404 



NATURE 



\Sept. I, 1 88 1 



the support of the hypothesis that the Ascidian is an earlier 

 stage ill the phylogeiietic history of the mamcaal and other 

 vertebrates." 



The larval forms which occur in so many groups, and of which 

 the Insects afford us the most familiar example^, are, in the 

 words of Quatrefages, embryos, which lead an independent life. 

 In such cases a'; these external conditions act upon the larva; as 

 they do upon the mature form ; hence we have two classes of 

 changes, adaptational or adaptive, and developmental. These 

 and many other facts must be taken into consideration ; never- 

 theless naturalists are now generally agreed that euibryological 

 characters are of high vauie as guides in classification, and it 

 may, I think, be regarded as well-established that, just as the 

 contents and sequence of rocks teach us the past history of the 

 earth, so is the gradual development of the species indicated by 

 the structure of the embryo and its developmental changes. 



When the supporters of Darwin are told that his theory is 

 incredible, they may fairly ask why it is impossible that a species 

 in the course of hundreds of thousands of years should have 

 passed through changes which occupy only a few days or weeks 

 in the life-history of each individual ? 



The phenomena of yolk-segmentation, first observed byPrevost 

 and Dumas, are now known to be in some form or other 

 invariably the precursors of embryonic development ; while they 

 reproduce, as the first stages in the formation of the higher 

 animals, the main and essential features in the life-history of the 

 lowest forms. The "blastoderm" as it is called, or first germ of 

 the embryo in the egg, divides itself into two layers, corresponding, 

 as Huxley has shown, to the two layers into which the body of 

 the Ccelenterata may be divided. Not only so, but most embryos 

 at an early stage of development have the form of a cup, the 

 walls of w hich are formed by the two layers of the blastoderm. 

 Kowalevsky was the first to show the prevalence of this embry- 

 onic form, and subsequently Lankester and Hx'ckel put forward 

 the hypothesis that it was the embryonic repetition of an ancestral 

 type, from which all the higher forms are descended. The cavity 

 of the cup is supposed to be the stomach of this simple organism, 

 and the opening of the cup the mouth. The inner layer of the 

 wall of the cup constitutes the digestive membrane, and the outer 

 the skin. To this form H:eckel gave the name Gastraea. It is 

 perhaps doubtful whether the theory of Lankester and Hceckel 

 can be accepted in precisely the form they propounded it ; but it 

 has had an important influence on the progress of embryology. 

 I cannot quit the science of embryology without alluding to the 

 very admirable work on "Comparative Embryology" by our new 

 general secretary, Mr. Balfour, and also the " Elements of Em- 

 bryology " which he had previously published in conjunction with 

 Dr. M. Foster. 



In 1S42, Steenstrup published his celebrated work on the 

 "Alternation of Generations," in which he showed that many 

 species are represented by two perfectly distinct types or broods, 

 differing in form, structure, and habits; that in one of them 

 males are entirely wanting, and that the repioduction is effected 

 by fission, or by buds, which, however, are in some cases 

 structurally indistinguishable from eggs. Steenstrup's illustra- 

 tions were mainly taken from marine or parasitic species, of very 

 great interest, but not generally familiar, excepting to naturalists. 

 It has since been shown that the common Cynips or Gallfly is 

 also a case in point. It had long been known that in some 

 genera belonging to this group, males are entirely wanting, and 

 it has now been shown by Bassett, and more thoroughly by 

 Adler, that some of these species are double-brooded ; the two 

 broods liaving been considered as distinct genera. 



Thus an insect known as Neiirotertts lenticiilaris, of which 

 females only occur, produces the familiar oak-spangles so com- 

 mon on the under sides of oak leaves, from which emerge, not 

 Ncurotcrus lenticiilaris, but an insect hitherto considered as a 

 distinct species, belonging even to a different genus, Spathegaslcr 

 baccanim. In Spathegaster both sexes occur ; they produce the 

 currant-like galls found on oaks, and from the^e galls Neuroterus 

 is again developed. So also the King Charles oak-apples 

 produce a species known as Terns terminalis, which descends 

 to the ground, and makes small galls on the roots of the oak. 

 From these emerge an insect known as Biorhiza aptera, which 

 again gives rise to the common oak-apple. 



