178 Studies in Animal Behavior 



production and maintenance of the normal form of 

 the organism. The science of developmental me- 

 chanics is of very recent growth. Its already numer- 

 ous devotees endeavor to gain a deeper insight into 

 the nature of development than it is possible to 

 obtain by the older methods of descriptive embryol- 

 og}^ However full and exact our knowledge may 

 be regarding the cleavage of the egg, the formation x 

 of the germ layers, and the differentiation of or- 

 gans; however thoroughly we may come to know 

 the sequence of events which lead from the egg cell 

 to the adult animal, such knowledge does not neces- 

 sarily reveal the threads of causal connection that 

 govern the course of development. The develop- 

 ment of any organism is a wonderfully complex 

 process. We may describe what goes on with the 

 greatest fidelity to every detail, but the causal rela- 

 tions are so involved that we can seldom discover 

 them without special methods of analysis. The sci- 

 ence of developmental mechanics set itself the task 

 not merely of describing what takes place but of 

 explaining why it takes place. Description it re- 

 gards as subsidiary to explanation. Having for its 

 aim the search for causes, it naturally adopts ex- 

 perimentation as its chief method. Developing eggs 

 are shaken apart, subjected to heat and cold, to 

 chemicals and a great variety of other external 

 agents ; particular parts of the embryo are removed, 

 displaced, or replaced by other parts, and all sorts 

 of modifications of development are induced in or- 



