214 Darwin, and after Darwin. 
policy in question fails. In other words, where the 
inhabitants of an area are free to migrate to other 
areas, the policy of correlating affinity with distribu- 
tion is most significantly forgotten. In this case 
species wander away from their native homes. and 
the course of their wanderings is marked by the 
origination of new species springing up ez route. 
Now, is it reasonable to suppose that the mere cir- 
cumstance of some members of a species being able 
to leave their native home should furnish any occasion 
for creating newand allied species upon the tracts over 
which they travel, or the territories to which they go? 
When the 400 existing species of humming-birds 
have all been created on the same continent for some 
reason supposed to be unknown, why should this 
reason give way before the accident of any means 
of migration being furnished to humming-birds, so 
that they should be able to visit, say the continents 
of Africa and Asia, there gain a footing beside the 
sun-birds, and henceforth determine a new centre for 
the separate creation of additional species of hum- 
ming birds peculiar to the Old World-—as has hap- 
pened in the case of the majority of species which, 
unlike the humming-birds, have been at any time 
free to migrate from their original homes? 
Lastly, my third consideration is, that the supposed 
policy in question does not extend to affinities which 
are wider than those between species and genera— 
more rarely to families, scarcely ever to orders, and 
never to classes. In other words, nature shows a 
double correlation in her geographical distribution 
of organic types :—first, that which we have already 
considered between geographical restriction and 
