254 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 



have been so greatly modified through adaptation to 

 new habits of life, as to be no longer recognisable. Even 

 in groups, in which the adults have been modified to 

 an extreme degree, community of origin is often re- 

 vealed by the structure of the larvse; we have seen, for 

 instance, that cirripedes, though externally so like shell- 

 fish, are at once known by their larvffi to belong to the 

 great class of crustaceans. As the embryo often shows 

 us more or less plainly the structure of the less modi- 

 fied 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 may hope hereafter to see the law proved 

 true. It can, however, be proved true only in those 

 cases in which the ancient state of the progenitor of 

 the group has not been wholly obliterated, either by 

 successive variations having supervened at a very early 

 period of growth, or by such variations having been in- 

 herited at an earlier age than that at which they first 

 appeared. It should also be borne in mind, that the 

 law may be true, but yet, owing to the geological record 

 not extending far enough back in time, may remain for 

 a long period, or for ever, incapable of demonstration. 

 The law will not strictly hold good in those cases in 

 which an ancient form became adapted in its larval state 

 to some special line of life, and transmitted the same 

 larval state to a whole group of descendants; for such 

 larvae will not resemble any still more ancient form in 

 its adult state. 



Thus, as it seems to me, the leading facts in embry- 

 ology, which are second to none in importance, are ex- 

 plained on the principle of variations in the many de- 



