LITERARY NOTICES. 



699 



laid down. It is infinitely easy to form 

 a fanciful idea as to how this or that fact 

 may be hypothetically explained, and very 

 little trouble is needed to imagine some 

 process by which hypothetical fundamental 

 causes, equally fanciful, may have led to 

 the result which has been actually observed. 

 But, when we try to prove by experiment 

 that this imaginary process of development 

 is indeed the true and inevitable one, much 

 time and laborious research are indispen- 

 sable. We have here the clew to Semper's 

 position as a biologist. He thinks that the 

 school of speculative system-makers, repre- 

 sented by Haeckel, are given to an over-in- 

 dulgence in hypotheses, and might better 

 concentrate their efforts upon the work of 

 observation and experiment, and the more 

 rigorous investigation of facts. 



Of the problems brought into promi- 

 nence by the doctrine of evolution, none is 

 more fundamental than that of variability 

 in animal organisms. It has, of course, 

 long been known that animals possess this 

 property, but the critical and unsettled ques- 

 tion is, To what extent and under what con- 

 ditions is it manifested ? Variability is prob- 

 ably that trait of animate beings which may 

 be first and most easily traced by exact in- 

 vestigation, both to its limits and to its effi- 

 cient causes. There is, however, at present 

 much strife of opinion upon the subject, 

 and this can be only harmonized by closer 

 research. The present volume is devoted 

 to this inquiry. It is a study in organic va- 

 riation, and the author aims to present the 

 general facts and hypotheses, which are 

 either of universal significance or offer 

 favorable subjects for experimental treat- 

 ment. 



But it is desirable to still further illus- 

 trate the specialty of Dr. Semper's work. 

 The general science of zoology has two great 

 branches, morphology and physiology, which, 

 although closely connected, are yet so wide- 

 ly different, both as to their details and to 

 the paths they have struck out for solving 

 their respective problems, that it becomes 

 necessary to keep them separate as two in- 

 dependent divisions of science. Morphology, 

 or the science of form and structure, aims 

 to discover those affinities of relationship in 

 animals which actually exist, and to found 

 on them a natural system of the animal 



kingdom. It is a statical inquiry that is, 

 it delineates the conditions and relation- 

 ships of organic structures, their differences 

 and similarities, simply as existing facts, 

 with no necessary reference to the manner 

 in which they have been produced. Were 

 all life suddenly destroyed upon the earth, 

 and nothing left but dead organisms capable 

 of dissection, there would still remain the 

 material for morphological study. 



Physiology, on the other hand, deals 

 with the dynamics of life. It investigates 

 the functions or activities of living parts, 

 and elucidates the forces, causes, and con- 

 ditions that have produced existing forms. 

 Physiology explains what morphology de- 

 scribes ; and, in this large sense, it is the 

 task of physiology to give account of the 

 facts which morphology embraces in its nat- 

 ural system. 



But, from this point of view, physiology 

 itself has two broad divisions. Simple phys- 

 iology, as it is usually known, treats of the 

 activities or the functions of the organs, 

 such as the brain, stomach, heart, muscles, 

 spinal cord, lungs, kidneys, etc., which may 

 be considered as carrying on independent 

 processes, or in their vitally coordinated, 

 intimate, mutual relations. But, in contra- 

 distinction to this conception of the physi- 

 ology of organs, there is also a more com- 

 prehensive physiology of animal organisms, 

 which may be properly termed universal 

 physiology. It treats of the general causes, 

 conditions, and laws of the development of 

 organisms, and of the transmutation of one 

 form into others. Here we meet the ques- 

 tion of the relation of organisms to their 

 environing conditions, and of animals as 

 acted upon and modified by the external 

 forces of nature. The problem of the geo- 

 graphical distribution of animals, of their 

 extension into new habitats, of their ex- 

 termination, the acquirement of divergent 

 traits and new qualities, and of the origin 

 of species, is here presented. We have 

 a new order of dependences, analogous to 

 the mutual dependence of organs in common 

 physiology, but it is now a dependence upon 

 the conditions of external nature. Professor 

 Semper says: "If the American prairies 

 were to cease to produce grass, the first re- 

 sult would be the rapid .and utter extinction 

 of the now numerous herds of buffaloes, and 



