136 mr. darwin's work and 



call vera causes — true causes; — in the next place, we 

 should be prepared to show that the assumed causes of 

 the phenomena are competent to produce such pheno- 

 mena as those which we wish to explain by them ; and 

 in the last place, we ought to be able to show that no 

 other known causes are competent to produce these 

 phenomena. If we can succeed in satisfying these 

 three conditions we shall have demonstrated our hypo- 

 thesis ; or rather I ought to say, we shall have proved 

 it as far as certainty is possible for us ; for, after all, 

 there is no one of our surest convictions which may not 

 be upset, or at any rate modified by a further accession 

 of knowledge. It was because it satisfied these con- 

 ditions that we accepted the hypothesis as to the dis- 

 appearance of the tea-pot and spoons in the case I 

 supposed in a previous lecture; we found that our 

 hypothesis on that subject was tenable and valid, 

 because the supposed cause existed in nature, because 

 it was competent to account for the phenomena, and 

 because no other known cause was competent to ac- 

 count for them; and it is upon similar grounds that 

 any hypothesis you choose to name is accepted in 

 science as tenable and valid. 



What is Mr. Darwin's hypothesis ? As I apprehend 

 it — for I have put it into a shape more convenient for 

 common purposes than I could find verbatim in his 

 book — as I apprehend it, I say, it is, that all the phe- 

 nomena of organic nature, past and present, result 

 from, or are caused by, the inter-action of those pro- 

 perties of organic matter, which we have called 

 Atavism and Variability, with the Conditions of 

 Existence ; or, in other words, — given the existence of 



