inct
species.

How many generations are necessary for one species or race to absorb
another by repeated crosses has often been discussed;[188] 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 will be
only 1-1024th part of foreign blood in the offspring. Gaertner found,[189]
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 seventh generations. In one instance, however, Koelreuter[190]
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;"--he succeeded, as he says, "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 Gaertner and Koelreuter are of far higher worth than
those made without scientific aim by breeders. The most remarkable
statement which I have met with of the persistent endurance of the effects
of a single cross is given by Fleischmann,[191] who, in reference to German
sheep, says "that the original coarse sheep have 5500 fibres of wool on a
square inch; grades of the third or fourth Merino cross produced about
8000, the twentieth cross 27,000, the perfect pure Merino blood 40,000 to
48,000." So that in this case common German sheep crossed twenty times
successively with Merinos have not by any means acquired wool as fine as
that of the pure breed. In all cases, the rate of absorption will {89}
depend largely on the conditions of life being favourable to any particular
character; and we may suspect that there would be under the climate of
Germany a constant tendency to degeneration in the wool of Merinos, unless
prevented by careful selection; and thus perhaps the foregoing remarkable
case may be explained. The rate of absorption must also depend on the
amount of distinguishable difference between the two forms which are
crossed, and especially, as Gaertner insists, on prepotency of transmission
in the one form over the other. We have seen in th