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On The Generation Of Animals   


of judging the differences of quality in the objects he is the best of
all. The reason is that the sense-organ in man is pure and least
earthy and material, and he is by nature the thinnest-skinned of all
animals for his size.

The workmanship of Nature is admirable also in the seal, for
though a viviparous quadruped it has no ears but only passages for
hearing. This is because its life is passed in the water; now the
ear is a part added to the passages to preserve the movement of the
air at a distance; therefore an ear is no use to it but would even
bring about the contrary result by receiving a mass of water into
itself.

We have thus spoken of sight, hearing, and smell.

3

As for hair, men differ in this themselves at different ages, and
also from all other kinds of animals that have hair. These are
almost all which are internally viviparous, for even when the covering
of such animals is spiny it must be considered as a kind of hair, as
in the land hedgehog and any other such animal among the vivipara.
Hairs differ in respect of hardness and softness, length and
shortness, straightness and curliness, quantity and scantiness, and in
addition to these qualities, in their colours, whiteness and blackness
and the intermediate shades. They differ also in some of these
respects according to age, as they are young or growing old. This is
especially plain in man; the hair gets coarser as time goes on, and
some go bald on the front of the head; children indeed do not go bald,
nor do women, but men do so by the time their age is advancing.
Human beings also go grey on the head as they grow old, but this is
not visible in practically any other animal, though more so in the
horse than others. Men go bald on the front of the head, but turn grey
first on the temples; no one goes bald first on these or on the back
of the head. Some such affections occur in a corresponding manner also
in all animals which have not hair but something analogous to it, as
the feathers of birds and scales in the class of fish.

For what purpose Nature has made hair in general for animals has
been previously stated in the work dealing with the causes of the
parts of animals; it is the business of the present inquiry to show
under what circumstances and for what necessary causes each particular
kind of hair occurs. The principal cause then of thickness and
thinness is the skin, for this is thick in some animals and thin in
others, rare in some and dense in others. The different quality of the
included moisture is also a helping cause, for in some animals this is
greasy and in others watery. For generally speaking the substratum
of the skin is of an earthy nature; being on the surface of the body
it becomes solid and earthy as the moisture evaporates. Now the
hairs or their analogue are not formed out of the flesh but out of the
skin moisture evaporating and exhaling in them, and therefore thick
hairs arise from a thick skin and thin from thin. If then the skin
is rarer and thicker, the hairs are thick because of the quantity of
earthy matter and the size of the pores, but if it is denser they
are thin because of the narrowness of the pores. Further, if the
moisture be watery it dries up quickly and the hairs do not gain in
size, but if it be greasy the opposite happens, for the greasy is
not easily dried up. Therefore the thicker-skinned animals are as a
general rule thicker-haired for the causes mentioned; however, the
thickest-skinned are not more so than other thick-skinned ones, as
is shown by the class of swine compared to that of oxen and to the
elephant and many others. And for the same reason also the hairs of
the head in man are thickest, for this part of his skin is thickest
and lies over most moisture and besides is very porous.

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