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  a reasonable view also that the embryo being larger takes more

  nourishment, so that less is left over about this time, and the less

  is concocted more quickly.

  That milk has the same nature as the secretion from which each

  animal is formed is plain, and has been stated previously. For the

  material which nourishes is the same as that from which Nature forms

  the animal in generation. Now this is the sanguineous liquid in the

  sanguinea, and milk is blood concocted (not corrupted; Empedocles

  either mistook the fact or made a bad metaphor when he composed the

  line: 'On the tenth day of the eighth month the milk comes into being,

  a white pus', for putrefaction and concoction are opposite things, and

  pus is a kind of putrefaction but milk is concocted). While women are

  suckling children the catamenia do not occur according to Nature,

  nor do they conceive; if they do conceive, the milk dries up. This

  is because the nature of the milk and of the catamenia is the same,

  and Nature cannot be so productive as to supply both at once; if the

  secretion is diverted in the one direction it must needs cease in

  the other, unless some violence is done contrary to the general

  rule. But this is as much as to say that it is contrary to Nature, for

  in all cases where it is not impossible for things to be otherwise

  than they generally are but where they may so happen, still what is

  the general rule is what is 'according to Nature'.

  The time also at which the young animal is born has been well

  arranged. For when the nourishment coming through the umbilical cord

  is no longer sufficient for the foetus because of its size, then at

  the same time the milk becomes useful for the nourishment of the

  newly-born animal, and the blood-vessels round which the so-called

  umbilical cord lies as a coat collapse as the nourishment is no longer

  passing through it; for these reasons it is at that time also that the

  young animal enters into the world.

  9

  The natural birth of all animals is head-foremost, because the parts

  above the umbilical cord are larger than those below. The body then,

  being suspended from the cord as in a balance, inclines towards the

  heavy end, and the larger parts are the heavier.

  10

  The period of gestation is, as a matter of fact, determined

  generally in each animal in proportion to the length of its life. This

  we should expect, for it is reasonable that the development of the

  long-lived animals should take a longer time. Yet this is not the

  cause of it, but the periods only correspond accidentally for the most

  part; for though the larger and more perfect sanguinea do live a

  long time, yet the larger are not all longer-lived. Man lives a longer

  time than any animal of which we have any credible experience except

  the elephant, and yet the human kind is smaller than that of the

  bushy-tailed animals and many others. The real cause of long life in

  any animal is its being tempered in a manner resembling the environing

  air, along with certain other circumstances of its nature, of which we

  will speak later; but the cause of the time of gestation is the size

  of the offspring. For it is not easy for large masses to arrive at

  their perfection in a small time, whether they be animals or, one

  may say, anything else whatever. That is why horses and animals akin

  to them, though living a shorter time than man, yet carry their

  young longer; for the time in the former is a year, but in the

  latter ten months at the outside. For the same reason also the time is

  long in elephants; they carry their young two years on account of

  their excessive size.

  We find, as we might expect, that in all animals the time of

  gestation and development and the length of life aims at being

  measured by naturally complete periods. By a natural period I mean,

  e.g. a day and night, a month, a year, and the greater times

  measured by these, and also the periods of the moon, that is to say,

  the full moon and her disappearance and the halves of the times

  between these, for it is by these that the moon's orbit fits in with

  that of the sun [the month being a period common to both].

  The moon is a first principle because of her connexion with the

  sun and her participation in his light, being as it were a second

  smaller sun, and therefore she contributes to all generation and

  development. For heat and cold varying within certain limits make

  things to come into being and after this to perish, and it is the

  motions of the sun and moon that fix the limit both of the beginning

  and of the end of these processes. Just as we see the sea and all

  bodies of water settling and changing according to the movement or

  rest of the winds, and the air and winds again according to the course

  of the sun and moon, so also the things which grow out of these or are

  in these must needs follow suit. For it is reasonable that the periods

  of the less important should follow those of the more important. For

  in a sense a wind, too, has a life and birth and death.

  As for the revolutions of the sun and moon, they may perhaps

  depend on other principles. It is the aim, then, of Nature to

  measure the coming into being and the end of animals by the measure of

  these higher periods, but she does not bring this to pass accurately

  because matter cannot be easily brought under rule and because there

  are many principles which hinder generation and decay from being

  according to Nature, and often cause things to fall out contrary to

  Nature.

