372 RUDIMENTARY, ATROPHIED, [CHAP. XIV. 



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 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 riot strictly 

 hold good in those cases in which 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 descend- 

 ants; for such larval will not resemble any still more ancient 

 form in its adult state. 



Thus, as it seems to me, the leading facts in embryology, which 

 are second to none in importance, are explained on the principle 

 of variations in the many descendants from some one ancient 

 progenitor, having appeared at a not very early period of life, and 

 having been inherited at a corresponding period. Embryology 

 rises greatly in interest, when we look at the embryo as a picture, 

 more or less obscured, of the progenitor, either in its adult or 

 larval state, of all the members of the same great class. 



Rudimentary, Atrophied, and Aborted Organs. 



Organs or parts in this strange condition, bearing the plain 

 stamp of inutility, are extremely common, or even general, 

 throughout nature. It would be impossible to name one of the 

 higher animals in which some part or other is not in a rudimentary 

 condition. In the mammalia, for instance, the males possess 

 rudimentary mammae; in snakes one lobe of the lungs is rudi- 

 mentary ; in birds the " bastard-wing " may safely be considered 

 as a rudimentary digit, and in some species the whole wing is so 

 far rudimentary that it cannot be used for flight. What can be 

 more curious than the presence of teeth in foetal whales, which 



