4 GENERAL PRINCIPLES OF ZOOLOG Y. 



tenous phenomena 01 the organic world, the path to the 

 full explanation of which was first broken by the Theory 

 of Descent, as will be shown in another chapter. 



Ontogeny. To Morphology belongs, as an important 

 integral part, Ontogeny or Embryology. Only a few ani- 

 mals at the beginning of their individual existence are 

 completely formed in all their parts ; most of them arise 

 from the egg, a relatively simple body, and then step by 

 step attain their permanent form in the course of compli- 

 cated changes. The morphologist must, in the completest 

 possible series, determine by observation the different 

 stages, compare 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 

 systemization as for the causal explanation of the animal 

 form. The developmental stages of man show definite 

 regular agreements, not only with the t structure of the 

 adult human being which in and of itself would be intelli- 

 gible, but also with the structure of lower vertebrates, like 

 the fishes, and even with many of the still lower animals 

 of the invertebrated 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 energy peculiar to organisms, and thereby any attempt 

 at a logical explanation of the vital processes was re- 

 nounced. Modern physiology has abandoned this theory 

 of vital energy ; it has begun the attempt to explain life 

 as the summation of extremely complicated chemico- 

 physical processes, and thus to apply to the organic world 

 those explanatory principles which prevail in the inorganic 

 realm. The results attained by this course show that this 

 is the correct method. 



Since each organic form is the product of its develop- 

 ment, since, further, the development represents to us the 