  We have now spoken of the nourishment of animals within the mother

  and of their birth into the world, both of each kind separately and of

  all in common.

  Book V

  1

  WE must now investigate the qualities by which the parts of

  animals differ. I mean such qualities of the parts as blueness and

  blackness in the eyes, height and depth of pitch in the voice, and

  differences in colour whether of the skin or of hair and feathers.

  Some such qualities are found to characterize the whole of a kind of

  animals sometimes, while in other kinds they occur at random, as is

  especially the case in man. Further, in connexion with the changes

  in the time of life, all animals are alike in some points, but are

  opposed in others as in the case of the voice and the colour of the

  hair, for some do not grow grey visibly in old age, while man is

  subject to this more than any other animal. And some of these

  affections appear immediately after birth, while others become plain

  as age advances or in old age.

  Now we must no longer suppose that the cause of these and all such

  phenomena is the same. For whenever things are not the product of

  Nature working upon the animal kingdom as a whole, nor yet

  characteristic of each separate kind, then none of these things is

  such as it is or is so developed for any final cause. The eye for

  instance exists for a final cause, but it is not blue for a final

  cause unless this condition be characteristic of the kind of animal.

&nbs
p; In fact in some cases this condition has no connexion with the essence

  of the animal's being, but we must refer the causes to the material

  and the motive principle or efficient cause, on the view that these

  things come into being by Necessity. For, as was said originally in

  the outset of our discussion, when we are dealing with definite and

  ordered products of Nature, we must not say that each is of a

  certain quality because it becomes so, but rather that they become

  so and so because they are so and so, for the process of Becoming or

  development attends upon Being and is for the sake of Being, not

  vice versa.

  The ancient Nature-philosophers however took the opposite view.

  The reason of this is that they did not see that the causes were

  numerous, but only saw the material and efficient and did not

  distinguish even these, while they made no inquiry at all into the

  formal and final causes.

  Everything then exists for a final cause, and all those things which

  are included in the definition of each animal, or which either are

  means to an end or are ends in themselves, come into being both

  through this cause and the rest. But when we come to those things

  which come into being without falling under the heads just

  mentioned, their course must be sought in the movement or process of

  coming into being, on the view that the differences which mark them

  arise in the actual formation of the animal. An eye, for instance, the

  animal must have of necessity (for the fundamental idea of the animal

  is of such a kind), but it will have an eye of a particular kind of

  necessity in another sense, not the sense mentioned just above,

  because it is its nature to act or be acted on in this or that way.

  These distinctions being drawn let us speak of what comes next in

  order. As soon then as the offspring of all animals are born,

  especially those born imperfect, they are in the habit of sleeping,

  because they continue sleeping also within the mother when they

  first acquire sensation. But there is a difficulty about the

  earliest period of development, whether the state of wakefulness

  exists in animals first, or that of sleep. Since they plainly wake

  up more as they grow older, it is reasonable to suppose that the

  opposite state, that of sleep, exists in the first stages of

  development. Moreover the change from not being to being must pass

  through the intermediate condition, and sleep seems to be in its

  nature such a condition, being as it were a boundary between living

  and not living, and the sleeper being neither altogether

  non-existent nor yet existent. For life most of all appertains to

  wakefulness, on account of sensation. But on the other hand, if it

  is necessary that the animal should have sensation and if it is then

  first an animal when it has acquired sensation, we ought to consider

  the original condition to be not sleep but only something resembling

  sleep, such a condition as we find also in plants, for indeed at

  this time animals do actually live the life of a plant. But it is

  impossible that plants should sleep, for there is no sleep which

  cannot be broken, and the condition in plants which is analogous to

  sleep cannot be broken.

  It is necessary then for the embryo animal to sleep most of the time

  because the growth takes place in the upper part of the body, which is

  consequently heavier (and we have stated elsewhere that such is the

  cause of sleep). But nevertheless they are found to wake even in

  the womb (this is clear in dissections and in the ovipara), and then

  they immediately fall into a sleep again. This is why after birth also

  they spend most of their time in sleep.

