254 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 



suppressed, or may 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, com- 

 munity of origin is often revealed by the structure of 

 the larvae ; we have seen, for instance, that cirripedes, 

 though externally so like shell-fish, are at once known 

 by their larvae 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 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 super- 

 vened at a very early period of growth, or by such 

 variations having been inherited 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 winch an ancient form 

 became adapted in its larvae state to some special line 

 of life, and transmitted the same larval state to a whole 

 group of descendants ; for such larval will not resemble 

 any still more ancient form in its adult state. 



Thus, as it seems to me, the leading fact3 in 

 embryology, which are second to none in importance, 

 are explained on the principle of variations in the 



