Chap. XIL 
OCEANIC ISLANDS. 
405 
enormous ranges. It is not meant that in world¬ 
ranging genera all the species have a wide range, or 
even that they have on an average a wide range; but 
only that some of the species range very widely ; for the 
facility with which widely-ranging species vary and give 
rise to new forms will largely determine their average 
range. For instance, two varieties of the same species 
inhabit America and Europe, and the species thus has 
an immense range; but, if the variation had been a little 
greater, the two varieties would have been ranked as dis¬ 
tinct species, and the common range would have been 
greatly reduced. Still less is it meant, that a species 
which apparently has the capacity of crossing barriers 
and ranging widely, as in the case of certain powerfully- 
winged birds, will necessarily range widely; for we 
should never forget that to range widely implies not 
only the power of crossing barriers, but the more im¬ 
portant power of being victorious in distant lands in 
the struggle for life with foreign associates. But 
on the view of all the species of a genus having de¬ 
scended from a single parent, though now distributed 
to the most remote points of the world, we ought to 
find, and I believe as a general rule we do find, that 
some at least of the species range very widely ; for it is 
necessary that the unmodified parent should range 
widely, undergoing modification during its diffusion, and 
should place itself under diverse conditions favourable 
for the conversion of its offspring, firstly into new varie¬ 
ties and ultimately into new species. 
In considering the wide distribution of certain genera, 
we should bear in mind that some are extremely ancient, 
and must have branched off from a common parent 
at a remote epoch; so that in such cases there will 
have been ample time for great climatal and geographical 
changes and for accidents of transport; and conse¬ 
quently for the migration of some of the species into all 
