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 admitted--ultrasceptical 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 think,Mr.
Darwin's view is pretty strong.I really believe that the alternative is either Darwinism or nothing,for I do not know of any rational conception or theory of the organic universe which has any scientific position at all beside Mr.Darwin's.I do not know of any proposition that has been put before us with the intention of explaining the phenomena of organic nature,which has in its favour a thousandth part of the evidence which may be adduced in favour of Mr.Darwin's views.
Whatever may be the objections to his views,certainly all others are absolutely out of court.
