Chap. IV. 
MANNER OF DEVELOPMENT. 
151 
to the os coccyx, which in man and the higher apes 
manifestly consists of the few basal and tapering seg- 
ments of an ordinary tail, I have heard it asked liow 
could these have become completely embedded within 
the body ; but there is no difficulty in this respect, 
for in many monkeys the basal segments of the true 
tail are thus embedded. 1 or instance, Mr. Murie in- 
forms me that in the skeleton of a not full-grown 
Naiacus inormtus , he counted nine or ten caudal ver- 
tebra, which altogether were only 1’8 inch in length. 
Of these the three basal ones appeared to have been 
embedded ; the remainder forming the free part of the 
tail, which was only one inch in length, and half an 
inch in diameter. Here, then, the three embedded 
caudal vertebrae plainly correspond with the four coal- 
esced vertebrae of the human os coccyx. 
I have now endeavoured to shew that some of the 
most distinctive characters ol man have in all proba- 
bility been acquired, either directly, or more commonly 
indirectly, through natural selection. We should bear 
in mind that modifications in structure or constitution, 
which are of no service to an organism in adapt- 
ing it to its habits of life, to the food which it con- 
sumes, or passively to the surrounding conditions, can- 
not have been thus acquired. W e must not, however, 
be too confident in deciding what modifications are of 
service to each being : we should remember how little 
We know about the use of many parts, or what changes 
in the blood or tissues may serve to fit an organism for 
a new climate or some new kind ot food. Nor must 
we forget the principle of correlation, by which, as 
Isidore Geoffroy has shewn in the case of man, many- 
strange deviations of structure are tied together. Inde- 
pendently of correlation, a change in one part often leads 