  When awake infants do not laugh, but while asleep they both laugh

  and cry. For animals have sensations even while asleep, not only

  what are called dreams but also others besides dreams, as those

  persons who arise while sleeping and do many things without

  dreaming. For there are some who get up while sleeping and walk

  about seeing just like those who are awake; these have perception of

  what is happening, and though they are not awake, yet this

  perception is not like a dream. So infants presumably have

  sense-perception and live in their sleep owing to previous habit,

  being as it were without knowledge of the waking state. As time goes

  on and their growth is transferred to the lower part of the body, they

  now wake up more and spend most of their time in that condition.

  Children continue asleep at first more than other animals, for they

  are born in a more imperfect condition than other animals that are

  produced in anything like a perfect state, and their growth has

  taken place more in the upper part of the body.

  The eyes of all children are bluish immediately after birth; later

  on they change to the colour which is to be theirs permanently. But in

  the case of other animals this is not visible. The reason of this is

  that the eyes of other animals are more apt to have only one colour

  for each kind of animal; e.g. cattle are dark-eyed, the eye of all

  sheep is pale, of others again the whole kind is blue or grey-eyed,

  and some are yellow (goat-eyed), as the majority of goats

  themselves, whereas the eyes of men happen to be of many colours,

  for they are blue or grey or dark in some cases and yellow in

  others. Hence, as the individuals in other kinds of animals do not

  differ from one another in the colour, so neither do they differ

  from themselves, for they are not of a nature to have more than one

  colour. Of the other animals the horse has the greatest variety of

  colour in the eye, for some of them are actually heteroglaucous;

  this phenomenon is not to be seen in any of the other animals, but man

  is sometimes heteroglaucous.

  Why then is it that there is no visible change in the other

  animals if we compare their condition when newly born with their

  condition at a more advanced age, but that there is such a change in

  children? We must consider just this to be a sufficient cause, that

  the part concerned has only one colour in the former but several

  colours in the latter. And the reason why the eyes of infants are

  bluish and have no other colour is that the parts are weaker in the

  newly born and blueness is a sort of weakness.

  We must also gain a general notion about the difference in eyes, for

  what reason some are blue, some grey, some yellow, and some dark. To

  suppose that the blue are fiery, as Empedocles says, while the dark

  have more water than fire in them, and that this is why the former,

  the blue, have not keen sight by day, viz. owing to deficiency of

  water in their composition, and the latter are in like condition by

  night, viz. owing to deficiency of fire- this is not well said if

  indeed we are to assume sight to be connected with water, not fire, in

  all cases. Moreover it is possible to render another account of the

  cause of the colours, but if indeed the fact is as was stated before

  in the treatise on the senses, and still earlier than that in the

  investigations c
oncerning soul- if this sense organ is composed of

  water and if we were right in saying for what reason it is composed of

  water and not of air or fire- then we must assume the water to be

  the cause of the colours mentioned. For some eyes have too much liquid

  to be adapted to the movement, others have too little, others the

  due amount. Those eyes therefore in which there is much liquid are

  dark because much liquid is not transparent, those which have little

  are blue; (so we find in the sea that the transparent part of it

  appears light blue, the less transparent watery, and the

  unfathomable water is dark or deep-blue on account of its depth).

  When we come to the eyes between these, they differ only in degree.

  We must suppose the same cause also to be responsible for the fact

  that blue eyes are not keen-sighted by day nor dark eyes by night.

  Blue eyes, because there is little liquid in them, are too much

  moved by the light and by visible objects in respect of their

  liquidity as well as their transparency, but sight is the movement

  of this part in so far as it is transparent, not in so far as it is

  liquid. Dark eyes are less moved because of the quantity of liquid

  in them. And so they see less well in the dusk, for the nocturnal

  light is weak; at the same time also liquid is in general hard to move

  in the night. But if the eye is to see, it must neither not be moved

  at all nor yet more than in so far as it is transparent, for the

  stronger movement drives out the weaker. Hence it is that on

  changing from strong colours, or on going out of the sun into the

  dark, men cannot see, for the motion already existing in the eye,

  being strong, stops that from outside, and in general neither a strong

  nor a weak sight can see bright things because the liquid is acted

  upon and moved too much.

  The same thing is shown also by the morbid affections of each kind

  of sight. Cataract attacks the blue-eyed more, but what is called

  'nyctalopia' the dark-eyed. Now cataract is a sort of dryness of the

  eyes and therefore it is found more in the aged, for this part also