250 ENTOMOLOGY 



when assisted by some form of discriminate isolation, which 

 determines the exclusive breeding of like with like, can hered- 

 ity make in favour of change of type, or lead to what we un- 

 derstand by organic evolution." (Romanes.) 



" As soon as a portion of a species is separated from the 

 rest of that species, so that breeding between the two portions 

 is no longer possible, the general average of characters in the 

 separated portion not being in all respects precisely the same 

 as it is in the other portion, the result of in-breeding among 

 all individuals of the separated portion will eventually be dif- 

 ferent from that which obtains in the other portion; so that, 

 after a number of generations, the separated portion may 

 become a distinct species from the effect of isolation alone. 

 Even without the aid of isolation, any original difference of 

 average characters may become, as it were, magnified in suc- 

 cessive generations, provided that the divergence is not harm- 

 ful to the individuals presenting it, and that it occurs in a 

 sufficient proportional number of individuals not to be imme- 

 diately swamped by intercrossing." (Romanes.) 



Of the many modes of isolation, the most important are the 

 geographical and the physiological, both of which have re- 

 ceived elaborate treatment by Romanes. 



The doctrine of geographical isolation offers a partial ex- 

 planation of the origin of the peculiar faunae and florae of 

 remote islands. These island species, however peculiar, 

 doubtless came originally from the mainlands where their 

 nearest allies now occur ; thus the endemic insects of the Gala- 

 pagos Islands are most nearly related to species of western 

 South America. 



The first individuals of Schistocerca doubtless reached the 

 Galapagos Islands by means of the wind or on driftwood. 

 These individuals, separated from the main body of their spe- 

 cies, would interbreed and might thereby give rise to a new 

 variety or species, if we may assume that the average of charac- 

 ters of the detached portion of the species differed from that 

 of the main body of individuals ; in other words, that the iso- 



