6 JOURNAL OF THE [January, 



Professor Huxley is one of those whose minds had what may. 

 without disparagement, be termed a natural bias towards the 

 whole philosophy of mechanical causation, or monism, as Profes- 

 sor Haeckel calls it; and yet he felt very strongly the inadequacy 

 of Darwin's experimental evidence at one point. On this subject 

 he has said: — "There is, in fact, one set of * * * peculiari- 

 ties which the theory of selective modification, as it stands at pres- 

 ent, is not wholly competent to explain, and that is the group of 

 phenomena which I mentioned to you under the name of Hybrid- 

 ism, 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 ex- 

 plain, 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 is as powerful in negativing the hypothesis, as five 

 hundred. If I am right in thus defining the obligations of a 

 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 ex- 

 periments have gone, it has not been found possible to produce 

 this complete physiological divergence by selective breeding. 

 * * *' If it could be proved, not only that this has 

 not been done, but that it cannot be done; if it could be demon- 

 strated 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 



