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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 characteristic of the original wild stock； the Rock
Pigeon。
Now； this is certainly a very remarkable circumstance； but I do not see
myself how it tells very strongly either one way or the other。 I
think； in fact； that this argument in favour of recurrence to the
primitive type might prove a great deal too much for those who so
constantly bring it forward。 For example； Mr。 Darwin has very forcibly
urged； that nothing is commoner than if you examine a dun horseand I
had an opportunity of verifying this illustration lately； while in the
islands of the West Highlands； where there are a great many dun
horsesto find that horse exhibit a long black stripe down his back；
very often stripes on his shoulder； and very often stripes on his
legs。 I； myself； saw a pony of this description a short time ago； in a
baker's cart； near Rothesay； in Bute： it had the long stripe down the
back； and stripes on the shoulders and legs； just like those of the
Ass； the Quagga； and the Zebra。 Now； if we interpret the theory of
recurrence as applied to this case； might it not be said that here was
a case of a variation exhibiting the characters and conditions of an
animal occupying something like an intermediate position between the
Horse； the Ass； the Quagga； and the Zebra； and from which these had
been developed？ In the same way with regard even to Man。 Every
anatomist will tell you that there is nothing commoner； in dissecting
the human body； than to meet with what are called muscular
variationsthat is； if you dissect two bodies very carefully； you will
probably find that the modes of attachment and insertion of the muscles
are not exactly the same in both； there being great peculiarities in
the mode in which the muscles are arranged； and it is very singular；
that in some dissections of the human body you will come upon
arrangements of the muscles very similar indeed to the same parts in the
Apes。 Is the conclusion in that case to be； that this is like the
black bars in the case of the Pigeon； and that it indicates a
recurrence to the primitive type from which the animals have been
probably developed？ Truly； I think that the opponents of modification
and variation had better leave the argument of recurrence alone； or it
may prove altogether too strong for them。
To sum up；the evidence as far as we have gone is against the argument
as to any limit to divergences； so far as structure is concerned； and
in favour of a physiological limitation。 By selective breeding we can
produce structural divergences as great as those of species； but we
cannot produce equal physiological divergences。 For the present I leave
the question there。
Now； the next problem that lies before usand it is an extremely
important oneis this： Does this selective breeding occur in nature？
Because； if there is no proof of it； all that I have been telling you
goes for nothing in accounting for the origin of species。 Are natural
causes competent to play the part of selection in perpetuating
varieties？ Here we labour under very great difficulties。 In the last
lecture I had occasion to point out to you the extreme difficulty of
obtaining evidence even of the first origin of those varieties which we
know to have occurred in domesticated animals。 I told you； that almost
always the origin of these varieties is overlooked； so that I could
only produce two of three cases； as that of Gratio Kelleia and of the
Ancon sheep。 People forget； or do not take notice of them until they
come to have a prominence； and if that is true of artificial cases；
under our own eyes； and in animals in our own care； how much more
difficult it must be to have at first hand good evidence of the origin
of varieties in nature！ Indeed； I do not know that it is possible by
direct evidence to prove the origin of a variety in nature； or to prove
selective breeding； but I will tell you what we can proveand this
comes to the same thingthat varieties exist in nature within the
limits of species； and； what is more； that when a variety has come into
e