STRUCTURE AND DEVELOPMENT 57 



life-history of a chick or a cat than we need to assume 

 that gravitation and the radiation of hght demand 

 immediate supernatural direction. The embryology 

 of no form is fully understood or described or explained, 

 but no intelligent person would be willing to assert 

 that because complete knowledge is lacking, it is 

 unnatural for organic transformation to take place 

 during growth. Whatever may be the ultimate origin 

 and nature of the directing powers behind gravitation 

 and development and other phenomena, we have 

 no concern with such matters because they cannot be 

 handled by scientific methods and one belief about them 

 is on the same plane with any other. Our task is to 

 deal with the everyday phenomena of life and the 

 production of living species. 



It is not necessary to go far afield to find an animal 

 which will introduce us to the general principles of 

 embryology. In the present instance as in the case 

 of comparative anatomy almost any form will disclose 

 the meaning of development, for animate nature is 

 uniform and consistent in its methods of operation 

 throughout its wide range. We shall begin with the 

 familiar frog which every one knows is a product of a 

 tadpole ; passing on to the chick we will learn more facts 

 that will enable us to formulate the main principle of 

 comparative embryology in definite terms ; we will then 

 be prepared to extend our survey so as to include some- 

 what less familiar facts and animals that are even more 

 significant than the first illustrations. 



If we should visit a woodland pond in early spring, we 



