250 DARWINISM TO-DAY. 



in the Hawaiian Islands, he formulated, in 1872, an im- 

 portant principle concerning the species-differentiating ef- 

 fects of indiscriminate isolation. As Romanes 2 " well points 

 out, isolation may not only admit of degrees, that is, 

 may be either total or partial, and, if partial, may occur 

 in numberless grades of efficiency, but it may be either dis- 

 criminate or indiscriminate. If it be discriminate, the isola- 

 tion has reference to the resemblance of the separated indi- 

 viduals to one another ; if it be indiscriminate, it has no such 

 reference. For example, if a shepherd divides a flock of 

 sheep without regard to their characters, he is isolating one 

 section from the other indiscriminately; but if he places all 

 the white sheep in one field and all the black sheep in another 

 field, he is isolating one section from the other discriminately. 

 Or, if geological subsidence divides a species into two parts, 

 the isolation will be indiscriminate ; but if the separation be 

 due to one of the sections developing, for example, a change 

 of instinct determining migration to another area, or occu- 

 pation of a different habitat on the same area, then the isola- 

 tion will be discriminate, so far as the resemblance of instinct 

 is concerned. Discriminate isolation has been called by 

 'Gulick segregate breeding, and indiscriminate isolation sepa- 

 rate breeding. 



Now the effectiveness of discriminate isolation or segre- 

 gate breeding, however effected, to produce species-differ- 

 entiation is of course obvious. In fact, as Romanes points 

 out, it is only when assisted by some form of discriminate 

 isolation which determines the exclusive breeding of like 

 with like, that heredity can make in favour of change of 

 type or lead to what we understand by organic evolution. 

 But what about indiscriminate isolation ? Does it not seem, 

 at first sight at least, that this kind of isolation must count 

 for nothing in the process of evolution ? Is it not apparently 

 self-evident that the farmer who separated his stock into two 

 or more parts indiscriminately, would not effect any more 



