Embryology and Phylogeny 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. Thus, community in embryonic structure 
reveals community of descent; but dissimilarity in embryonic develop- 
ment does not prove discommunity 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 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 in enabling us to 
arrive at a natural system of classification. With regard to the 
latter point, he quotes with approval the opinion that “the structure 
of the embryo is even more important for classification than that of 
the adult.” What justification is there for this view? The phase of 
life chosen for the ordinary anatomical and physiological studies, 
namely, the adult phase, is merely one of the large number of stages 
of structure through which the organism passes. 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 period, for 
the developmental changes are more numerous and take place with 
