248 
THE DESCENT OF MAN. 
Part I.. 
jecting supra-orbital ridges. It is not improbable that 
the texture of the hair, which differs much in the dif- 
ferent races, may stand in some kind of correlation with 
the structure of the skin ; for the colour of the hair 
and skin are certainly correlated, as is its colour and 
texture with the Mandans . 53 The colour of the skin 
and the odour emitted by it are likewise in some man- 
ner connected. With the breeds of sheep the number 
of hairs within a given space and the number of the 
excretory pores stand in some relation to each other . 54 
If we may judge from the analogy of our domesticated 
animals, many modifications of structure in man pro- 
bably come under this principle of correlated growth. 
We have now seen that the characteristic differences 
between the races of man cannot be accounted for in a 
satisfactory manner by the direct action of the condi- 
tions of life, nor by the effects of the continued use of 
parts, nor through the principle of correlation. We 
are therefore led to inquire whether slight individual 
differences, to which man is eminently liable, may 
not have been preserved and augmented during a long 
series of generations through natural selection. But 
here we are at once met by the objection that beneficial 
variations alone can be thus preserved ; and as far as 
w r e are enabled to judge (although always liable to 
error on this head) not one of the external differ- 
ences between the races of man are of any direct or 
53 Mr. Catlin states (‘ N. American Indians/ 3rd edit. 1842, vol. i. p. 
49) t hat in the whole tribe of the Mandans, about one in ten or twelve 
of the members of all ages and both sexes have bright silvery grey hair, 
which is hereditary. Now this hair is as coarse and harsh as that of 
a horse’s mane, whilst the hair of other colours is fine and soft. 
54 On the odour of the skin, Godron, 1 Sur l’Espece/ tom. ii. p. 217. 
On the pores in the skin, Dr. Wilckens, 4 Die Aufgaben der landwirth. 
Zootjchnik,’ 1869, s. 7. 
