242 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
instincts, but as small consequences of one general law leading to the 
advancement of all organic beings, namely, multiply, vary, let the 
strongest live and the weakest die. 
DARWIN’S SUMMARY OF HIS ANSWER TO THE DIFFICULTY AS TO THE INABILITY OF 
NATURAL SELECTION TO ACCOUNT FOR THE FACT THAT SPECIES WHEN CROSSED 
ARE STERILE OR PRODUCE STERILE OFFSPRING, WHEREAS WHEN VARIETIES 
ARE CROSSED THEIR FERTILITY IS UNIMPAIRED 
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 
most careful experimentalists have arrived at diametrically opposite 
conclusions in ranking forms by this test. The sterility is innately 
variable in individuals of the same species, and is eminently suscept- 
ible to the action of favorable and unfavorable 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 
hybrids produced from this cross. 
In the same manner as in grafting trees, the capacity in one species 
or variety to take on another, is incidental on differences, generally 
of an unknown nature, 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 their crossing and blending in 
nature, than to think that trees have been specially endowed with 
various and somewhat analogous degrees of difficulty in being grafted 
together in order to prevent their inarching in our forests. 
The sterility of first crosses and of their hybrid progeny has not 
been acquired through natural selection. In the case of first crosses 
it seems to depend on several circumstances; in some instances in 
chief part on the early death of the embryo. In the case of hybrids, 
it apparently depends on their whole organization having been dis- 
turbed by being compounded from two distinct forms; the sterility 
being closely allied to that which so frequently affects pure species, 
when exposed to new and unnatural conditions of life. He who will 
explain these latter cases will be able to explain the sterility of hybrids. 
This view is strongly supported by a parallelism of another kind: 
namely, that, firstly, slight changes in the conditions of life add to the 
