404 ON THE ORIGIN OF SPECIES 



can at once be recognised by their larvae as belonging 

 to the great class of crustaceans. As the embryonic 

 state of each species and group of species partially shows 

 us the structure of their less modified ancient progeni- 

 tors, we can clearly see why ancient and extinct forms 

 of life should resemble the embryos of their descend- 

 ants, — our existing species. Agassiz believes this to 

 be a law of nature ; but I am bound to confess that I 

 only hope to see the law hereafter proved true. It can 

 be proved true in those cases alone in which the ancient 

 state, now supposed to be represented in existing em- 

 bryos, has not been obliterated, either by the successive 

 variations in a long course of modification having super- 

 vened at a very early age, or by the variations having 

 been inherited at an earlier period than that at which 

 they first appeared. It should also be borne in mind, 

 that the supposed law of resemblance of ancient forms 

 of life to the embryonic stages of recent forms, may be 

 true, but yet, owing to the geological record not ex- 

 tending far enough back in time, mayTemain for a loi?g 

 period, or for ever, incapable of demonstration. 



Thus, as it seems to me, the leading facts in embryo- 

 logy, which are second in importance to none in natural 

 history, are explained on the principle of slight modifi- 

 cations not appearing, in the many descendants froi 

 some one ancient progenitor, at a very early period ii 

 the life of each, though perhaps caused at the earliest, 

 and being inherited at a corresponding not earlj 

 period. Embryology rises greatly in interest, when w€ 

 thus look at the embryo as a picture, more or less 

 obscured, of the common parent-form of each great class 

 of animals. 



Rudimentary , atrophied, or aborted Organs. — Organs 

 or parts in this strange condition, bearing the stamp of 

 inutility, are extremely common throughout nature. 

 For instance, rudimentary mammae are very general 

 the males of mammals : I presume that the " bastard- 

 wing" in birds may be safely considered as a digit ii 

 a rudimentary state : in very many snakes one lobe of 



