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The Conditions of Existence
by Thomas H。 Huxley
IN the last Lecture I endeavoured to prove to you that； while； as a
general rule； organic beings tend to reproduce their kind； there is in
them； also； a constantly recurring tendency to varyto vary to a
greater or to a less extent。 Such a variety； I pointed out to you；
might arise from causes which we do not understand； we therefore called
it spontaneous； and it might come into existence as a definite and
marked thing； without any gradations between itself and the form which
preceded it。 I further pointed out； that such a variety having once
arisen； might be perpetuated to some extent； and indeed to a very
marked extent； without any direct interference； or without any exercise
of that process which we called selection。 And then I stated further；
that by such selection； when exercised artificiallyif you took care to
breed only from those forms which presented the same peculiarities of
any variety which had arisen in this mannerthe variation might be
perpetuated； as far as we can see； indefinitely。
The next question； and it is an important one for us； is this： Is there
any limit to the amount of variation from the primitive stock which can
be produced by this process of selective breeding？ In considering this
question； it will be useful to class the characteristics； in respect of
which organic beings vary； under two heads： we may consider structural
characteristics； and we may consider physiological characteristics。
In the first place； as regards structural characteristics； I endeavoured
to show you； by the skeletons which I had upon the table； and by
reference to a great many well…ascertained facts； that the different
breeds of Pigeons； the Carriers； Pouters； and Tumblers； might vary in
any of their internal and important structural characters to a very
great degree； not only might there be changes in the proportions of the
skull； and the characters of the feet and beaks； and so on； but that
there might be an absolute difference in the number of the vertebrae of
the back； as in the sacral vertebrae of the Pouter； and so great is the
extent of the variation in these and similar characters that I pointed
out to you； by reference to the skeletons and the diagrams； that these
extreme varieties may absolutely differ more from one another in their
structural characters than do what naturalists call distinct SPECIES of
pigeons； that is to say； that they differ so much in structure that
there is a greater difference between the Pouter and the Tumbler than
there is between such wild and distinct forms as the Rock Pigeon or the
Ring Pigeon； or the Ring Pigeon and the Stock Dove； and indeed the
differences are of greater value than this； for the structural
differences between these domesticated pigeons are such as would be
admitted by a naturalist； supposing he knew nothing at all about their
origin； to entitle them to constitute even distinct genera。
As I have used this term SPECIES； and shall probably use it a good deal；
I had better perhaps devote a word or two to explaining what I mean by
it。
Animals and plants are divided into groups； which become gradually
smaller； beginning with a KINGDOM； which is divided into SUB…KINGDOMS；
then come the smaller divisions called PROVINCES； and so on from a
PROVINCE to a CLASS from a CLASS to an ORDER； from ORDERS to FAMILIES；
and from these to GENERA； until we come at length to the smallest
groups of animals which can be defined one from the other by constant
characters； which are not sexual； and these are what naturalists call
SPECIES in practice； whatever they may do in theory。
If； in a state of nature； you find any two groups of living beings；
which are separated one from the other by some constantly…recurring
characteristic； I don't care how slight and trivial； so long as it is
defined and constant； and does not depend on sexual peculiarities； then
all naturalists agree in calling them two species； that is what is
meant by the use of the word speciesthat is to say； it is； for the
practical naturalist； a mere question of structural differences。*
＇footnote＇* I lay stress here on the 'practical'
signification of 〃Species。〃 Whether a physiological test
between species exist or not； it is hardly ever applicable
by the practical naturalist。
We have seen nowto repeat this point once more； and it is very
essential that we should rightly understand itwe have seen that
breeds； known to have been derived from a common stock by selection；
may be as different in their structure from the original stock as
species may be distinct from each other。
But is the like true of the physiological characteristics of animals？
Do the physiological differences of varieties amount in degree to those
observed between forms which naturalists call distinct species？ This
is a most important point for us to consider。
As regards the great majority of physiological characteristics； there is
no doubt that they are capable of being developed； increased； and
modified by selection。
There is no doubt that breeds may be made as different as species in
many physiological characters。 I have already pointed out to you very
briefly the different habits of the breeds of Pigeons； all of which
depend upon their physiological peculiarities；as the peculiar habit
of tumbling； in the Tumblerthe peculiarities of flight； in the
〃homing〃 birds；the strange habit of spreading out the tail； and
walking in a peculiar fashion； in the Fantail；and； lastly； the habit
of blowing out the gullet； so characteristic of the Pouter。 These are
all due to physiological modifications； and in all these respects these
birds differ as much from each other as any two ordinary species do。
So with Dogs in their habits and instincts。 It is a physiological
peculiarity which leads the Greyhound to chase its prey by sight；that
enables the Beagle to track it by the scent；that impels the Terrier
to its rat…hunting propensity；and that leads the Retriever to its
habit of retrieving。 These habits and instincts are all the results of
physiological differences and peculiarities； which have been developed
from a common stock； at least there is every reason to believe so。 But
it is a most singular circumstance； that while you may run through
almost the whole series of physiological processes； without finding a
check to your argument； you come at last to a point where you do find a
check； and that is in the reproductive processes。 For there is a most
singular circumstance in respect to natural speciesat least about some
of themand it would be sufficient for the purposes of this argument
if it were true of only one of them； but there is； in fact； a great
number of such casesand that is； that； similar as they may appear to
be to mere races or breeds； they present a marked peculiarity in the
reproductive process。 If you breed from the male and female of the same
race； you of course have offspring of the like kind； and if you make
the offspring breed together； you obtain the same result； and if you
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 g