22 



Darwin, and after Darwin. 



evolution," or an arborescent multiplication of types 

 arising simultaneously in space. 



For example, let us again take the geographical 

 form of isolation. Where a single small intergenerant 

 group of individuals is separated from the rest of 

 its species — say, on an oceanic island — monotypic 

 evolution may take place through a continuous and 

 cumulative course of independent variation in a 

 single line of change : all the individuals composing 

 any one given generation will closely resemble one 

 another, although the type may be progressively 

 altering through a long series of generations. But if 

 the original species had had two small colonies 

 separated from itself (one on each of two different 

 islands, so giving rise to two cases of isolation), then 

 polytypic evolution would have ensued to the extent 

 of there having been two different lines of evolu- 

 tion going on simultaneously (one upon each of 

 the two islands concerned). Similarly, of course, if 

 there had been three or four such colonies, there 

 would have been three or four divergent lines of 

 evolution, and so on. 



5. In the cases of isolation just supposed there 

 is only one fo7'm of isolation ; and it is thus shown 

 that under one form of isolation there may be as 

 many lines of divergence as there are separate cases 

 of such isolation. But now suppose that there are 

 two or more forms of isolation — for instance, that 

 on the same oceanic island the original colony has 

 begun to segregate into secondary groups under 

 the influence of natural selection, sexual selection, 

 physiological selection, or any of the other forms 

 of isolation — then there will be as many lines of 