It might seem that such inquiries as these could hardly have 

 any practical bearing. Yet it is not improbable that they may 

 lead to very important results. For instance, it would appear 

 that the fluk>? which produces the rot in sheep, passes one phase 

 of its existence in the black slug, and we are not without hopes 



that the researches, in which our lamented friend Prof. RoUeston 

 was engaged at the time of his death, which we all so much 

 deplore, will lead, if not to the extirpation, at any rate to the 

 diminution, of a pest from which our farmers have so grievously 

 suffered. 



It was in the year 1839 that Schwann and Schleiden demon- 

 strated the intimate relation in which animals and plants stand 

 to each other, by showing the identity of the laws of development 

 of the elementary parts in the two kingdoms of organic nature. 



As regards descriptive biology, by far the greater number of 

 species now recorded have beeen named and described within 

 the last half-century. 



Dr. Giinther has been good enough to make a calculation for 

 me. The numbers, of course, are only approximate, but it 

 appears that while the total number of animals described up 

 to 1831 was not more than 70,000, the number now is at least 

 320,000. 



Lastly, to show how large a field still remains for exploration, 

 I may add that Mr. Waterhouse estimates that the] British 

 Mu'cum alone contains not fewer than 12,000 species of insects 

 which have not yet been described, while our collections do not 

 probably contain anything like one-half of those actually in 

 existence. Further than this, the anatomy and habits even of 

 those which have been described offer an inexhaustible field for 

 research, and it is not going too far to say that there is not a 

 single species which would not amply repay the devotion of a 

 lifetime. 



One remarkable feature in the modern progress of biological 

 science has been the application of improved methods of observa- 

 tion and experiment ; and the employment in physiological 

 research of the exact measurements employed by the experi- 

 mental physicist. Our microscopes have been greatly improved. 

 The use of chemical re-agents in microscopical investigations has 

 proved most instructive, and another very important method of 

 investigation has been the power of obtaining very thin slices by 

 imbedding the object to be examined in paraffin or some other 

 soft substance. In this manner we can now obtain, say, 

 fifty separate sections of the egg of a beetle, or the brain of 

 a bee. 



At the close of the last century, Sprengel published a most 

 suggestive work on flowers, in which he pointed out the curious 

 relation existing between these and insects, and showed that the 

 latter carry the pollen from flower to flower. His observations, 

 however, attracted little notice until Darwin called attention to 

 the subject in 1S62. It had long been known that the cowslip 

 and primrose exist under two. forms, about equally numerous, 

 and diftering from one another in the arrangements of their 

 stamens and pistils ; the one form having the stamens on the 

 summit of the flower and the stigma half-way down ; while in 

 the other the relative positions are reversed, the stigma being 

 at the summit of the tube and the stamens half-way down. This 

 <lifference had, however, been regarded as a case of mere varia- 

 bility ; but Darwin showed it to be a beautiful provision, the 

 result of which is that insects fertilise each flower with pollen 

 brought from a different plant ; and he proved that flowers 

 fertilised with pollen from the other form yield more seed than 

 if fertilised with pollen of the same form, even if taken from a 

 different plant. 



Attention having been thus directed to the question, an 

 astonishing variety of most beautiful contrivances have been 

 observed and described by many botanists, especially Hooker, 

 Axel, Delpino, Hildebrand, Bennett, Fritz Miiller, and above 

 all Hermann Miiller and Darwin himself. The general result 

 is that to insects, and especially to bees, we owe the beauty of 

 our gardens, the sweetness of our fields. To their beneficent, 

 though unconscious action, flowers owe their scent and colour, 

 their honey — nay, in many cases, even their form. Their present 

 shape and varied arrangements, their brilliant colours, their 

 honey, and their sweet scent are all due to the selection exercised 

 by insects. 



In these cases the relation between plants and insects is one of 

 mutual advantage. In many species, however, plants present us 

 with complex arrangements adapted to protect them from insects ; 

 such, for instance, are in many cases the resinous glands which 

 render leaves unpalatable ; the thickets of hairs and other pre- 

 cautions which prevent flowers from being robbed of their honey 

 by ants. Again, more than a century ago, our countryman, Ellis, 

 described an American plant, Dionrea, in which the leaves are 

 somewhat concave, with long lateral spines and a joint in the 

 middle ; close up w ith a jerk, like a rat-trap, the moment any 



