affected.

The period of gestation was formerly thought to be of so unalterable a
character, that a supposed difference of this kind between the wolf and the
dog was esteemed a sure sign of specific distinction; but we have seen that
the period is shorter in the improved breeds of the pig, and in the larger
breeds of the ox, than in other breeds of these two animals. And now we
know, on the excellent authority of Hermann von Nathusius (3/90. A
translation of his paper is given in 'Bull. Soc. Imp. d'Acclimat.' tome 9
1862 page 723.), that Merino and Southdown sheep, when both have long been
kept under exactly the same conditions, differ in their average period of
gestation, as is seen in the following Table

Merinos 150.3 days.
Southdowns 144.2 "
Half-bred Merinos and Southdowns 146.3 "
3/4 blood of Southdown 145.5 "
7/8 blood of Southdown 144.2 "

In this graduated difference in cross-bred animals having different
proportions of Southdown blood, we see how strictly the two periods of
gestation have been transmitted. Nathusius remarks that, as Southdowns grow
with remarkable rapidity after birth, it is not surprising that their
foetal development should have been shortened. It is of course possible
that the difference in these two breeds may be due to their descent from
distinct parent-species; but as the early maturity of the Southdowns has
long been carefully attended to by breeders, the difference is more
probably the result of such attention. Lastly, the fecundity of the several
breeds differs much; some generally producing twins or even triplets at a
birth, of which fact the curious Shangai sheep (with their truncated and