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is added to cold or cold to hot so as to mix, the result is that the

  embryo itself arising from these is preserved and thus these animals

  are fertile when crossed with one another, but the animal produced

  by them is no longer fertile but unable to produce perfect offspring.

  And in general each of these animals naturally tends towards

  sterility. The ass has all the disadvantages already mentioned, and if

  it should not begin to generate after the first shedding of teeth,

  it no longer generates at all; so near is the constitution of the

  ass to being sterile. The horse is much the same; it tends naturally

  towards sterility, and to make it entirely so it is only necessary

  that its generative secretion should become colder; now this is what

  happens to it when mixed with the corresponding secretion of the

  ass. The ass in like manner comes very near generating a sterile

  animal when mated with its own species. Thus when the difficulty of

  a cross contrary to nature is added, (when too even in the other case

  when united with their own species they with difficulty produce a

  single young one), the result of the cross, being still more

  sterile and contrary to nature, will need nothing further to make it

  sterile, but will be so of necessity.

  We find also that the bodies of female mules grow large because

  the matter which is secreted in other animals to form the catamenia is

  diverted to growth. But since the period of gestation in such

  animals is a year, the mule must not only conceive, if she is to be

  fertile, but must also nourish the embryo till birth, and this is

  impossible if there are no catamenia. But there are none in the

  mule; the useless part of the nutriment is discharged with the

  excretion from the bladder- this is why male mules do not smell to the

  pudenda of the females, as do the other solid-hoofed ungulates, but

  only to the evacuation itself- and the rest of the nutriment is used

  up to increase the size of the body. Hence it is sometimes possible

  for the female to conceive, as has been known to happen before now,

  but it is impossible for her to complete the process of nourishing the

  embryo and bringing it to birth.

  The male, again, may sometimes generate, both because the male sex

  is naturally hotter than the female and because it does not contribute

  any material substance to the mixture. The result in such cases is a

  'ginnus', that is to say, a dwarf mule; for 'ginni' are produced

  also from the crossing of horse and ass when the embryo is diseased in

  the uterus. The ginnus is in fact like the so-called 'metachoera' in

  swine, for a 'metachoerum' also is a pig injured in the uterus; this

  may happen to any pig. The origin of human dwarfs is similar, for

  these also have their parts and their whole development injured during

  gestation, and resemble ginni and metachoera.

  Book III

  1

  WE have now spoken about the sterility of mules, and about those

  animals which are viviparous both externally and within themselves.

  The generation of the oviparous sanguinea is to a certain extent

  similar to that of the animals that walk, and all may be embraced in

  the same general statement; but in other respects there are

  differences in them both as compared with each other and with those

  that walk. All alike are generated from sexual union, the male

  emitting semen into the female. But among the ovipara (1) birds

  produce a perfect hard-shelled egg, unless it be injured by disease,

  and the eggs of birds are all two-coloured. (2) The cartilaginous

  fishes, as has been often said already, are oviparous internally but

  produce the young alive, the egg changing previously from one part

  of the uterus to another; and their egg is soft-shelled and of one

  colour. One of this class alone does not produce the young from the

  egg within itself, the so-called 'frog'; the reason of which must be

  stated later. (3) All other oviparous fishes produce an egg of one

  colour, but this is imperfect, for its growth is completed outside the

  mother's body by the same cause as are those eggs which are

  perfected within.

  Concerning the uterus of these classes of animals, what

  differences there are among them and for what reasons, has been stated

  previously. For in some of the viviparous creatures it is high up near

  the hypozoma, in others low down by the pudenda; the former in the

  cartilaginous fishes, the latter in animals both internally and

  externally viviparous, such as man and horse and the rest; in the

  ovipara it is sometimes low, as in the oviparous fish, and sometimes

  high, as in birds.

