30 THE DESCENT OF MAN. Part I. 



extensor of the tail, which is so largely developed in 

 many mammals. 



The spinal cord in man extends only as far down- 

 wards as the last dorsal or first lumbar vertebra ; but a 

 thread-like structure (the filum terminate) runs down the 

 axis of the sacral part of the spinal canal, and even 

 along the back of the coccygeal bones. The upper 

 part of this filament, as Prof. Turner informs me, is 

 undoubtedly homologous with the spinal cord ; but the 

 lower part apparently consists merely of the pia mater, 

 or vascular investing membrane. Even in this case the 

 os coccyx may be said to possess a vestige of so im- 

 portant a structure as the spinal cord, though no longer 

 enclosed within a bony canal. The following fact, for 

 which I am also indebted to Prof. Turner, shews how 

 closely the os coccyx corresponds with the true tail in 

 the lower animals: Luschka has recently discovered at 

 the extremity of the coccygeal bones a very peculiar 

 convoluted bodv, which is continuous with the middle 

 sacral artery ; and this discovery led Krause and Meyer 

 to examine the tail of a monkey (Macacus) and of a cat, 

 in both of which they found, though not at the extre- 

 mity, a similarly convoluted body. 



The reproductive system offers various rudimentary 

 structures ; but these differ in one important respect 

 from the foregoing cases. We are not here concerned 

 with a vestige of a part which does not belong to the 

 species in an efficient state; but with a part which 

 is always present and efficient in the one sex, being 

 represented in the other by a mere rudiment. Never- 

 theless, the occurrence of such rudiments is as diffi- 

 cult to explain on the belief of the sejmrate creation 

 of each species, as in the foregoing cases. Hereafter 

 I shall have to recur to these rudiments, and shall 

 shew that their presence generally depends merely on 

 inheritance ; namely, on parts acquired by one sex 



