Emhnjology and Phylogemj 173 



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. JThus^ community in embryonic structure 

 reveals community of descent; but dissimilarity in embryonic develop- 

 lifent does not prove discommunity of descent, for in one of tAvo 

 gi'oups 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 recognisable. 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 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 

 supervened at a very early period of growth, or by such variations 

 having been inherited at an earlier stage 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." 



As this passage shows, Darwin held that embryology was of 

 interest because of the light it seems to throw upon ancestral history 

 (phylogeny) and because of the help it would give hi enabling us to 

 arrive at a natural system of classification. With regard to the 

 latter point, he ([uotes with approval the opinion that "the structure 

 of the embryo is even more important for classification than that of 

 the adult." AVluit justification is there for this view? The phase of 

 life chosen for the ordinary anatomical and physiological studies, 

 namely, the adult i)hHse, is merely one of the large number of stages 

 of structure through which the organism pfissos. By far the greater 

 number of these are included in what is specially called the develop- 

 mental or (if we include larvae with embryos) embryonic i)ori()d, for 

 the deveh^pmental changes arc iiioic numerous and take place with 



