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On The Generation Of Animals   


effected in animals by the nature of the vital heat, in yeasts by
the heat of the juice commingled with them. The eggs then grow of
necessity through this cause (for they have in them superfluous
yeasty matter), but also for the sake of a final cause, for it is
impossible for them to attain their whole growth in the uterus because
these animals have so many eggs. Therefore are they very small when
set free and grow quickly, small because the uterus is narrow for
the multitude of the eggs, and growing quickly that the race may not
perish, as it would if much of the time required for the whole
development were spent in this growth; even as it is most of those
laid are destroyed before hatching. Hence the class of fish is
prolific, for Nature makes up for the destruction by numbers. Some
fish actually burst because of the size of the eggs, as the fish
called 'belone', for its eggs are large instead of numerous, what
Nature has taken away in number being added in size.

So much for the growth of such eggs and its reason.

5

A proof that these fish also are oviparous is the fact that even
viviparous fish, such as the cartilaginous, are first internally
oviparous, for hence it is plain that the whole class of fishes is
oviparous. Where, however, both sexes exist and the eggs are
produced in consequence of impregnation, the eggs do not arrive at
completion unless the male sprinkle his milt upon them. Some
erroneously assert that all fish are female except in the
cartilaginous fishes, for they think that the females of fish differ
from what are supposed to be males only in the same way as in those
plants where the one bears fruit but the other is fruitless, as
olive and oleaster, fig and caprifig. They think the like applies to
fish except the cartilaginous, for they do not dispute the sexes in
these. And yet there is no difference in the males of cartilaginous
fishes and those belonging to the oviparous class in respect of the
organs for the milt, and it is manifest that semen can be squeezed out
of males of both classes at the right season. The female also has a
uterus. But if the whole class were females and some of them
unproductive (as with mules in the class of bushy-tailed animals),
then not only should those which lay eggs have a uterus but also the
others, only the uterus of the latter should be different from that of
the former. But, as it is, some of them have organs for milt and
others have a uterus, and this distinction obtains in all except
two, the erythrinus and the channa, some of them having the milt
organs, others a uterus. The difficulty which drives some thinkers
to this conclusion is easily solved if we look at the facts. They
say quite correctly that no animal which copulates produces many
young, for of all those that generate from themselves perfect
animals or perfect eggs none is prolific on the same scale as the
oviparous fishes, for the number of eggs in these is enormous. But
they had overlooked the fact that fish-eggs differ from those of birds
in one circumstance. Birds and all oviparous quadrupeds, and any of
the cartilaginous fish that are oviparous, produce a perfect egg,
and it does not increase outside of them, whereas the eggs of fish are
imperfect and do so complete their growth. Moreover the same thing
applies to cephalopods also and crustacea, yet these animals are
actually seen copulating, for their union lasts a long time, and it is
plain in these cases that the one is male and the other has a
uterus. Finally, it would be strange if this distinction did not exist
in the whole class, just as male and female in all the vivipara. The
cause of the ignorance of those who make this statement is that the
differences in the copulation and generation of various animals are of
all kinds and not obvious, and so, speculating on a small induction,
they think the same must hold good in all cases.

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