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paired were unproductive, yet neither impotent. For instance, 
[ had this morning a letter with a case of a Hereford 
heifer, which seemed to be, after repeated trials, sterile with 
one particular and far from impotent bull, but not with 
another bull. But it is too long a story—it is to attempt to 
make two strains, both fertile, and yet sterile when one of one 
strain is crossed with one of the other strain. But the 
difficulty . . . would be beyond calculation.” * 
The experiment was evidently unsuccessful,—perhaps was 
never seriously undertaken,—and a few years later Darwin 
added the following postscript to a letter to Huxley, January 7 
[1867]. 
“P.S.—Nature never made species mutually sterile by 
selection, nor will men.” t 
This was probably only an offhand expression of opinion, 
not intended to be taken seriously. An altogether hopeless 
attitude would not be reasonable until the suggested scheme 
had been applied many times, and in several parts of the 
animal and vegetable kingdoms. 
But the positive results demanded by Huxley, even if 
obtained, would by no means justify his far-reaching 
conclusions, If the barrier of sterility were thus artificially 
produced, we should be very far from the proof that its exist- 
ence in nature is due to the same kind of cause, viz. selection. 
If Darwin was right in his controversy with Wallace, if 
“Nature never made species mutually sterile by selection,” 
the suggested experiment would merely do by artificial selection 
what is not done by natural selection. 
It is by no means difficult to understand the mutual sterility 
which is usual between natural species as an incidental result 
of their separation by asyngamy for a long period of time. 
In the process of fertilisation a portion of a single cell nucleus 
from one individual fuses with a portion from another in- 
dividual, the two combining to form the complete nucleus of 
the first cell of the offspring, from which all the countless 
cells of the future individual will arise by division. Each 
part-nucleus contains the whole of the hereditary qualities 
* “* More Letters,” vol. i, pp. 225, 226, Letter 154. 
+ Ibid. vol. i, p. 277, Letter 197. 
