270 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
will be that, if they differ in any definite way from the main body of 
the species, a new elementary species will at once gain a foothold and 
will evolve independently of the parent-species. If a certain area of 
land is cut off from a continent so as to form a continental island, the 
members of each species that have become isolated will evolve independ- 
ently of the main body of the species and will have their own peculiar 
lines of variation preserved from back-crossing with the parent-species. 
Professor David Starr Jordan,* the leading proponent of the theory 
of geographic isolation in America says: 
“Tt is now nearly forty years since Moritz Wagner (1868) first 
made it clear that geographic isolation (rdumlicke Sonderung) was a 
factor or condition in the formation of every species, race, or tribe of 
animal or plant we know on the face of the earth. This conclusion 
is accepted as almost self-evident by every competent student of 
species or of the geographical distribution of species. But to those 
who approach the subject of evolution from some other side the 
principles set forth by Wagner seem less clear. They have never been 
confuted, scarcely ever attacked, so far as the present writer remem- 
bers, but in the literature of evolution of the present day they have 
been almost universally ignored. Nowadays much of our discussion 
turns on the question of whether or not minute favorable variations 
would enable their possessors little by little to gain on the parent stock, 
so that a new race would be established side by side with the old, or on 
whether a wide fluctuation or mutation would give rise to a new species 
which would hold its own in competition with the parent. In theory, 
either of these conditions might exist. In fact, both of them are 
virtually unknown. In nature a closely related distinct species is not 
often quite side by side with the old. It is simply next to it, geo- 
graphically or geologically speaking, and the degree of distinction 
almost always bears a relation-to the importance or the permanence 
of the barrier separating the supposed new stock from the parent 
stock. 
“A flood of light may be thrown on the theoretical problem of the 
origin of species by the study of the probable, actual origin of species 
with which we are familiar or of which the actual history or the actual 
ramifications may in some degree be traced. 
“Tn regions broken by few barriers, migration and interbreeding 
being ‘allowed, we find widely distributed species, homogeneous in their 
character, the members showing individual fluctuation and climatic 
t Science, N.S., Vol. XXII (1905). 
