The Complete Aristotle (eng.)

the genus is an element in a thing’s nature),
nor is it so with reference to things which are not in the same
genus, but it will differ in genus from them, and in species from
things in the same genus. For a thing’s difference from that from
which it differs in species must be a contrariety; and this belongs
only to things in the same genus.
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9
    One might raise the question, why woman does not differ from man
in species, when female and male are contrary and their difference
is a contrariety; and why a female and a male animal are not
different in species, though this difference belongs to animal in
virtue of its own nature, and not as paleness or darkness does;
both ‘female’ and ‘male’ belong to it qua animal. This question is
almost the same as the other, why one contrariety makes things
different in species and another does not, e.g. ‘with feet’ and
‘with wings’ do, but paleness and darkness do not. Perhaps it is
because the former are modifications peculiar to the genus, and the
latter are less so. And since one element is definition and one is
matter, contrarieties which are in the definition make a difference
in species, but those which are in the thing taken as including its
matter do not make one. And so paleness in a man, or darkness, does
not make one, nor is there a difference in species between the pale
man and the dark man, not even if each of them be denoted by one
word. For man is here being considered on his material side, and
matter does not create a difference; for it does not make
individual men species of man, though the flesh and the bones of
which this man and that man consist are other. The concrete thing
is other, but not other in species, because in the definition there
is no contrariety. This is the ultimate indivisible kind. Callias
is definition + matter, the pale man, then, is so also, because it
is the individual Callias that is pale; man, then, is pale only
incidentally. Neither do a brazen and a wooden circle, then, differ
in species; and if a brazen triangle and a wooden circle differ in
species, it is not because of the matter, but because there is a
contrariety in the definition. But does the matter not make things
other in species, when it is other in a certain way, or is there a
sense in which it does? For why is this horse other than this man
in species, although their matter is included with their
definitions? Doubtless because there is a contrariety in the
definition. For while there is a contrariety also between pale man
and dark horse, and it is a contrariety in species, it does not
depend on the paleness of the one and the darkness of the other,
since even if both had been pale, yet they would have been other in
species. But male and female, while they are modifications peculiar
to ‘animal’, are so not in virtue of its essence but in the matter,
ie. the body. This is why the same seed becomes female or male by
being acted on in a certain way. We have stated, then, what it is
to be other in species, and why some things differ in species and
others do not.
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10
    Since contraries are other in form, and the perishable and the
imperishable are contraries (for privation is a determinate
incapacity), the perishable and the imperishable must be different
in kind.
    Now so far we have spoken of the general terms themselves, so
that it might be thought not to be necessary that every
imperishable thing should be different from every perishable thing
in form, just as not every pale thing is different in form from
every dark thing. For the same thing can be both, and even at the
same time if it is a universal (e.g. man can be both pale and
dark), and if it is an individual it can still be both; for the
same man can be, though not at the same time, pale and dark. Yet
pale is contrary to dark.
    But while some contraries belong to certain things by accident
(e.g. both those now mentioned and many others), others cannot, and
among these are ‘perishable’ and ‘imperishable’. For nothing is by
accident perishable. For what is accidental is capable of not being
present, but perishableness is one of the attributes that belong of
necessity to the things to which they belong; or else one and the
same thing may be perishable and imperishable, if perishableness is
capable of not belonging to it. Perishableness then must either be
the essence or be present in the essence of