escaped in the Zoological Gardens of London "and for a long time afterwards
the keepers frequently caught cross-bred rats, at first half-breds, afterwards
with less of the character of the snake-rat, till at length all traces of it
disappeared." (15/8. Mr. S.J. Salter 'Journal Linn. Soc.' volume 6 1862 page
71.) On the other hand, in some parts of London, especially near the docks,
where fresh rats are frequently imported, an endless variety of intermediate
forms may be found between the brown, black, and snake rat, which are all
three usually ranked as distinct species.

How many generations are necessary for one species or race to absorb another
by repeated crosses has often been discussed (15/9. Sturm 'Ueber Racen, etc.'
1825 s. 107. Bronn 'Geschichte der Natur' b. 2 s. 170 gives a table of the
proportions of blood after successive crosses. Dr. P. Lucas 'L'Heredite Nat.'
tome 2 page 308.); and the requisite number has probably been much
exaggerated. Some writers have maintained that a dozen or score, or even more
generations, are necessary; but this in itself is improbable, for in the tenth
generation there would be only 1-1024th part of foreign blood in the
offspring. Gartner found (15/10. 'Bastarderzeugung' s. 463, 470.), that with
plants, one species could be made to absorb another in from three to five
generations, and he believes that this could always be effected in from six to
seven generations. In one instance, however, Kolreuter (15/11. 'Nova Acta
Petrop.' 1794 page 393: see also previous volume.) speaks of the offspring of
Mirabilis vulgaris, crossed during eight successive generations by M.
longiflora, as resembling this latter species so closely, that the most
scrupulous observer could detect "vix aliquam notabilem differentiam" or, as
he says, he succeeded, "ad plenariam fere transmutationem." But this
expression shows that the act of absorption was not even then absolutely
complete, though these crossed plants contained only the 1-256th part of M.
vulgaris. The conclusions of such accurate observers as Gartner and Kolreuter
are of far higher worth than those made without scientific aim by breeders.