110 MR. DAEWIN's WORK 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 this — to 

 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 repect 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 spe- 

 cies 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 inexpli- 

 cable 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 hun- 

 dred. 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 



