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extirpate the primitive one； or 'vice versa'。
Now palaeontology shows us many facts which are perfectly harmonious
with these observed effects of the process by which Mr。 Darwin supposes
species to have originated； but which appear to me to be totally
inconsistent with any other hypothesis which has been proposed。 There
are some groups of animals and plants； in the fossil world； which have
been said to belong to 〃persistent types；〃 because they have persisted；
with very little change indeed； through a very great range of time；
while everything about them has changed largely。 There are families of
fishes whose type of construction has persisted all the way from the
carboniferous rock right up to the cretaceous； and others which have
lasted through almost the whole range of the secondary rocks； and from
the lias to the older tertiaries。 It is something stupendous thisto
consider a genus lasting without essential modifications through all
this enormous lapse of time while almost everything else was changed
and modified。
Thus I have no doubt that Mr。 Darwin's hypothesis will be found
competent to explain the majority of the phenomena exhibited by species
in nature； but in an earlier lecture I spoke cautiously with respect to
its power of explaining all the physiological peculiarities of species。
There is； in fact； one set of these peculiarities which the theory of
selective modification； as it stands at present； is not wholly
competent to explain； and that is the group of phenomena which I
mentioned to you under the name of Hybridism； and which I explained to
consist in the sterility of the offspring of certain species when
crossed one with another。 It matters not one whit whether this
sterility is universal； or whether it exists only in a single case。
Every hypothesis is bound to explain； or； at any rate； not be
inconsistent with； the whole of the facts which it professes to account
for； and if there is a single one of these facts which can be shown to
be inconsistent with （I do not merely mean inexplicable by； but contrary
to） the hypothesis； the hypothesis falls to the ground；it is worth
nothing。 One fact with which it is positively inconsistent is worth as
much； and as powerful in negativing the hypothesis； as five hundred。 If
I am right in thus defining the obligations of an hypothesis； Mr。
Darwin； in order to place his views beyond the reach of all possible
assault； ought to be able to demonstrate the possibility of developing
from a particular stock by selective breeding； two forms； which should
either be unable to cross one with another； or whose cross…bred
offspring should be infertile with one another。
For； you see； if you have not done that you have not strictly fulfilled
all the conditions of the problem； you have not shown that you can
produce； by the cause assumed； all the phenomena which you have in
nature。 Here are the phenomena of Hybridism staring you in the face；
and you cannot say； 'I can； by selective modification； produce these
same results。' Now； it is admitted on all hands that； at present； so
far as experiments have gone； it has not been found possible to produce
this complete physiological divergence by selective breeding。 I stated
this very clearly before； and I now refer to the point； because； if it
could be proved； not only that this 'has' not been done； but that it
'cannot' be done； if it could be demonstrated that it is impossible to
breed selectively； from any stock； a form which shall not breed with
another； produced from the same stock； and if we were shown that this
must be the necessary and inevitable results of all experiments； I hold
that Mr。 Darwin's hypothesis would be utterly shattered。
But has this been done？ or what is really the state of the case？ It is
simply that； so far as we have gone yet with our breeding； we have not
produced from a common stock two breeds which are not more or less
fertile with one another。
I do not know that there is a single fact which would justify any one in
saying that any degree of sterility has been observed between breeds
absolutely known to have been produced by selective breeding from a
common stock。 On the other hand； I do not know that there is a single
fact which can justify any one in asserting that such sterility cannot
be produced by proper experimentation。 For my own part； I see every
reason to believe that it may； and will be so produced。 For； as Mr。
Darwin has very properly urged； when we consider the phenomena of
sterility； we find they are most capricious； we do not know what it is
that the sterility depends on。 There are some animals which will not
breed in captivity； whether it arises from the simple fact of their
being shut up and deprived of their liberty； or not； we do not know；
but they certainly will not breed。 What an astounding thing this is； to
find one of the most important of all functions annihilated by mere
imprisonment！
So； again； there are cases known of animals which have been thought by
naturalists to be undoubted species； which have yielded perfectly
fertile hybrids； while there are other species which present what
everybody believes to be varieties* which are more or less infertile
with one another。 There are other cases which are truly extraordinary；
there is one； for example； which has been carefully examined；of two
kinds of sea…weed； of which the male element of the one； which we may
call A； fertilizes the female element of the other； B； while the male
element of B will not fertilize the female element of A； so that； while
the former experiment seems to show us that they are 'varieties'； the
latter leads to the conviction that they are 'species'。
*＇footnote＇ And as I conceive with very good reason； but if
any objector urges that we cannot prove that they have been
produced by artificial or natural selection； the objection
must be admittedultrasceptical as it is。 But in science；
scepticism is a duty。
When we see how capricious and uncertain this sterility is； how unknown
the conditions on which it depends； I say that we have no right to
affirm that those conditions will not be better understood by and by；
and we have no ground for supposing that we may not be able to
experiment so as to obtain that crucial result which I mentioned just
now。 So that though Mr。 Darwin's hypothesis does not completely
extricate us from this difficulty at present； we have not the least
right to say it will not do so。
There is a wide gulf between the thing you cannot explain and the thing
that upsets you altogether。 There is hardly any hypothesis in this
world which has not some fact in connection with it which has not been
explained； but that is a very different affair to a fact that entirely
opposes your hypothesis； in this case all you can say is； that your
hypothesis is in the same position as a good many others。
Now； as to the third test； that there are no other causes competent to
explain the phenomena； I explained to you that one should be able to
say of an hypothesis； that no other known causes than those supposed by
it are competent to give rise to the phenomena。 Here； I th