THE EPIC OF EVOLUTION 301 



The presence of useless vestigial structures, left behind during the 

 forward march of individual development, furnishes a hint of former 

 differences in the ancestral make-up, and is evidence that evolution 

 has occurred. For example, the useless downy hair (lanugo) that 

 covers the entire body, including even the face, of the human embryo 

 during its earlier stages is at least a reminder of other mammals 

 that are clothed all over with hair. It is hard to explain such cases 

 except upon the supposition of relationship and the evolution of one 

 form from another. 



A garage with horse stalls and mangers in it would obviously be a 

 horse barn made over to meet the modern demands of the automo- 

 bile. The stalls and mangers, like the vestigial organs that it has 

 not been imperative to remove, indicate not a "specially created" 

 garage, but the evolution of a horse barn into a garage. Embry- 

 ology is rich in instances of vestigial organs which point to the fact 

 that evolution has been going on. 



But the idea that ontogeny, or the development of the individual, 

 faithfully repeats phylogeny, that is, the ancestral evolution of the 

 race, though certainly very suggestive, nevertheless has its limita- 

 tions. In the first place it is too much to ask of a hen's egg, which 

 can develop into a chick in three weeks, to rehearse word for word 

 a phylogenetic story that has required a million years to accom- 

 plish. Countless episodes would naturally have to be omitted. 

 Certain embryonic structures, moreover, such as the yolk sac, the 

 amnion, and the allantois, have no counterpart in the adult ancestry 

 of the race. Another limitation is that the larval stages, exposed to 

 environmental adjustments, may become modified into temporary 

 emergency devices having no phylogenetic significance. 



There have been various attempts to make sense out of the obvi- 

 ous parallel between embryonic development and organic evolution. 

 The Recapitulation Theory, or as Haeckel (1834-1919) named it, the 

 ''Biogenetic Law," assumes that higher forms of life during their 

 embryonic development pass through stages attained by adult organ- 

 isms of the lower orders. Fishes, amphibians, reptiles, birds, and 

 mammals, for instance, beginning alike at the egg stage, become 

 adult by stopping at various levels, as shown in the figure at the 

 bottom of page 502, in which the vertical lines represent embryonic 

 development, and the horizontal lines the attainment of the adult 

 condition. The vertical line of mammals at the right represents 

 ontogeny repeating phylogeny. 