  Some embryos are formed in birds spontaneously, which are called

  wind-eggs and 'zephyria' by some; these occur in birds which are not

  given to flight nor rapine but which produce many young, for these

  birds have much residual matter, whereas in the birds of prey all such

  secretion is diverted to the wings and wing-feathers, while the body

  is small and dry and hot. (The secretion corresponding in hen-birds

  to catamenia, and the semen of the cock, are residues.) Since then

  both the wings and the semen are made from residual matter, nature

  cannot afford to spend much upon both. And for this same reason the

  birds of prey are neither given to treading much nor to laying many

  eggs, as are the heavy birds and those flying birds whose bodies are

  bulky, as the pigeon and so forth. For such residual matter is

  secreted largely in the heavy birds not given to flying, such as

  fowls, partridges, and so on, wherefore their males tread often and

  their females produce much material. Of such birds some lay many

  eggs at a time and some lay often; for instance, the fowl, the

  partridge, and the Libyan ostrich lay many eggs, while the pigeon

  family do not lay many but lay often. For these are between the

  birds of prey and the heavy ones; they are flyers like the former, but

  have bulky bodies like the latter; hence, because they are flyers

  and the residue is diverted that. way, they lay few eggs, but they lay

  often because of their having bulky bodies and their stomachs being

  hot and very active in concoction, and because moreover they can

  easily procure their food, whereas the birds of prey do so with

  difficulty.

  Small birds also tread often and are very fertile, as are

  sometimes small plants, for what causes bodily growth in others turn

  in them to a seminal residuum. Hence the Adrianic fowls lay most eggs,

  for because of the smallness of their bodies the nutriment is used

  up in producing young. And other birds are more fertile than

  game-fowl, for their bodies are more fluid and bulkier, whereas

  those of game-fowl are leaner and drier, since a passionate spirit

  is found rather in such bodies as the latter. Moreover the thinness

  and weakness of the legs contribute to making the former class of

  birds naturally inclined to tread and to be fertile, as we find also

  in the human species; for the nourishment which otherwise goes to

  the legs is turned in such into a seminal secretion, what Nature takes
/>   from the one place being added at the other. Birds of prey, on the

  contrary, have a strong walk and their legs are thick owing to their

  habits, so that for all these reasons they neither tread nor lay much.

  The kestrel is the most fertile; for this is nearly the only bird of

  prey which drinks, and its moisture, both innate and acquired, along

  with its heat is favourable to generative products. Even this bird

  does not lay very many eggs, but four at the outside.

  The cuckoo, though not a bird of prey, lays few eggs, because it

  is of a cold nature, as is shown by the cowardice of the bird, whereas

  a generative animal should be hot and moist. That it is cowardly is

  plain, for it is pursued by all the birds and lays eggs in the nests

  of others.

  The pigeon family are in the habit of laying two for the most

  part, for they neither lay one (no bird does except the cuckoo, and

  even that sometimes lays two) nor yet many, but they frequently

  produce two, or three at the most generally two, for this number

  lies between one and many.

  It is plain from the facts that with the birds that lay many eggs

  the nutriment is diverted to the semen. For most trees, if they bear

  too much fruit, wither away after the crop when nutriment is not

  reserved for themselves, and this seems to be what happens to annuals,

  as leguminous plants, corn, and the like. For they consume all their

  nutriment to make seed, their kind being prolific. And some fowls

  after laying too much, so as even to lay two eggs in a day, have

  died after this. For both the birds the plants become exhausted, and

  this condition is an excess of secretion of residual matter. A similar

  condition is the cause of the later sterility of the lioness, for at

  the first birth she produces five or six, then in the next year

  four, and again three cubs, then the next number down to one, then

  none at all, showing that the residue is being used up and the

  generative secretion is failing along with the advance of years.

