N@2 i] ORGANIC VARIATION. 261 
range of the species. Now if the Darwinian doctrine of evolution 
be true, all such races have descended from one ancestral form ; 
but I think that we may go still further, and postulate that 
wherever one geographical race grades insensibly into another, 
there the agency of development must be still continuing. 
For supposing A, 4, C to be three intergrading geographical 
races inhabiting contiguous areas a, b,c. In area a, together 
with the individuals of race A, will always occur some individuals 
of races B and C, which have migrated from @ and ¢ into a. 
Now these individuals, which have migrated from 4 and ¢ into 
a, must adapt themselves to the new environment of the area a, 
if they would compete successfully with the individuals of race 
A ; and thus a continuous development of a considerable num- 
ber of the individuals of the species must proceed, tending to 
produce favorable adaptations in the struggle for existence, — 
this struggle being probably keenest where the areas a, 6, and 
c overlap. It is still a bone of contention between systematic 
ornithologists whether individuals of a race B are ever found 
in the area proper to a race A, or vice versa; but the cases 
where this is so, as e.g. Dendroica palmarum and its variety 
hypochrysea, are so numerous as to warrant the conclusion that, 
wherever the geographical lines of demarcation between the 
respective breeding areas are not strongly marked, there 
must be a considerable interchange of wandering individuals. 
And it seems to be always the rule, that the indefinitely broad 
area of demarcation is peopled promiscuously by individuals of 
the contiguous races. Only when the lines of demarcation 
are formed by high mountain ranges, deserts, or great water 
expanses would there be little or no interchange of individuals ; 
but when the boundaries of the areas of the several forms are 
so comparatively impassable, the various forms usually do not 
perfectly intergrade, and hence are to be classed rather as sep- 
arate species than as races of one species. Therefore it is cor- 
rect to conclude that in a widely ranging species, split up into 
a number of intergrading geographical races, a continuous 
agency of development is at work, leading to the readaptation 
of the migrating individuals to new environments. 
Darwin (“Origin of Species”) has ably argued the point 
