come unhealthy.

I have studied M. Tessier's careful and elaborate experiments,[328]
made to disprove the common belief that good is derived from a change
of seed; and he certainly shows that the same seed may with care be
cultivated on the same farm (it is not stated whether on exactly the
same soil) for ten consecutive years without loss. Another excellent
observer, Colonel Le Couteur,[329] has come to the same conclusion; but
then he expressly adds, if the same seed be used, "that which is grown
on land manured from the mixen one year becomes seed for land prepared
with lime, and that again becomes seed for land dressed with ashes,
then for land dressed with mixed manure, and so on." But this in effect
is a systematic exchange of seed, within the limits of the same farm.

On the whole the belief, which has long been held by many skilful
cultivators, that good follows from exchanging seed, tubers, &c., seems to
be fairly well founded. Considering the small size of most seeds, it seems
hardly credible that the advantage thus derived can be due to the seeds
obtaining in one soil some chemical element deficient in the other soil. As
plants after once germinating naturally become fixed to the same spot, it
might have been anticipated that they would show the good effects of a
change more plainly than animals, which continually wander about; and this
apparently is the {148} case. Life depending on, or consisting in, an
incessant play of the most complex forces, it would appear that their
action is in some way stimulated by slight changes in the circumstances to
which each organism is exposed. All forces throughout nature, as Mr.
Herbert Spencer[330] remarks, tend towards an equilibrium, and for the life
of each being it is necessary that this tendency should be checked. If
these views and the foregoing facts can be trusted, they probably throw
light, on the one hand, on the good effects of crossing the breed, for the
germ will be thus slightly modified or acted on by new forces; and on the
other hand, on the evil effects of close interbreeding prolonged during
many generations, during which the germ will be acted on by a male having
almost identically the same constitution.

_Sterility from changed Conditions of Life._

I will now attempt to show that animals and plants, when removed from their
natural conditions, are often rendered in some degree infertile or
completely barren