90 EXPERIMENTAL EVOLUTION LECT. 



racters sometimes go to such an extent that animals 

 of the same species have been at times considered as 

 belonging to different species, and even to different 

 genera. Such has been the case, for instance, with 

 two fishes Abramis versicolor and Stilbe americana 

 which C. C. Abbott recognizes as one and the same 

 species which has a great tendency to variation, 

 not only as concerns colour, but in respect of fins and 

 scales, according to its environment. 1 This is doubt- 

 less an extreme case, but its interest is considerable, 

 in that it exemplifies, on the one hand, the importance 

 of variation, while, on the other, it shows once more 

 how very artificial and unsound our specific and even 

 generic distinctions in some cases are. While we 

 ascribe most of the superficial, or integumentary, 

 variations to that general and complex factor which 

 we call change of climate although we cannot in all 

 cases tell which particular factor of the complex 

 operates there are cases where we can trace the 

 variation to one determined cause. Such is the case 

 with the variation in length of wool. There is a direct 

 relation between the abundance of food and the length 

 of the wool of sheep, for instance. Krocker, 2 in 

 Proskau, has shown that the amount of wool yielded 



1 C. C. Abbott : Notes on the Cyprinoias of Central New Jersey. 

 American Nattiralist, vol. viii. p. 326. 



- His paper has been published in the Annalen der Landwirthschaft 

 in den Koeniglich Preussischen Staaten for 1869. 



