36 WHAT EVOLUTION IS 



The peculiarity of development, 

 that higher animals pass in a tempo- 

 rary way through stages that are per- 

 manent in lower forms, has long been 

 recognized as a characteristic feature 

 of general growth. It has sometimes 

 been dignified as a law of develop- 

 ment and has been designated, in 

 honor of the father of modern em- 

 bryology, von Baer's law. As such it 

 was strongly advocated by Louis 

 Agassiz. In a more descriptive way 

 it has been spoken of as the law of 

 recapitulation, for the reason that 

 such features in the development of 

 an animal as those already alluded to 

 recapitulate, in a rough way, the ra- 

 cial history of the animal concerned. 

 Thus the presence of gill slits in the 

 embryo of the human being indicates 

 that a gill-breathing animal is to be 

 included in our remote ancestry. As 

 Huxley facetiously remarked in dis- 



