30 
THE DESCENT OF MAN. 
Part I. 
described by Theile as a rudimentary repetition of the 
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 j)ia 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 body, 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 oilers 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, beinn 
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 separate 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 
