NATURE 



405 



THURSDAY, MARCH 3, 1S81 



NATURAL CONDITIONS AND ANIMAL LIFE 



The Natural Conditions of Existence as they Affect 



Animal Life. By Karl Semper, Professor in the 



University of Wiirzburg. International Scientific 



Series. (London : Kegan Paul and Co., 1881.) 



THIS is in many respects one of the most interesting 

 contributions to zoological literature which has 

 appeared for some time. The author is well known 

 as an accomplished anatomist and microscopist who, 

 after spending some years in exploring the fauna of 

 the Philippine and neighbouring islands, returned to 

 Europe, and having been appointed to the Chair of 

 Zoology in Wiirzburg, set himself to work at the mor- 

 phological problems which so largely occupy at pre- 

 sent the attention of anatomists. His most remarkable 

 productions in this department have been his speculations 

 and observations on the segmentation of animals and on 

 the origin of the vertebrate kidney. But Prof. Semper 

 has the advantage of being something more than an 

 anatomist ; as a traveller and one who has seen and 

 studied life under most varied conditions, he has thought 

 much and collected many facts bearing upon the problem 

 of the influence of changed conditions of life in modifying 

 the structure of animals submitted to those conditions. 



With the leading theoretical consideration advanced by 

 Prof. Semper no naturalist who knows the history of evo- 

 lutional theory will agree, but the large collection of well- 

 described and well-illustrated facts for which he claims 

 attention in consequence of his theoretical preconceptions, 

 are none the less interesting. The book has the great 

 merit of being one v/liich will be found equally readable 

 by the professed zoologist and by the general reader. 



Prof. Semper, whilst accepting the doctrine of the 

 origin of new forms of life by the natural selection of 

 fittest varieties of pre-existing forms, is unable to conceive 

 of the " fittest varieties" in question, being such slightly 

 divergent forms as are normally to be found in the 

 offspring of all parents. Though he does not explicitly 

 deny the physiological importance of even such minute 

 variations as are not readily perceived by the human eye, 

 and consequently does not openly controvert Mr. Darwin's 

 theory to the effect that such of these minute variations 

 as are fitted to given conditions of existence, are per- 

 petuated and intensified by the survival of those animals 

 in which they occur, and the failure and death of those in 

 which they do not occur, yet Prof. Semper is among those 

 who look for a more rapid and conspicuous method of the 

 production of new species than that taught by pure 

 Darwinism. He thinks that Mr. Darwin has overlooked 

 or underrated the importance of " directfy-transfoiining 

 agents." He is no doubt aware that it is equally possible 

 to over-estimate the importance of such action, and that 

 this was done by Mr. Darwin's predecessors. Accordingly 

 he examines in the volume before us such cases as may 

 tend to give evidence on the subject. 



Such cases are to be found when an animal living 



upon special food, or in given temperature, or light, or 



in water (still or running, fresh or saline), or air (dry or 



moist, still or breezy), or in isolation, or as parasite, is 



Vol. XXIII. — No. 592 



subjected to a change in those conditions either by natural 

 processes or by experiment. A large series of natural 

 instances are afforded by pairs of representative species 

 of one genus, the one living under one set of condi- 

 tions, the other under conditions in which the factor, 

 the influence of which is sought, is removed or altered. 

 Very few experiments, as Prof. Semper remarks, have 

 been made upon this subjecl, but some of remarkable 

 interest are cited. 



The result of the examination of the instances which 

 have been gathered together in this volume is not 

 such as to lead to the conclusion that directly trans- 

 forming agents play an important part in the produc- 

 tion of new species. " Changed conditions," Mr Darwin 

 has said, " induce an almost indefinite amount of 

 fluctuating variability, by which the whole organism 

 is rendered in some degree plastic," and it is to the 

 non-significant variations so produced which are selected 

 by survival and fixed by heredity that new forms are 

 due, and not to those direct adaptations effected in 

 the individual by changed conditions, which are remark- 

 ably rare, and moreover, as Prof. Semper recognises 

 (p. 38), are not transmitted, as a rule, to offspring. In 

 order to establish his point Prof. Semper should have 

 been able to give us, firstly, numerous instances of 

 change of structure in the individual brought about in 

 adaptation to a change in that individual's conditions of 

 life. He produces very few, whilst the most striking and 

 numerous facts whicli he records are instances of physio- 

 logical adaptation to or toleration of new conditions 

 without any corresponding change of structure. Secondly, 

 he should have been able to give instances of the trans- 

 mission to offspring of pecuUarities acquired by the parent 

 by undoubted action of the environment on the individual 

 parent. Such instances are excessively rare, though a 

 few aic on record; but none are cited by Prof. Semper, 

 and indeed the evidence as at present before us is such 

 as to warrant the conclusion that such transmission 

 cannot be in any way an important factor in the produc- 

 tion of new races. 



In his concluding paragraph (p. 405) Prof. Semper 

 states that "there is a universal difficulty of deciding 

 whether a modification which has taken place is to be 

 ascribed to some direct determining and modifying cause, 

 or to the enhancing of a previously modified character 

 which is frequently connected with selection," and then 

 deprecates the habit of theoretical e.xplanations from 

 general propositions. He holds apparently that we are 

 not to seek an explanation of such modifications in those 

 truths of heredity and adaptation, of variation and selec- 

 tion, which have been actually demonstrated and esta- 

 blished by Mr. Darwin, but must, if we would behave as 

 right-minded philosophers, keep before us the possibility 

 of these modifications being due to — what ? Not to a 

 cause which has been shown to be necessarily or even 

 usually at work, as have those to which Mr. Darwin 

 points, but to a cause which has always proved illusory, 

 namely, the "directly-transforming" action of the en- 

 vironment. It was because they appealed to this cause 

 and could not show~that it had a real existence that the 

 " transformists " of the beginning of this century failed, 

 where Mr. Darwin, appealing to another cause which he 

 showed was an existing cause, has succeeded. Prof. 



