26 
DOMESTIC PIGEONS. 
Chap. I. 
that each breed, even the purest, has within a dozen or, 
at most, within a score of generations, been crossed by 
the rock-pigeon: I say within a dozen or twenty genera¬ 
tions, for we know of no fact countenancing the belief 
that the child ever reverts to some one ancestor, removed 
by a greater number of generations. In a breed which 
has been crossed only once with some distinct breed, the 
tendency to reversion to any character derived from such 
cross will naturally become less and less, as in each suc¬ 
ceeding generation there will be less of the foreign blood; 
but when there has been no cross with" a distinct breed, 
and there is a tendency in both parents to revert to a 
character, which has been lost during some former gene¬ 
ration, this tendency, for all that we can see to the 
contrary, may be transmitted undiminished for an indefi¬ 
nite number of generations. These two distinct cases are 
often confounded in treatises on inheritance. 
Lastly, the hybrids or mongrels from between all the 
domestic breeds of pigeons are perfectly fertile. I can 
state this from my own observations, purposely made, 
on the most distinct breeds. Now, it is difficult, per¬ 
haps impossible, to bring forward one case of the hybrid 
offspring of two animals clearly distinct being themselves 
perfectly fertile. Some authors believe that long-con¬ 
tinued domestication eliminates this strong tendency to 
sterility: from the history of the dog I think there is 
some probability in this hypothesis, if applied to species 
closely related together, though it is unsupported by a 
single experiment. But to extend the hypothesis so 
far as to suppose that species, aboriginally as distinct, 
as carriers, tumblers, pouters, and fantails now are, 
should yield offspring perfectly fertile, inter se, seems to 
me rash in the extreme. 
From these several reasons, namely, the improbability 
of man having formerly got seven or eight supposed 
