EMBRYOLOGY ^68 



classes; and as all within each class have, according to 

 our theory, been connected together by fine gradations, 

 the best, and, if our collections were nearly perfect, the 

 only possible arrangement, would be genealogical; descent 

 being the hidden bond of connection which naturalists 

 have been seeking under the term of the Natural Sys- 

 tem. 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. In two or more groups of animals, however much 

 they may differ from each other in structure and habits 

 in their adult condition, if they pass through closely 

 similar embryonic stages, we may feel assured that they 

 all are descended from one parent form, and are therefore 

 closely related. Thus, community in embryonic structure 

 reveals community of descent; but dissimilarity in em- 

 bryonic development does not prove discomm unity of 

 descent, for in one of two groups the developmental 

 stages may have been suppressed, or may have been 

 so greatly modified through adaptation to new habits of 

 life as to be no longer recognizable. Even in groups, 

 in which the adults have been modified to an extreme 

 degree, community of origin is often revealed by the 

 structure of the larvae; we have seen, for instance, that 

 cirripeds, though externally so like shell-fish, are at 

 once known by their larvse to belong to the great class 

 of crustaceans. As the embryo often shows us more or 

 less plainly the structure of the less modified and ancient 

 progenitor of the group, we can see why ancient and 

 extinct forms so often resemble in their adult state the 

 embryos of existing species of the same class. Agassiz 

 believes this to be a universal law of nature; and we 



