170 SCIENCE REMAKING THE WORLD 



might be a fact had to do with what was called the 

 "power of adaptation," which we now call "responses.'* 

 It was observed that plants and animals respond to 

 changes in environment, often in a striking way. I 

 have seen what were regarded as two good species 

 changed into one another by changing from a moist 

 habitat to a dry one, or the reverse. This ability to 

 respond to changing conditions seemed to indicate that 

 species are not so rigid and invariable as had been 

 supposed. 



As technique developed, and the internal structures 

 of plants and animals became known, it often happened 

 that rudimentary structures were found, which never 

 developed to a functioning stage, but which occurred 

 fully developed in related forms. For example, it was 

 found that in the developing parrot a set of embryo 

 teeth begins, but never matures. The inference was 

 natural that these structures had been functional in the 

 ancestors, but had been abandoned by some of their 

 descendants. In these days, it has become the habit to 

 call these rudimentary structures "vestiges." Plants 

 and animals are full of these vestiges. One illustration 

 in the human body is the vermiform appendix. It 

 seems safe to say that we are walking museums of 

 antiquity. As technique developed still further, the 

 embryology of plants and animals began to be studied 

 in detail, the whole progress from egg to adult being 

 observed. In very many cases, during this progress, 

 glimpses of fleeting structures and resemblances were ob- 

 tained, which had disappeared when the adult stage was 

 reached, but which related the form to other species. 



