EMBRYOLOGY 403 



explanation, however, of the embryo not undergoing 

 any metamorphosis is perhaps requisite. If, on the 

 other hand, it profited the young to follow habits of 

 life in any degree different from those of their parent, 

 and consequently to be constructed in a slightly dif- 

 ferent manner, then, on the principle of inheritance at 

 corresponding ages, the active young or larvae might 

 easily be rendered by natural selection different to any 

 conceivable extent from their parents. Such differ- 

 ences might, also, become correlated with successive 

 stages of development ; so that the larvae, in the first 

 stage, might differ greatly from the larvae in the second 

 stage, as we have seen to be the case with cirripedes. 

 The adult might become fitted for sites or habits, in 

 which organs of locomotion or of the senses, etc. , would 

 be useless ; and in this case the final metamorphosis 

 would be said to be retrograde. 



As all the organic beings, extinct and recent, which 

 have ever lived on this earth have to be classed together, 

 a'ad as all have been connected by the finest gradations, 

 the best, or indeed, if our collections were nearly perfect, 

 the only possible arrangement, would be genealogical. 

 Descent being on my view the hidden bond of con- 

 nection which naturalists have been seeking under 

 the term of the natural system. On this view we 

 can understand how it is that, in the eyes of most 

 naturalists, the structure of the embryo is even more 

 important for classification than that of the adult. For 

 the embryo is the animal in its less modified state ; 

 and in so far it reveals the structure of its progenitor. 

 In two groups of animals, however much they may at 

 present differ from each other in structure and habits, if 

 they pass through the same or similar embryonic stages., 

 we may feel assured that they have both descended 

 from the same or nearly similar parents, and are there- 

 fore in that degree closely related. Thus, community in 

 embryonic structure reveals community of descent. It 

 will reveal this community of descent, however much 

 the structure of the adult may have been modified and 

 obscured ; we have seen, for instance, that cirripedes 



