization and before ontogeny has begun, is added to them’, and well marked atavism is only found as a pathological occurrence when the assimilating forces fail. It is next of interest to inquire how the various structures of the gall came to be evolved. It may be taken as perfectly certain that the tree does not form them in a disinterested manner for the sake of the Cynips. Darwin says: ‘If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection’. So far as galls are concerned, Darwin’s theory is perfectly safe. The ‘excitatory emanations,’ as Professor Romanes® aptly calls them, which lead to gall-growth, can only have arisen by gradual and increasing improvements in the initial stages of their formation, acting through natural selection, over an unlimited period of time, and through numerous consecutive species. 1 Weismann, Germ-plasm, p. 192. 2 Darwin, Origin of Species, chap. vi. > Romanes, JVature, vol. xli. p. 80, 1889. Classification of Galls. p.6.0.41 1 Galls may be arranged in groups of gradually increasing complexity, beginning with those like Spathe- gaster baccarum, and leading up to the complicated structure of Cyzips Kollart. Beyerinck’s classification following that of Lacaze-Duthiers, is into five groups :— 1. Simple galls, consisting of nutritive tissue enclosed in thin-walled parenchyma with vascular bundles : Neuroterus ostreus, Spathegaster albipes, S. baccarum, S. Aprilinus. 2. Galls similar to these, but having the nutritive tissue first enclosed in sclerenchyma, which forms an ‘inner gall’: Neuroterus lenticularis, N. laeviusculus, N. numismatis, N. fumipennis, Aphilotrix Sreboldt, A. autumnalis, A. radicis, A. globult, Andricus curvator, Biorhiza renum, Bb, aptera. 3. Galls possessing an inner gall like the last, but having it surrounded by thick-walled parenchyma : Dryophanta longiventris, D. divisa. 4. Galls with the inner gall enclosed in a spongy layer of branched parenchyma, with wide intercellular spaces, and having the surface covered with a differentiated epidermis : Unilocular. Dryophanta scutellaris. Multilocular. Teras terminalis. 5. Galls which have the inner gall enclosed in thick- walled parenchyma, and then in spongy tissue ; and which have a differentiated epidermis : Cynips Kollart. Besides these histological differences, the outward characters are also of varying complexity ; each infini- tesimal improvement, which has been of service as a protection against parasites, or has been successful in securing natural conditions favourable to the life and € XXX1V : Introduction. growth of the larva, has been preserved, and has formed the starting-point of further beneficial variations. It is always that larva which has been able to induce successful morphological abnormalities, which is re- produced to continue the race; the unsuccessful perish. The ruling force is natural selection; it is impossible that intelligence or memory can be of any use in guiding the Cynipidae; no Cynips ever sees its young, and none ever pricks buds a_ second season, or lives to know the results that follow the act. Natural selection alone has preserved an impulse which is released by seasonally recurring feelings, sights, or smells’, and by the simultaneous ripening of the eggs within the fly. These set the whole physiological apparatus in motion, and secure the insertion of eggs at the right time, and in the right place. The number of eggs placed is instinctively proportionate to the space suitable for oviposition, to the size of the fully grown galls, and to the food sup- plies available for their nutrition. Dyryophanta scutellaris will only place from one to six eggs on a leaf which Neuroterus lenticularts would probably prick a hundred times. The veins and petiole of the leaf carry onwards water and salts derived from the soil, and return the organic products of the leaf-cells; and these food currents render them desirable situations for gall- growth. The under surface of the leaf is folded out- wards in the bud (vernation conduplicate), so that it is the first part reached, when buds are pricked. When expanded leaves are pricked, the spongy meso- phyll of the under surface is much more easily penetrated than the upper surface, which is covered with the cuticle of an epidermis, that rests on closely + Weismann, Essays on Heredity, vol. i. p. 95. Evolution of Galls. XXXV packed palisade cells ; the lower surface is consequently the situation most in favour. Whatever form the gall takes, the potentialities of the tissue-growth exhibited by it, must be present at the spot pricked by the fly. It is not necessary to assume, with De Vries, that every vegetable cell contains the potentialities of every other cell in a latent condition. The conical growing point in every bud contains the germ-plasm of the next shoot, and consequently of the whole plant. The potentialities of growth being present, they are called into activity by the larva, a result advantageous to the larva and sometimes described as disinterested and self-sacrificing on the part of the plant’. We have just seen that, so far as the larva is concerned, the peculiar structures of the gall owe their origin to their success in feeding and defending it; and so far as the plant is concerned, these structures have been evolved in consequence of their value in enabling the plant to repair injuries: in general, and the injuries inflicted by larvae in particular. If John Doe raises a cane to strike Richard Roe, and Richard throws up his arms intuitively to parry the stroke, the action does not indicate a prophetic arrangement of molecules to frustrate John in particular, but an inherited action of defence. The first act of an injured plant is to 1 St. George Mivart, Nature, Nov. 14, 1889. ‘Now surely it is too much to ask us to believe that the germ-plasm of the plant, in the first instance, before even, say, a single cynips had visited it, had in the complex collocation of its molecules, an arrangement such as would compel the plant which was to grow from it, to grow those cells and form a gall.’ And in a note he adds: ‘It would be very interesting to know how natural selection could have caused this plant to perform actions which, if not self-sacrificing (and there must be some expenditure of energy), are at least so disinterested.’ C2 XXXVI Introduction. throw out a blastem, and only those larvae survive to hand down their art, which emerge from an egg so cunningly placed as to excite the growth of a nu- tritive blastem. It is not always possible to keep the besiegers from using the waters of the moat, although there is no disinterested thought of the besiegers’ wants when the ditches are planned. So in the war- game that goes on between insect and plant, natural selection directs the moves of both players, but there is nothing generous or altruistic on either side. The means of defence against parasites which have been evolved by galls are many of them very curious. A philotrix Szeboldi and some others’ secrete a sweet glutinous secretion which, like honey dew, is particu- larly attractive to ants, and leads them to act as senti- nels in guarding the galls from parasites. Occasionally ants will go so far as to cover the secreting galls over with a concrete made of sand, leaving only a tunnel by which they pass up to reach the stores of honey dew. A glutinous secretion is sometimes found on long tufts of hair growing from the gall.