2l6 



SCIENCE. 



BOOKS RECEIVED. 

 Animal Life as affected by the Natural Con- 

 ditions of Existence, by Karl Semper, Professor 

 of the University of Wiirzbjrg. With two maps and 

 one hundred and six wood cuts. D. Appleton & Co., 

 New York, 1881. 

 Naturalists have been more than once taunted with too 

 much philosophizing over the Darwinian theory, that they 

 were content to form fanciful ideas as to how this or that 

 difficulty could be hypothetically explained, and that fun- 

 damental causes — equally fanciful — -were imagined to 

 account for results which were actually observed. 



We apprehend that if the Darwinian theory is to be- 

 come a scientific dogma, the future course of naturalists 

 must lie in the direction of applying the test of exact in- 

 vestigation to the hypotheses already laid down. The 

 task is doubtless a laborious one, and Professor Semper 

 himself says that to prove by experiment the truth of 

 many of these hypotheses long and deep researches are 

 indispensable, or the student will find himself wrecked 

 upon insurmountable difficulties. 



There are a number of eminent naturalists whose works 

 tend in this direction, and Professor Semper now leads 

 the van of those who would systematically apply them- 

 selves to this task. 



Considering that Variability is one of the properties of 

 the animal kingdom which might be most easily traced by 

 exact investigation to its efficient causes, Professor Semper 

 has made it the subject matter of his book, and to facili- 

 tate the task of himself and others, has presented a gen- 

 eral view of those facts and hypotheses which bear upon 

 the subject, and which are either of universal significance 

 or appear to offer favorable subjects for experimental 

 treatment. 



ft is not claimed that this work is a complete review of 

 even this branch of the enquiry, but it lays out a plan for- 

 tified by a long array of facts, showing how the enquiry 

 may be systematically conducted. It is thus a protest 

 against casual and disconnected observation, and as such 

 may be read with profit by every student. 



The introductory chapters are of much interest, deal- 

 ing with some of the salient points of the Darwinian theory. 

 The plan of the work is also explained and the reader in- 

 troduced to the subject. 



The main body of the book is divided into two sec- 

 tions. The first treats of the influence of inanimate sur- 

 roundings, and in th ; s division Professor Semper directs 

 attention to the influence of food, light, and temperature 

 upon organisms. The results attributable to svater, both 

 still, and in motion, are explained, and finally other influ- 

 ences are considered. 



In the concluding portion of this work, the influence 

 of living surroundings is discussed in such a masterly 

 manner, as to be of the highest service to those studying 

 this subject. 



We notice that the subject of the geographical distribu- 

 tion of animals is discussed by Professor Semper, who 

 points out the chief difficulties in bringing into accord 

 the various hypotheses, suggested to explain the un- 

 doubted fact that certain species overstep the limits 

 apparently assigned to them by Nature. 



Whenever any extensive resemblance between the 

 faunas of two distinct countries is discovered or imagined, 

 a hypothetical history of upheavals and subsidences is 

 suggested, to form a bridge of mainland, as a mode ol 

 accounting for this resemblance. This appears to be a 

 favorite theory of Mr. Wallace, and Professor Semper 

 himself admits that such must have been the case 

 in some instances, as he himself found an Indian 

 elephant on Mindanao, the most southerly of the Philip- 

 pines, for such an animal could scarcely have made the 

 passage by sea. Nevertheless, Professor Semper con- 

 siders these hypothetical connections of the islands and 



mainland as not sufficient by themselves to explain even 

 those facts which are already known, as to the distribu- 

 tion of Indian and Australian forms on the islands lying 

 between the two continents. 



He further states that " until the question is finally set- 

 tled whether two parallel series of animal development 

 might not have proceded independently in two countries 

 remote from each other, we can never venture to regard 

 the resemblance of two faunas as conclusive evidence of 

 their primaeval actual connection ; nay, it even seems to 

 me that the two historical series of species of the horse, 

 recently discovered both in Europe and America, may, 

 on the contrary, be regarded almost as a proof that each 

 series was developed independently on the two continents, 

 and yet led to the same result : namely, the production of 

 the horse." 



Leaving this, however, as an open question. Professor 

 Semper advances a theory for accounting generally for 

 irregularities in the geographical distribution of animals, 

 by suggesting that the action of currents and winds co- 

 operated in a large degree in producing the results which 

 are found to exist. 



As a means of distributing animal life it is evident that 

 winds and currents conveyed certain animals from place 

 to place, but Professor Semper points out that these influ- 

 ences frequently acted as a hindrance to the distribution 

 of species. Every navigator is familiar with the fact that 

 currents have a dividing power, shown by the tendency of 

 objects to drift to the edge of the stream, although they may 

 have fallen into the middle of it. This tendency of the 

 current to clear itself — or clean itself — is stronger in pro- 

 portion to its rapidity and strength. Hence, objects torn 

 by a stream flowing between two islands from the one 

 lying to the left of it, could be borne to that on the right 

 side only under specially favoring circumstances ; and 

 vice versa, those brought from the right could never, or 

 very rarely, be carried to the opposite side. Thus a mix- 

 ture of the faunas of the two islands might be hindered, 

 simply by the action of the current flowing between 

 them, except in the case of free swimming animals having 

 the power to overcome the mechanical resistance of the 

 current. In considering the striking circumstance that 

 the islands lying close to Africa have quite a different 

 fauna from that of the neighboring continent, this influ- 

 ence is mentioned as a factor. 



Between these theories, offered respectively by Mr. 

 Wallace and Professor Semper, no positive conclusions 

 can be drawn, for want of sufficient evidence based on 

 general conclusions, and while neither can be rejected as 

 erroneous, both must remain open for future discussion. 

 Professor Semper, however, claims one advantage that his 

 hypothesis appeals for proof only to such elements as can 

 be brought under direct observation, while that of Mr. 

 Wallace is intrinsically incapable of demonstration by ob- 

 servation. 



The work concludes with sixty pages of closely-printed 

 notes, containing much useful information, and a long 

 array of facts bearing on the subject matter of the work. 



We have probably shown by this review that Professor 

 Semper has presented a work of the highest value to 

 every naturalist, and we can assure the general reader 

 that he will find in it material that will engross his atten- 

 tion, and cause him to regret the moment when he ar- 

 rives at the last pages. 



ERRATUM. 



Mr. Dopp desires to make the following correction in his 

 paper in the last issue : 



"In my article on page 200 of " Science," the expres- 

 sion A = — and X' = — should have been X ■=» — and A' =■ 

 n n' n 



y 



— V being the velocity of light. 

 n 