  We have now stated in which birds wind-eggs are found, and also what

  sort of birds lay many eggs or few, and for what reasons. And

  wind-eggs, as said before, come into being because while it is the

  material for generation that exists in the female of all animals,

  birds have no discharge of catamenia like viviparous sanguinea (for

  they occur in all these latter, more in some, less in others, and in

  some only enough in quantity just to mark the class). The same

  applies to fish as to birds, and so in them as in birds is found an

  embryonic formation without impregnation, but it is less obvious

  because their nature is colder. The secretion corresponding to the

  catamenia of vivipara is formed in birds at the appropriate season for

  the discharge of superfluous matter, and, because the region near

  the hypozoma is hot, it is perfected so far as size is concerned,

  but in birds and fishes alike it is imperfect for generation without

  the seminal fluid of the male; the cause of this has been previously

  given. Wind-eggs are not formed in the flying birds, for the same

  reason as prevents their laying many eggs; for the residual matter

  in birds of prey is small, and they need the male to give an impulse

  for the discharge of it. The wind-eggs are produced in greater numbers

  than the impregnated but smaller in size for one and the same

  reason; they are smaller in size because they are imperfect, and

  because they are smaller in size they are more in number. They are

  less pleasant for food because they are less concocted, for in all

  foods the concocted is more agreeable. It has been sufficiently

  observed, then, that neither birds' nor fishes' eggs are perfected for

  generation without the males. As for embryos being formed in fish also

  (though in a less degree) without the males, the fact has been

  observed especially in river fish, for some are seen to have eggs from

  the first, as has been written in the Enquiries concerning them. And

  generally speaking in the case of birds even the impregnated eggs

  are not wont for the most part to attain their full growth unless

  the hen be trodden continually. The reason of this is that just as

  with women intercourse with men draws down the secretion of the

  catamenia (for the uterus being heated attracts the moisture and

  the passages are opened), so this happens also with birds; the

  residual matter corresponding to the catamenia advances a little at

  a time, and is not discharged externally, because its amount is

  small and the uterus is high up by the hypozoma, but trickles together

  into the uterus itself. For as the embryo of the vivipara grows by

  means of the umbilical cord, so the egg grows through this matter

  flowing to it through the uterus. For when once the hens have been

  trodden, they all continue to have eggs almost without intermission,

  though very small ones. Hence some are wont to speak of wind-eggs as

  not coming into being independently but as mere relics from a previous

  impregnation. But this is a false view, for sufficient observations

  have been made of their arising without impregnation in chickens and

  goslings. Also the female partridges which are taken out to act as

  decoys, whether they have ever been impregnated or not, immediately on

  smelling the male and hearing his call, become filled with eggs in the

  latter case and lay them in the former. The reason why this happens is

  the same as in men and quadrupeds, for if their bodies chance to be in

  rut they emit semen at the mere sight of the female or at a slight

  touch. And such birds are of a lascivious and fertile nature, so

  that the impulse they need is but small when they are in this

  excited condition, and the secreting activity takes place quickly in

  them, wind-eggs forming in the unimpregnated and the eggs in those

  which have been impregnated growing and reaching perfection swiftly.

  Among creatures that lay eggs externally birds produce their egg

  perfect, fish imperfect, but the eggs of the latter complete their

  growth outside as has been said before. The reason is that the fish

  kind is very fertile; now it is impossible for many eggs to reach

  completion within the mother and therefore they lay them outside. They

  are quickly discharged, for the uterus of externally oviparous

  fishes is near the generative passage. While the eggs of birds are

  two-coloured, those of all fish are one-coloured. The cause of the

  double colour may be seen from considering the power of each of the

  two parts, the white and the yolk. For the matter of the egg is

  secreted from the blood [No bloodless animal lays eggs,] and that

  the blood is the material of the body has been often said already. The

  one part, then, of the egg is nearer the form of the animal coming

  into being, that is the hot part; the more earthy part gives the

  substance of the body and is further removed. Hence in all

  two-coloured eggs the animal receives the first principle of

  generation from the white (for the vital principle is in that which

  is hot), but the nutriment from the yolk. Now in animals o
f a

  hotter nature the part from which the first principle arises is

  separated off from the part from which comes the nutriment, the one

  being white and the other yellow, and the white and pure is always

  more than the yellow and earthy; but in the moister and less hot the

  yolk is more in quantity and more fluid. This is what we find in

  lake birds, for they are of a moister nature and are colder than the

  land birds, so that the so-called 'lecithus' or yolk in the eggs of

  such birds is large and less yellow because the white is less

  separated off from it. But when we come to the ovipara which are

  both of a cold nature and also moister (such is the fish kind) we

  find the white not separated at all because of the small size of the

  eggs and the quantity of the cold and earthy matter; therefore all

  fish eggs are of one colour, and white compared with yellow, yellow

  compared with white. Even the wind-eggs of birds have this distinction

  of colour, for they contain that out of which will come each of the

  two parts, alike that whence arises the principle of life and that

  whence comes the nutriment; only both these are imperfect and need the

  influence of the male in addition; for wind-eggs become fertile if

  impregnated by the male within a certain period. The difference in

  colour, however, is not due to any difference of sex, as if the

  white came from the male, the yolk from the female; both on the

  contrary come from the female, but the one is cold, the other hot.

  In all cases then where the hot part is considerable it is separated

  off, but where it is little it cannot be so; hence the eggs of such

  animals, as has been said, are of one colour. The semen of the male

  only puts them into form; and therefore at first the egg in birds

  appears white and small, but as it advances it is all yellow as more

  of the sanguineous material is continually mixed with it; finally as

  the hot part is separated the white takes up a position all round it

  and equally distributed on all sides, as when a liquid boils; for

  the white is naturally liquid and contains in itself the vital heat;

  therefore it is separated off all round, but the yellow and earthy