2 Darwin, and after Darwin. 



that in the principle of Isolation we have a principle 

 so fundamental and so universal, that even the great 

 principle of Natural Selection lies less deep, and 

 pervades a region of smaller extent. Equalled only 

 in its importance by the two basal principles of 

 Heredity and Variation, this principle of Isolation 

 constitutes the third pillar of a tripod on which is 

 reared the whole superstructure of organic evolution. 



By isolation I mean simply the prevention of inter- 

 crossing between a separated section of a species or 

 kind and the rest of that species or kind. Whether 

 such a separation be due to geographical barriers, to 

 migration, or to any other state of matters leading 

 to exclusive breeding within the separated group, 

 I shall indifferently employ the term isolation for the 

 purpose of designating what in all cases is the same 

 result — namely, a prevention of intercrossing between 

 A and B, where A is the separated portion and B the 

 rest of the species or kind. 



The importance of isolation as against dissimilar 

 forms has always been fully appreciated by breeders 

 fanciers, horticulturists, &c., who are therefore most 

 careful to prevent their pedigree productions from 

 intercrossing with any other stock. Isolation is indeed, 

 as Darwin has observed, " the corner-stone of the 

 breeder's art." And similarly with plants and animals 

 in a state of nature : unless intercrossing with allied 

 (i.e. dissimilar) forms is prevented, the principle of 

 heredity is bound to work for uniformity, by blend- 

 ing the dissimilar types in one : only when there is 

 exclusive breeding of similarly modified forms can the 

 principle of heredity work in the direction of change 

 — i. e. of evolution. 



