96 Darwin, and after Darwin. 



parent form. Then another long period of stability 

 ensued until another eruption of new variations took 

 place ; and these variations, after having affected 

 a greater and greater number of individuals, eventu- 

 ally blended together by intercrossing and sup- 

 planted their parent form. So the process went on, 

 comparatively short periods of variation alternating 

 with comparatively long periods of stability, the 

 variations, moreover, always occurring suddenly in 

 crops, then multiplying, blending together, and in 

 their finally blended type eventually supplanting their 

 parent form. 



Now, the remarkable fact here is that whenever the 

 variations arose, they only intercrossed between them- 

 selves, they did not intercross with their parent form ; 

 for, if they had, not only could they never have 

 survived (having been at first so few in number and 

 there having been no geographical barriers in the 

 small lake), but we should have found evidence of 

 the fact in the half-bred progeny. Moreover, natural 

 selection can have had nothing to do with the process, 

 because not only are the variations in the form of the 

 shells of no imaginable use in themselves ; but it 

 would be preposterous to suppose that at each of these 

 " variation periods " several different variations should 

 always have occurred simultaneously, all of which were 

 of some hidden use, although no one of them ever 

 occurred during any of the prolonged periods of 

 stability. How, then, are we to explain the fact that 

 the individuals composi, g each crop of varieties, while 

 able to breed among themselves, never crossed with 

 their parent form? These varieties, each time that 

 they arose, were intimately commingled with their 



