Petersburg' 1780 part 2 pages 84, 100.), and has been adopted by several
authors. I can find hardly any direct facts in its support; but unfortunately
no one has compared, in the case of either animals or plants, the fertility of
anciently domesticated varieties, when crossed with a distinct species, with
that of the wild parent-species when similarly crossed. No one has compared,
for instance, the fertility of Gallus bankiva and of the domesticated fowl,
when crossed with a distinct species of Gallus or Phasianus; and the
experiment would in all cases be surrounded by many difficulties. Dureau de la
Malle, who has so closely studied classical literature, states (16/29.
'Annales des Sc. Nat.' tome 21 1st series page 61.) that in the time of the
Romans the common mule was produced with more difficulty than at the present
day; but whether this statement may be trusted I know not. A much more
important, though somewhat different, case is given by M. Groenland (16/30.
'Bull. Bot. Soc. de France' December 27, 1861 tome 8 page 612.), namely, that
plants, known from their intermediate character and sterility to be hybrids
between Aegilops and wheat, have perpetuated themselves under culture since
1857, WITH A RAPID BUT VARYING INCREASE OF FERTILITY IN EACH GENERATION. In
the fourth generation the plants, still retaining their intermediate
character, had become as fertile as common cultivated wheat.

The indirect evidence in favour of the Pallasian doctrine appears to me to be
extremely strong. In the earlier chapters I have shown that our various breeds
of the dog are descended from several wild species; and this probably is the
case with sheep. There can be no doubt that the Zebu or humped Indian ox
belongs to a distinct species from European cattle: the latter, moreover, are
descended from two forms, which may be called either species or races. We have
good evidence that our domesticated pigs belong to at least two specific
types, S. scrofa and indicus. Now a widely extended analogy leads to the
belief that if these several allied species, when first reclaimed, had been
crossed, they would have exhibited, both in their first unions and in their