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1859
1860
1861
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we plainly see that 1859 1860 1861 1866
OMIT 1869 1872

that is between members of different strains or sub-breeds, 1859 1860 1861 1866
which differ to a certain extent, 1869 1872

offspring. 1859 1860 1861 1866
offspring; and that close interbreeding continued during several generations between the nearest relations, especially if these be kept under the same conditions of life, almost always induces weakness and sterility. 1869
offspring; 1872

I believe, indeed, from the facts alluded to in our fourth chapter, that a certain amount of crossing is indispensable even with hermaphrodites; 1859 1860 1861 1866
OMIT 1872

always induces weakness and sterility in the progeny. 1859 1860 1861 1866
almost always leads to decreased size, weakness, or sterility. 1872

varied and become 1859 1860 1861 1866 1869
been subjected to 1872

give 1859 1860 1861 1866 1869
conditions, or which have slightly varied, give 1872

that greater changes, or changes of a particular nature, often render 1859 1860 1861 1866 1869
OMIT 1872

in some degree 1859 1860 1861 1866 1869
long habituated to certain uniform conditions under a state of nature, when subjected, as under confinement, to a considerable change in their conditions, very frequently are rendered more or less 1872

that greater crosses, that is 1859 1860 1861 1866 1869
we know that a 1872

males and females which 1859 1860 1861 1866 1869
two forms, that 1872

cannot persuade myself 1859 1860 1861 1866 1869
am fully persuaded 1872

parallelism is 1859 1860 1861 1866 1869
double parallelism is by no means 1872

3 blocks not present in 1859 1860 1861 1866 1869; present in 1872
He who is able to explain why the elephant and a multitude of other animals are incapable of breeding when kept under only partial confinement in their native country, will be able to explain the primary cause of hybrids being so generally sterile. He will at the same time be able to explain how it is that the races of some of our domesticated animals, which have often been subjected to new and not uniform conditions, are quite fertile together, although they are descended from distinct species, which would probably have been sterile if aboriginally crossed. The above two parallel series of facts seem to be connected together by some common but unknown bond, which is essentially related to the principle of life; this principle, according to Mr. Herbert Spencer, being that life depends on, or consists in, the incessant action and reaction of various forces, which, as throughout nature, are always tending towards an equilibrium; and when this tendency is slightly disturbed by any change, the vital forces gain in power.

life; this principle apparently being that life, as Mr. Herbert Spencer has remarked, depends on, or consists in, the incessant action and reaction of various forces, which, as throughout nature, are always tending towards an equilibrium; and when this ten- dency is slightly disturbed by any change, the vital forces apparently gain in power. 1866
life. 1859 1860 1861
life; this principle apparently being that life, as Mr. Herbert Spencer has remarked, depends on, or consists in, the incessant action and reaction of various forces, which, as throughout nature, are always tending towards an equilibrium; and when this tendency is slightly disturbed by any change, the vital forces apparently gain in power. 1869

seed, tubers, &c., from one soil or climate to another, and back again. During the convalescence of animals, we plainly see that great benefit is derived from almost any change in
their
the
habits of life. Again, both with plants and animals, there is
the clearest
abundant
evidence,
evidence
that a cross between
very distinct
very distinct
individuals of the same species, that is between members of different strains or sub-breeds, gives vigour and fertility to the offspring. I believe, indeed, from the facts alluded to in our fourth chapter, that a certain amount of crossing is indispensable even with hermaphrodites; and that close
inter-breeding
interbreeding
continued during several generations between the nearest relations,
especially
especially
if these be kept under the same conditions of life, always induces weakness and sterility in the progeny.
Hence it seems that, on the one hand, slight changes in the conditions of life benefit all organic beings, and on the other hand, that slight crosses, that is crosses between the males and females of the same
species,
species
which have varied and become slightly
different
different,
give vigour and fertility to the offspring.
But,
But
as we
we
have
seen,
seen
that greater changes, or changes of a particular nature, often render organic beings in some degree sterile; and that greater crosses, that is
cross
crosses
between males and females which have become widely or specifically different, produce hybrids which are
almost always
generally sterile
in some
degree sterile.
degree.
I cannot persuade myself that this parallelism is an accident or an illusion. Both series of facts seem to be connected together by some common but unknown bond, which is essentially related to the principle of life; this principle apparently being that life, as Mr. Herbert Spencer has remarked, depends on, or consists in, the incessant action and reaction of various forces, which, as throughout nature, are always tending towards an equilibrium; and when this ten- dency is slightly disturbed by any change, the vital forces apparently gain in power.