same species rarely or never exists. In former times, even in the United
States, there were no distinct races of sheep, for all had been mingled
together. (15/1. 'Communications to the Board of Agriculture' volume 1 page
367.) The celebrated agriculturist Marshall (15/2. 'Review of Reports, North
of England' 1808 page 200.) remarks that "sheep that are kept within fences,
as well as shepherded flocks in open countries, have generally a similarity,
if not a uniformity, of character in the individuals of each flock;" for they
breed freely together, and are prevented from crossing with other kinds;
whereas in the unenclosed parts of England the unshepherded sheep, even of the
same flock, are far from true or uniform, owing to various breeds having
mingled and crossed. We have seen that the half-wild cattle in each of the
several British parks are nearly uniform in character; but in the different
parks, from not having mingled and crossed during many generations, they
differ to a certain small extent.

We cannot doubt that the extraordinary number of varieties and sub-varieties
of the pigeon, amounting to at least one hundred and fifty, is partly due to
their remaining, differently from other domesticated birds, paired for life
once matched. On the other hand, breeds of cats imported into this country
soon disappear, for their nocturnal and rambling habits render it hardly
possible to prevent free crossing. Rengger (15/3. 'Saugethiere von Paraguay'
1830 s. 212.) gives an interesting case with respect to the cat in Paraguay:
in all the distant parts of the kingdom it has assumed, apparently from the
effects of the climate, a peculiar character, but near the capital this change
has been prevented, owing, as he asserts, to the native animal frequently
crossing with cats imported from Europe. In all cases like the foregoing, the
effects of an occasional cross will be augmented by the increased vigour and
fertility of the crossed offspring, of which fact evidence will hereafter be
given; for this will lead to the mongrels increasing more rapidly than the
pure parent-breeds.