Chap. XIII. EMBRYOLOGY. 449 



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

 and 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- 

 nexion 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 animal, 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 

 can at once be recognised by their larvae as belonging 

 to the great class of crustaceans. As the embryonic 

 state of each species and group of species partially shows 

 us the structure of their less modified ancient progeni- 

 tors, we can clearly see why ancient and extinct forms 

 of life should resemble the embryos of their descend- 

 ants, — our existing species. Agassiz believes this to 

 be a law of nature ; but I am bound to confess that I 

 only hope to see the law hereafter proved true. It can 

 be proved true in those cases alone in which the ancient 

 state, now supposed to be represented in many embryos, 

 has not been obliterated, either by the successive varia- 

 tions in a long course of modification having super- 



