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for

species

within the

same

country

could

hardly

have kept

distinct

had they

been

capable of

crossing

freely.

The

importance

of the

fact that

hy- brids

are very

generally

sterile,

has, I

think,

been much

underrated

by some

late

writers.

On the

theory of

natural

selection

the case

is

especially

important,

inasmuch

as the

sterility

of hybrids

could not

possibly

be of any

advantage

to them,

and

therefore

could not

have been