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breed from these again； you will still have the same kind of offspring；
there is no check。 But if you take members of two distinct species；
however similar they may be to each other and make them breed together；
you will find a check； with some modifications and exceptions； however；
which I shall speak of presently。 If you cross two such species with
each other； then；although you may get offspring in the case of the
first cross； yet； if you attempt to breed from the products of that
crossing； which are what are called HYBRIDSthat is； if you couple a
male and a female hybridthen the result is that in ninety…nine cases
out of a hundred you will get no offspring at all； there will be no
result whatsoever。
The reason of this is quite obvious in some cases； the male hybrids；
although possessing all the external appearances and characteristics of
perfect animals； are physiologically imperfect and deficient in the
structural parts of the reproductive elements necessary to generation。
It is said to be invariably the case with the male mule； the cross
between the Ass and the Mare； and hence it is； that； although crossing
the Horse with the Ass is easy enough； and is constantly done； as far
as I am aware； if you take two mules； a male and a female； and endeavour
to breed from them； you get no offspring whatever； no generation will
take place。 This is what is called the sterility of the hybrids
between two distinct species。
You see that this is a very extraordinary circumstance； one does not see
why it should be。 The common teleological explanation is； that it is
to prevent the impurity of the blood resulting from the crossing of one
species with another； but you see it does not in reality do anything of
the kind。 There is nothing in this fact that hybrids cannot breed with
each other； to establish such a theory； there is nothing to prevent the
Horse breeding with the Ass； or the Ass with the Horse。 So that this
explanation breaks down； as a great many explanations of this kind do；
that are only founded on mere assumptions。
Thus you see that there is a great difference between 〃mongrels；〃 which
are crosses between distinct races； and 〃hybrids；〃 which are crosses
between distinct species。 The mongrels are； so far as we know； fertile
with one another。 But between species； in many cases； you cannot
succeed in obtaining even the first cross： at any rate it is quite
certain that the hybrids are often absolutely infertile one with
another。
Here is a feature； then； great or small as it may be； which
distinguishes natural species of animals。 Can we find any
approximation to this in the different races known to be produced by
selective breeding from a common stock？ Up to the present time the
answer to that question is absolutely a negative one。 As far as we
know at present； there is nothing approximating to this check。 In
crossing the breeds between the Fantail and the Pouter； the Carrier and
the Tumbler； or any other variety or race you may nameso far as we
know at presentthere is no difficulty in breeding together the
mongrels。 Take the Carrier and the Fantail； for instance； and let them
represent the Horse and the Ass in the case of distinct species； then
you have； as the result of their breeding； the Carrier…Fantail
mongrel；we will say the male and female mongrel；and； as far as we
know； these two when crossed would not be less fertile than the
original cross； or than Carrier with Carrier。 Here； you see； is a
physiological contrast between the races produced by selective
modification and natural species。 I shall inquire into the value of
this fact； and of some modifying circumstances by and by； for the
present I merely put it broadly before you。
But while considering this question of the limitations of species； a
word must be said about what is called RECURRENCEthe tendency of
races which have been developed by selective breeding from varieties to
return to their primitive type。 This is supposed by many to put an
absolute limit to the extent of selective and all other variations。
People say； 〃It is all very well to talk about producing these
different races； but you know very well that if you turned all these
birds wild； these Pouters； and Carriers； and so on； they would all
return to their primitive stock。〃 This is very commonly assumed to be
a fact； and it is an argument that is commonly brought forward as
conclusive； but if you will take the trouble to inquire into it rather
closely； I think you will find that it is not worth very much。 The
first question of course is； Do they thus return to the primitive
stock？ And commonly as the thing is assumed and accepted； it is
extremely difficult to get anything like good evidence of it。 It is
constantly said； for example； that if domesticated Horses are turned
wild； as they have been in some parts of Asia Minor and South America；
that they return at once to the primitive stock from which they were
bred。 But the first answer that you make to this assumption is； to ask
who knows what the primitive stock was； and the second answer is； that
in that case the wild Horses of Asia Minor ought to be exactly like the
wild Horses of South America。 If they are both like the same thing；
they ought manifestly to be like each other！ The best authorities；
however； tell you that it is quite different。 The wild Horse of Asia
is said to be of a dun colour； with a largish head； and a great many
other peculiarities； while the best authorities on the wild Horses of
South America tell you that there is no similarity between their wild
Horses and those of Asia Minor； the cut of their heads is very
different； and they are commonly chestnut or bay…coloured。 It is quite
clear； therefore； that as by these facts there ought to have been two
primitive stocks； they go for nothing in support of the assumption that
races recur to one primitive stock； and so far as this evidence is
concerned； it falls to the ground。
Suppose for a moment that it were so； and that domesticated races； when
turned wild； did return to some common condition； I cannot see that
this would prove much more than that similar conditions are likely to
produce similar results； and that when you take back domesticated
animals into what we call natural conditions； you do exactly the same
thing as if you carefully undid all the work you had gone through； for
the purpose of bringing the animal from its wild to its domesticated
state。 I do not see anything very wonderful in the fact； if it took
all that trouble to get it from a wild state； that it should go back
into its original state as soon as you removed the conditions which
produced the variation to the domesticated form。 There is an important
fact； however； forcibly brought forward by Mr。 Darwin； which has been
noticed in connection with the breeding of domesticated pigeons； and it
is； that however different these breeds of pigeons may be from each
other； and we have already noticed the great differences in these
breeds； that if； among any of those variations； you chance to have a
blue pigeon turn up； it will be sure to have the black bars across the
wings； which are charact