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and

crossing.

Fertility

of

varieties

when

crossed

and of

their

mongrel

offspring

not

universal.

Hybrids

and

mongrels

compared

indepen-

dently of

their

fertility.

Summary.

The view

generally

entertained

by

naturalists

is that

species,

when

inter-

crossed,

have been

specially

endowed

with the

quality of

sterility,

in order

to prevent

the

confusion

of all

organic

forms.

This view

certainly

seems at

first

probable,