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 havé re- 
ceived elaborate treatment by Romanes. 
The doctrine of geographical isolation offers a partial ex- 
planation of. the origin of the peculiar faunze and flore 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- 
