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Updated: June 29, 2025
When hybrids are able to breed inter se, they transmit to their offspring from generation to generation the same compounded organisation, and hence we need not be surprised that their sterility, though in some degree variable, rarely diminishes.
In both, the tendency goes to a certain extent with systematic affinity, for whole groups of animals and plants are rendered impotent by the same unnatural conditions; and whole groups of species tend to produce sterile hybrids.
The towns stood like ruins in a vast desert, and one might write musing epitaphs at every wind-beaten dwelling, whence the wretched denizens had fled in cold and poverty to a doubtful hospitality in the far South. Fences there were none, nor any living animals save the braying hybrids which limped across the naked plains to eke out existence upon some secluded patches of grass.
The general fertility of varieties does not seem to me sufficient to overthrow the view which I have taken with respect to the very general, but not invariable, sterility of first crosses and of hybrids, namely, that it is not a special endowment, but is incidental on slowly acquired modifications, more especially in the reproductive systems of the forms which are crossed.
This is what is called the sterility of the hybrids between two distinct species." The mongrels are, so far as we know, fertile with one another. But between species, in many cases, you cannot succeed in obtaining even the first cross; at any rate it is quite certain that the hybrids are often absolutely infertile one with another.
The hybrids, moreover, produced from reciprocal crosses often differ in fertility. Now do these complex and singular rules indicate that species have been endowed with sterility simply to prevent their becoming confounded in nature? I think not.
In the second place, some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive generations of hybrids, which were at first only slightly sterile; and if this be so, we surely ought not to expect to find sterility both appearing and disappearing under nearly the same conditions of life.
The hybrids raised from two species which are very difficult to cross, and which rarely produce any offspring, are generally very sterile; but the parallelism between the difficulty of making a first cross, and the sterility of the hybrids thus produced two classes of facts which are generally confounded together is by no means strict.
Mephisto 4- 100% Mephisto | / | / | 77.5 % Dom. | / \ > All Mephisto \ | \ 9- all hybrids with 83-68% | 22.5 % Rec. dominants and 17-32% | recessives. 100% Danebrog. Danebrog Examining these figures we find one-fourth of constant recessives, as has already been said, further one-fourth of constant dominants, and the rest or one half as unstable hybrids.
Both of the pure groups have therefore reappeared in the same numbers. Calling A the specimens with the pure active mark, L those with the latent mark, and H the hybrids, these proportions may be expressed as follows: 1A+2H+1L. This simple law for the constitution of the second generation of varietal hybrids with a single differentiating mark in their parents is called the law of Mendel.
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