INTRODUCTION. 3 



sufficient to give the subject the character of a science ; an addi- 

 tional mental elaboration of this material is necessary. Such a 

 result is reached by comparison. The morphologist compares 

 animals with each other according to their structure, in order to 

 ascertain what parts of the organization recur everywhere, what 

 only within narrow limits, possibly restricted to the representatives 

 of a single species. He thus gains a double advantage: (1) an 

 insight into the relationships of animals, and hence the foundation 

 for a Natural System ; (2) the evidence of the laws which govern 

 organisms. Any organism is not a structure which has arisen 

 independently and which is hence intelligible by itself: it stands 

 rather in a regular dependent relation to the other members of the 

 animal kingdom. We can only understand its structure when we 

 compare it with the closely and the more distantly related animals, 

 e.g., when we compare man with the other vertebrates and with 

 many lower invertebrate forms. Here we have to consider one of 

 the most mysterious phenomena of the organic world, the path to 

 the full explanation of which was first broken by the Theory of 

 Evolution, as will be shown in another chapter. 



Ontogeny. To morphology belongs, as an important integral 

 part, Ontogeny or Embryology. Only a few animals are com- 

 pletely formed in all their parts at the beginning of their individual 

 existence; most of them arise from the egg, a relatively simple 

 body, and then step by step attain their permanent form by com- 

 plicated changes. The morphologist must, with the completest 

 possible series, determine by observation the different stages, com- 

 pare them with the mature animals, and with the structure and 

 developmental stages of other animals. Here is revealed to him 

 the same conformity to law which dominates the mature animals, 

 and a knowledge of this conformity is of fundamental importance 

 as well for classification as for the causal explanation of the animal 

 form. The df3velopmental stages of man show definite regular 

 agreements, not only with the structure of the adult human being, 

 which in and of itself would be intelligible, but also with the 

 structure of lower vertebrates, like the fishes, and even with many 

 of the still lower animals of the invertebrate groups. 



Physiology. In the same way as the morphologist studies the 

 structure, the physiologist studies the vital phenomena of animals 

 and the functions of their organs. Formerly life was regarded as 

 the expression of a special vital force peculiar to organisms, and 

 any attempt at a logical explanation of the vital processes was 

 thereby renounced. Modern physiology has abandoned this theory 



