ssing the relative numbers of the sexes in insects, and has brought
out some very curious results.

Is the orang polygamous? But I daresay I shall find that in your papers
in (I think) the _Annals and Magazine of Natural History_.

* * * * *

The following group of letters deals with the causes of the sterility of
hybrids (_see_ note in "More Letters," p. 287). Darwin's final view is
given in the "Origin," 6th edit., 1900, p. 384. He acknowledges that it
would be advantageous to two incipient species if, by physiological
isolation due to mutual sterility, they could be kept from blending; but
he continues: "After mature reflection, it seems to me that this could
not have been effected through Natural Selection." And finally he
concludes (p. 386): "But it would be superfluous to discuss this
question in detail; for with plants we have conclusive evidence that the
sterility of crossed species must be due to some principle quite
independent of Natural Selection. Both Gaeartner and Kolreuter have
proved that in genera including numerous species a series can be formed
from species which, when crossed, yield fewer and fewer seeds, to
species which never produce a single seed, but yet are affected by the
pollen of certain other species, for the germen swells. It is here
manifestly impossible to select the more sterile individuals, which have
already ceased to yield seeds; so that this acme of sterility, when the
germen alone is affected, cannot have been gained through selection; and
from the laws governing the various grades of sterility being so uniform
throughout the animal and vegetable kingdoms, we may infer that the
cause, whatever it may be, is the same or nearly the same in all cases."

Wallace still adhered to his view (_see_ "Darwinism," 1889, p. 174,
_also_ p. 292 of "More Letters," note 1, and Letter 211, p. 299). The
discussion of 1868 began with a letter from Wallace, written towards the
end of February, giving his opinion on the "Variation of Animals and
Plants"; the discussion on the sterility of hybrids is at p. 185, Vol.
II., 1st edit.

* * * * *

(_Second and third sheets of a letter from Wallace, apparently of
February, 1868._)

I am in the second volume of your book, and I have been astonished at
the immense number of interesting facts you have brought together. I
read the chapter on Pangenesis first, for I could not wait. I can hardly
t