276 
HYBKIDISM. 
Chap. VIII, 
in all other respects there seems to be a general and 
close similarity in the offspring of crossed species, and of 
crossed varieties. If we look at species as having been 
specially created, and at varieties as having been pro¬ 
duced by secondary laws, this similarity would be an 
astonishing fact. But it harmonises perfectly with the 
view that there is no essential distinction between spe¬ 
cies and varieties. 
Summary of Chapter ,—First crosses between forms 
sufficiently distinct to be ranked as species, and their 
hybrids, are very generally, but not universally, sterile. 
The sterility is of all degrees, and is often so slight that 
the two most careful experimentalists who have ever 
lived, have come to diametrically opposite conclusions 
in ranking forms by this test. The sterility is innately 
variable in individuals of the same species, and is 
eminently susceptible of favourable and unfavourable 
conditions. The degree of sterility does not strictly 
follow systematic affinity, but is governed by several 
curious and complex laws. It is generally different, 
and sometimes widely different, in reciprocal crosses 
between the same two species. It is not always equal 
in degree in a first cross and in the hybrid produced 
from this cross. 
In the same manner as in grafting trees, the capacity 
of one species or variety to take on another, is incidental 
on generally unknown differences in their vegetative 
systems, so in crossing, the greater or less facility of one 
species to unite with another, is incidental on unknown 
differences in their reproductive systems. There is no 
more reason to think that species have been specially 
endowed with various degrees of sterility to prevent 
them crossing and blending in nature, than to think 
that trees have been specially endowed with various and 
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