166 THE AMERICAN NATURALIST [VouXLIX 



possibly undergo the first stages of development, the 

 preliminary segmentation, within the oviduct of the 

 parent just as well as in the water, for in both cases 

 they would be enclosed in their envelopes, and the 

 morphological differences between the early stages in 

 the two cases might be expected to be quite insignificant. 

 But it must be the same at each term of the series, for 

 each term is built upon the foundation of the preceding 

 one, and the whole process takes place by slow and im- 

 perceptible degrees. 



It is true that by the time we reach the formation of 

 the vestigial gill-slits in the embryo of one of the higher 

 vertebrates the environmental conditions are very dif- 

 ferent from those under which gill-slits were developed 

 in their aquatic ancestors. But what then? Are not 

 the gill-slits also very different! The changed environ- 

 ment has had its effect. The gills themselves are never 

 developed, and the gill-slits never become functional; 

 moreover, they disappear completely at later stages of 

 development, when the conditions of life become still 

 more different and their presence would be actually det- 

 rimental to their possessor. The embryo with the ves- 

 tigial gill-slits is, as a whole, perfectly well adapted to 

 its environment, though the gill-slits themselves have 

 ceased to be adaptive characters. They still appear be- 

 cause the environmental conditions, and especially the 

 internal conditions, which have now become far more 

 important than the external ones, are still such as to 

 cause them to do so. 



I think the chief difficulty in forming a mental picture 

 of the manner in which evolution has taken place, and 

 especially in accounting for the phenomenon of recapitu- 

 lation in ontogeny, which is merely another aspect of 

 the same problem, arises from attempting to take in too 

 much at once. There is no difficulty in understanding 

 how any particular stage is related to the corresponding 

 stage in the previous generation, and the whole series of 

 stages, whether looked at from the ontogenetic or from 



