ORIGIN OF SPECIES 117 



rated which might serve to create, as it were, a new 

 species from an old one, yet they are perfectly natural 

 ones, and such as I think must occur, have occurred, 

 and possibly be occurring still. We know so very little 

 of the causes which, in by far the majority, if not in 

 nearly all cases, make species rare or common, that 

 there may be hundreds of others at work, some even more 

 powerful than these, that go to perpetuate certain forms 

 in Darwin's words according to natural means of selec- 

 tion. You may have a mere individual difference in 

 the organs of digestion, and in this way produce a 

 Gillaroo Trout with his gizzard-like stomach, out of a 

 common Salmo fario. But for your paper you must 

 first consult Darwin and Wallace, and you will under- 

 stand that nothing that I have advised here is my own, 

 but theirs, except the application of their theory to 

 Algerian Larks and Irish trout. You should also get a 

 little book of Vernon Wollaston's on the " Variation of 

 Species," published a year or two ago by Van Voorst, 

 the price of which is 5s. or so.* 



Thirty years later, when writing the article for 

 Macmillan's Magazine, from which the above extracts 

 have been taken, Newton asked Tristram to lend him 

 the last quoted letter and recalled the circumstances in 

 which it had been written 



With many thanks I return the old letter you have 

 sent me. The particular one, or more than one, that I 

 wanted to see must be much earlier. I think you will 

 find I mentioned the Darwin and Wallace paper to you 

 as soon as I became acquainted with it, and that was in 

 August, 1858, just after my return from Iceland, having 

 taken Castle Eden on my way home. During our stay 

 in Iceland Wolley and I had been continually discussing 

 what should be held to constitute a " species " and 

 how new " species " began. Of course, we came to no 



* Letter to H. B. Tristram, August 24, 1858. 



