STRUCTURE AND DEVELOPMENT 55 



true, while still fewer are aware that the peculiar 

 and characteristic early stages through which an 

 animal passes in becoming an adult are even more 

 striking than the fact of development itself. We shall 

 learn something of these earlier conditions in the 

 development of some of our most familiar animals, 

 but at the outset nothing can be more important than 

 an appreciation of the first great lesson of this depart- 

 ment of natural history namely that organic trans- 

 formation is real and natural. We do not need to 

 employ the methods of formal logic to know that in 

 growing up a human infant undergoes the changes 

 of childhood and adolescence, that kittens become cats, 

 and that an oak tree is produced by an acorn, for we 

 know these things directly by observing them. It is 

 natural for development to take place under normal 

 conditions, and if it does not, then something has inter- 

 fered with nature. Inasmuch as ''growing up" is 

 accomplished by the alteration of an organic mechanism 

 with one structure into an individual with a changed 

 plan of body, it is in essence the actual process of 

 evolution which the comparative study of grown ani- 

 mals of to-day demonstrates in the way we have 

 learned. The study of animal structure discovers 

 the process of evolution because the most reasonable 

 interpretation of the similarities and minor differences 

 exhibited everywhere by the various groups of animals 

 is that descent with adaptive and divergent modification 

 has taken place ; the result is reached by inference, 

 it is true, but by scientific and logical inference. 

 With development it is otherwise. No reasoning is 

 necessary to tell us that organic transformation is 



