138 MR. DARWIN S WORK AND 



organic matter, its tendency to transmit its properties, 

 and its tendency occasionally to vary ; and, lastly, given 

 the conditions of existence by which organic matter is 

 surrounded — that these put together are the causes of 

 the Present and of the Past conditions of Organic 

 Nature. 



Such is the hypothesis as I understand it. Now let 

 us see how it will stand the various tests which I laid 

 down just now. In the first place, do these supposed 

 causes of the phenomena exist in nature ? Is it the 

 fact that in nature these properties of organic matter 

 — atavism and variability — and those phenomena 

 which we have called the conditions of existence, — is it 

 true that they exist ? Well, of course, if they do not 

 exist, all that I have told you in the last three or 

 four lectures must be incorrect, because I have been 

 attempting to prove that they do exist, and I take it 

 that there is abundant evidence that they do exist ; so 

 far, therefore, the hypothesis does not break down. 



But in the next place comes a much more difficult 

 inquiry: — Are the causes indicated competent to give 

 rise to the phenomena of organic nature ? I suspect 

 that this is indubitable to a certain extent. It is de- 

 monstrable, I thinki as I have endeavoured to show 

 you, that they are perfectly competent to give rise to 

 all the phenomena which are exhibited by Races in 

 nature. Furthermore, I believe that they are quite 

 competent to account for all that we may call purely 

 structural phenomena which are exhibited by Species 

 in nature. On that point also I have already enlarged 

 somewhat. Again, I think that the causes assumed 

 are competent to account for most of the physio- 



