18.01.2013 Views

The Descent of Man

The Descent of Man

The Descent of Man

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

hair were determined by light or heat; and although<br />

it can hardly be denied that some effect<br />

is thus produced, almost all observers now<br />

agree that the effect has been very small, even<br />

after exposure during many ages. But this subject<br />

will be more properly discussed when we<br />

treat <strong>of</strong> the different races <strong>of</strong> mankind. With<br />

our domestic animals there are grounds for<br />

believing that cold and damp directly affect the<br />

growth <strong>of</strong> the hair; but I have not met with any<br />

evidence on this head in the case <strong>of</strong> man.<br />

EFFECTS OF THE INCREASED USE AND<br />

DISUSE OF PARTS.<br />

It is well known that use strengthens the muscles<br />

in the individual, and complete disuse, or<br />

the destruction <strong>of</strong> the proper nerve, weakens<br />

them. When the eye is destroyed, the optic nerve<br />

<strong>of</strong>ten becomes atrophied. When an artery is<br />

tied, the lateral channels increase not only in<br />

diameter, but in the thickness and strength <strong>of</strong>

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!