178 PROBLEMS OF EVOLUTION 



it has in consequence been elevated to the rank of a separate 

 species (Putorius bibernicus). 



I have studied the lengthy and somewhat pedantic paper on 

 "Divergent evolution through cumulative segregation" 1 in which 

 Mr Gulick has expounded the theories that he bases on his in- 

 vestigation of the Molluscan Fauna of the Sandwich Islands. 

 And though the actual conclusions seem to me to a great extent 

 warranted by the evidence, yet some of the sounding phrases 

 would, if taken literally, mean a great deal more than they 

 are, apparently, intended to mean. Such are "cumulative 

 segregation," " intensive segregation." It turns out, after all, 

 when we come to the facts, that the words "cumulative" 

 and "intensive" are misapplied, meaning no more than that 

 species are multiplied, or subdivided into many, the differ- 

 ences -between all of them being slight. They still 

 belong, all of them, to the same family or sub-family, the 

 Achatinellidae. The snails have somehow been transported to 

 new valleys, and, probably because there were no new arrivals 

 for a long time, intersterility with other varieties has arisen and 

 so a new species has been formed. This repeated division and 

 sub-division till one species has become three hundred or so, to 

 say nothing of numbers of varieties, is a fact to stimulate reflec- 

 tion. And I think the conclusion that we must draw is, that 

 slight variations that arise may become fixed, when there is 

 isolation, without the help of Natural Selection. But there is no 

 cumulation. The variations have not been piled one upon the 

 other till the three hundredth species bears little resemblance to 

 the first. There seems to have been no thought of putting any 

 of them in a separate family. But if evolution is a fact we have 

 to account for such things as the gradual development of a rudi- 

 mentary notochord into a backbone, of a mere pigment spot 

 beneath the skin into an eye, of a one-celled animal into a man. 



To these problems isolation, acting alone, has noth ng to say. 

 The kind of work it has accomplished among the molluscs of 

 the Sandwich Islands is the same, only on a large scale, as might 

 be achieved if our many varieties and sub-varieties of brambles 



1 Linnaean Society's Journal, vol. xx (1886-1890). 



