DEVELOPMENT AND EMBRYOLOGY. 467 



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, how- 

 ever, 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 not 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 descendants; 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 inter- 

 est, 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 mamma- 

 lia, for instance, the males possess rudimentary mammae; 

 in snakes one lobe of the lungs is rudimentary; in birds 

 the "bastard-wing'^ may safely be considered as a rudir 

 mentary digit, and in some species the wliole wing is so fa- 



