ISOLATED FORMS ANCESTRAL 47 



tently than in the crowded areas where Natural 

 Selection holds more potent sway. 



The grounds for this conclusion, stated by 

 Darwin half a century ago, should be seriously 

 considered by those who are inclined to follow 

 de Vries in his rash speculations on the periodic 

 mutation of species. The following statements 

 are to be found in Darwin's letters to Lyell : 



' A monad, if no deviation in its structure profitable to it 

 under its excessively simple conditions of life occurred, might 

 remain unaltered from long before the Silurian Age to the 

 present day.' 1 



'With respect to Lepidosiren, Ganoid fishes, perhaps 

 Ornithorliynchus, I suspect, as stated in the Origin, that they 

 have been preserved, from inhabiting fresh-water and isolated 

 parts of the world, in which there has been less competition 

 and less rapid progress in Natural Selection, owing to the 

 fewness of individuals which can inhabit small areas ; and 

 where there are few individuals variation at most must be 

 slower.' 2 



' I quite agree with you on the strange and inexplicable fact 

 of Ornithorhynchus having been preserved, and Australian 

 Trigonia, or the Silurian Lingula. I always repeat to myself 

 that we hardly know why any one single species is rare or 

 common in the best-known countries. I have got a set of 

 notes somewhere on the inhabitants of fresh water ; and it is 

 singular how many of these are ancient, or intermediate 

 forms ; which I think is explained by the competition having 

 been less severe, and the rate of change of organic forms 

 having been slower in small confined areas, such as all the 

 fresh waters make compared with sea or land.' s 



1 Oct. 11, 1859. Life and Letters, ii. 210. 



2 Feb. 18, 1860. More Letters, i. 143. See Origin of Species, 

 ed. vi, 83, 112. 



s Sept. 12, 1860. Life and Letters, ii. 340. See also Quarterly 

 Review, July, 1909, 21, 22. 



