374 ENTOMOLOGY 
from the Pacific to the Atlantic, exhibits many geographical 
varieties, some of which are clearly due to temperature, as 
experiments have shown. 
Geographical isolation is often followed by changes in the 
specific characters of an organism, as witness the endemic 
species and varieties of oceanic islands. Even in the same 
archipelago, the different islands may be characterized by dif- 
ferent varieties of one and the same species, or even by differ- 
ent but closely allied species of the same genus. ‘Thus Darwin 
and Alexander Agassiz found that in the Galapagos Islands 
each island had its own species of Tropidurus (a lizard) and 
had only one species, with almost no exceptions. ‘The same 
phenomenon occurs in the two Galapagan species of Schisto- 
cerca—S. melanocera and S. literosa. In melanocera, as 
Scudder discovered, “‘ Three or four distinct types are becom- 
ing gradually differentiated on the eight [now ten] islands 
from which they are known.” Snodgrass, who has recently 
made important additions to Scudder’s account, says, in regard 
to the two species, ‘‘ The specimens from the different islands 
show striking, though, in most cases, slight differences distin- 
euishing the individuals of each island as a race, from those 
inhabiting any other island. There are two exceptions. 
Abingdon and Bindloe have the same form, and Albemarle 
supports at least two races.’’ [Each of these two species pre- 
sents no less than five racial types, to which distinctive names 
have been applied. Though the relationships and evolution of 
these races have been ably discussed by Snodgrass, definite 
conclusions upon these subjects are still needed. Isolation in 
general we have considered briefly in Chapter VII. 
Faunal Realms.—The general distribution of life is such 
that naturalists divide the earth into several realms, each of 
which has its characteristic fauna and flora. As to the precise 
boundaries of these faunal realms, zoologists do not all agree, 
owing chiefly to the fact that faunz overlap one another to 
such an extent as to render their exact separation more or less 
arbitrary. Five realms, at least, are generally recognized: 
