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 indispensable. We have here the clew to Semper's position as a biologist. He thinks that the school of speculative system-makers, represented by Haeckel, are given to an over-indulgence 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 prominence 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 question is, To what extent and under what conditions is it manifested? Variability is probably that trait of animate beings which may be first and most easily traced by exact investigation, both to its limits and to its efficient 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 variation, and the author aims to present the general facts and hypotheses, which are either of universal significance or offer favorable subjects for experimental treatment.
But it is desirable to still further illustrate the specialty of Dr. Semper's work. The general science of zoölogy has two great branches, morphology and physiology, which, although closely connected, are yet so widely 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 independent 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 relationships 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 conditions that have produced existing forms. Physiology explains what morphology describes; and, in this large sense, it is the task of physiology to give account of the facts which morphology embraces in its natural system.
But, from this point of view, physiology itself has two broad divisions. Simple physiology, 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 contradistinction to this conception of the physiology of organs, there is also a more comprehensive 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 question 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 geographical distribution of animals, of their extension into new habitats, of their extermination, 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 result would be the rapid and utter extinction of the now numerous herds of buffaloes, and
