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acquired

by the

continued

preservation

of

successive

profitable

degrees of

sterility.

I hope,

however,

to be able

to show

that

sterility

is not a

specially

acquired

or endowed

quality,

but is

incidental

on other

acquired

differences.

In

treating

this

subject,

two

classes of

facts, to

a large

extent

fundamentally

different,

have

generally

been

confounded

together;

namely,

the

sterility

of two

species

when first

crossed,

and the

sterility