apparently comes into play, for otherwise all parts of the body would be
equally intermediate in character. So again, when the offspring of hybrids or
mongrels, which are themselves nearly intermediate in character, revert either
wholly or by segments to their ancestors, the principle of the affinity of
similar, or the repulsion of dissimilar atoms, must come into action. To this
principle, which seems to be extremely general, we shall recur in the chapter
on pangenesis.

It is remarkable, as has been strongly insisted upon by Isidore Geoffroy St.
Hilaire in regard to animals, that the transmission of characters without
fusion occurs very rarely when species are crossed; I know of one exception
alone, namely, with the hybrids naturally produced between the common and
hooded crow (Corvus corone and cornix), which, however, are closely allied
species, differing in nothing except colour. Nor have I met with any well-
ascertained cases of transmission of this kind, even when one form is strongly
prepotent over another, when two races are crossed which have been slowly
formed by man's selection, and therefore resemble to a certain extent natural
species. Such cases as puppies in the same litter closely resembling two
distinct breeds, are probably due to superfoetation,--that is, to the
influence of two fathers. All the characters above enumerated, which are
transmitted in a perfect state to some of the offspring and not to others,--
such as distinct colours, nakedness of skin, smoothness of leaves, absence of
horns or tail, additional toes, pelorism, dwarfed structure, etc.,--have all
been known to appear suddenly in individual animals and plants. From this
fact, and from the several slight, aggregated differences which distinguish
domestic races and species from one another, not being liable to this peculiar
form of transmission, we may conclude that it is in some way connected with
the sudden appearance of the characters in question.]

ON THE MODIFICATION OF OLD RACES AND THE FORMATION OF NEW RACES BY CROSSING.