NA TURE 



\_March 3, 188] 



cavity into a lung must necessarily be accompanied by 

 definite changes in the structure of that organ." 



After chapters expounding Prof. Semper's original 

 observations and special theory as to the formation of 

 coral islands, in which he characteristically see'.NS to 

 improve upon Mr. Darwin, and a chapter upon the 

 influence of parasitism, we come to a final chapter 

 entitled " The selective influence of living organisms 

 upon animals." Here new facts bearing upon the 

 competition for similar conditions, the relations of the 

 pursuer and the pursued, and mimicry, are set forth in 

 abundance. The curious dorsal eyes of the marine slug 

 Onchidium are described and figured, and an ingenious 

 attempt is made to account for their evolution in relation 

 to the pursuit of the Onchidium by the leaping-fish 

 Periophthalmus. Prof. Semper is not blundering when 

 he states that these eyes are constructed on what he calls 

 " a type identical with those of the vertebrata." At the 

 same time such a statement is very misleading, for these 

 eyes differ essentially in their origin and structure from 

 those of vertebrates, although having one superficial 

 resemblance to the vertebrate eye in the fact that the 

 retinal nerve is distributed to the anterior instead of to 

 the deep surface of the retinal cells. This arrangement 

 exists also in Pecten, contrary to Prof. Semper's statement 

 that Onchidium is a solitary example of its occurrence in 

 invertebrata. 



As to mimicry Prof. Semper brings forward a new 

 instance among land-snails where a Philippine Helicarion 

 ■which sheds its tail (metapodium) and so escapes when 

 seized by a bird or lizard, is imitated closely in appearance 

 by a Xesta which has not the power of shedding its tail, 

 but benefits by the reputation for elusiveness of the 

 Helicarion. On the general subject of mimicry Semper 

 does not consider the doctrine of selection adequate, but 

 thinks it necessary ta improve the current theory relating 

 to it by some original touches. He has made the not very 

 new discovery that " under some circumstances the most 

 perfect and complete resemblance between two creatures 

 not living associated may originate without its being 

 referable to the. selective power of mimicry, i.e. a protec- 

 tive resemblance." The resemblance referred to is of 

 course a superficial one of colour or appearance of 

 one part of the body, and not really "perfect" or "com- 

 plete." From this he goes on to suggest that sub- 

 sequently to this stage a necessity for protection may 

 arise, and the previously-established resemblance viay 

 become protective to one or other of the reciprocally 

 counterfeit organisms. On the strength of this suggestion 

 he proceeds further to question whether natural selection 

 has ever produced mimicry, and declares that some 

 causes " tiuisl have availed to produce by their direct 

 action an advantageous and protective change of colour- 

 ing " in the first instance. Similar to this, he states, is 

 the conclusion which is arrived at in each chapter of his 

 book in reference to other adaptations besides those 

 coming under the head of mimicry, viz. that natural 

 selection cannot operate until directly transforming 

 agencies have produced advantageous characters of a 

 definite and obvious kind upon which it may operate. 



With the whole of this reasoning, and especially with 

 the statement that any such conclusion can be derived 

 from the facts stated in earlier chapters, we disagree. 



On the contrary, we maintain that natural selection 

 operates upon advantageous variations which are exceed- 

 ingly small, and do not, by an immense interval, amount 

 to such coarse advantages as those assumed by Prof. 

 Semper. Such small variations are incessantly caused by 

 the action of external forces on the complex physiological 

 units of the parents and by the action of those of one 

 parent upon those of another. These causes of variation 

 are not transforming causes, but produce irrelative and 

 multifarious variations of small amount. It is upon these 

 that natural selection acts. The existence of such varia- 

 tions, the power of selection to intensify them, and so to 

 transform species and further the natural existence of a 

 necessary selection, have been established by Mr. Darwin 

 by an enormous mass of evidence. Prof. Semper, so far 

 from having brought his reader in each chapter to a con- 

 clusion favourable to his views, has not adduced any 

 evidence to show that natural selection cannot or does 

 not act as taught by Mr. Darwin, and has moreover 

 completely failed to adduce any evidence making it even 

 probable that large changes of structure are ever effected 

 by " directly transforming" agents," of the very existence 

 of which he can offer no evidence. Still less has he 

 succeeded in showing that natural selection does or even 

 that it could make use'of such large changes — concerning 

 which it is difficult to reason, since nothing is known about 

 them excepting that Prof. Semper believes in theni.i 



The supposed cases of minute resemblance without 

 mimicry which are given by Semper are either to be 

 explained as due to a protective resemblance to a third 

 object, or as due to like advantages secured independently 

 in each case by natural selection in a way which may 

 become apparent when we have more ample knowledge 

 of the particular cases, or lastly, as due to an accidental 

 superficial identity in two things having absolutely no 

 relations in common. To argue that the last account of 

 the matter is the true one, and that the elaborate mimicry 

 of insects is to be explained with the assumption of the 

 frequent occurrence of such coincidences rather than by 

 the doctrine of natural selection, is, it may be conceded, 



* It is necessary t^ plainly and emphatically state that Prof. Semper and a 

 few other writers of similar views (e.g.. the Rev. George Henslow in 

 Modem Thoiiglit, vol. ii. No. 5, 1881), are not adding to or 'building en 

 Mr. Darwin's theory, but are actually opposing all that is essential and 

 distinctive in that theory by the revival of the exploded notions of "directly 

 transforming agents "advocated by Lamarck and others. They do n -t 

 seem to be aware of this, for they make no attempt to seriously 

 ctamine Mr. Darwin's accumulated facts and arguments. The doctrine 

 of organic evolution has become an accepted truth entirely in con- 

 sequence of Mr. Darwin having demonstrated the mechanism by which 

 the evolution is possible : it was almost unanimously rejected, whilst 

 such undemonstrable agencies as those arbitrarily asserted to exist by Prof. 

 Semper and Mr. George Henslow were the only causes suggested by its 

 advocates. Mr. Darwin's argument rests on the prm'cd existence of minute 

 many-sided, irrelative variations not produced by directly transforming 

 agents, but showing themselves at each new act of reprotiuction as part of 

 the phenomenon of heredity. Such minute " sports " or "variations" are 

 due to constitutional disturbance, and appear not in individuals subjected to 

 new conditions, but in the offspring of all, though more freely in the off- 

 spring of those subjected to special causes of constitutional disturbance. Mr. 

 Darwin has further Jirovcd that these slight variations can be transmitted 

 and intensified by selective breeding. They have in reference to breeding 

 a remarkably tenacious or persistent character, as might be expected from 

 their origin in connection with the reproductive process. On the other hand 

 mutilations and other effects of directly transforming agents are rarely, if 

 ever, transmitted. , , , . 



It is little short of an absurdity for persons to come forward at this epoch, 

 when evolution is at length accepted solely because of Mr. Danvin's doctrine, 

 and coolly to propose to replace that doctrine by the old notion so often tried 

 and rejected. . . .,, ■ ,. • 1 



That such an attempt should be made is an illustration of a curious weak- 

 ness of humanity. Not unfrequently, after a long-contested cause ha,< 

 triumphed and all have yielded allegiance thereto, you will find when 

 few generations have passed that men have clean forgotten what or who it 

 was that made that cause triumphant, and ignorantly will set up for honour 

 the name of a traitor or of an impostor, or attribute to a great man as a merit, 

 deeds and thoughts which he spent a long life in opposing. 



