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Updated: August 25, 2024
In some parts of the world, as for instance in Spain, where a long-continued effort has been made to develop the animal for interbreeding with the horse, the result shows that the form is relatively inelastic. It is doubtful if any conceivable amount of care would develop such variations as the horse now exhibits.
For the moment we shrink from the interbreeding of races, but we do so in spite of some conspicuous examples of successful interbreeding in the past, and largely because of our complete ignorance of the conditions on which success depends.
I have collected so large a body of facts, showing that close interbreeding lessens fertility, and, on the other hand, that an occasional cross with a distinct individual or variety increases fertility, that I cannot doubt the correctness of this almost universal belief amongst breeders.
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 interbreeding continued during several generations between the nearest relations, especially if these be kept under the same conditions of life, always induces weakness and sterility in the progeny.
Again, with respect to the fertility in successive generations of the more fertile hybrid animals, I hardly know of an instance in which two families of the same hybrid have been raised at the same time from different parents, so as to avoid the ill effects of close interbreeding.
They are admirably suited for window and greenhouse culture and are interesting subjects for interbreeding, though no startling results should be expected. The winter-blooming varieties grown by florists, such as the Bride, Delicatissima, and Peach Blossom, belong to the hybrid section known as Gladiolus Colvillii, which is, without doubt, a hybrid between G. cardinalis and G. tristis.
To explain the way in which the type or physical features of people undergo great changes by interbreeding, Mtesa was instanced as having lost nearly every feature of his Mhuma blood, but the kings of Uganda having been produced, probably for several generations running, of Waganda mothers.
Distinction between the sterility of first crosses and of hybrids Sterility various in degree, not universal, affected by close interbreeding, removed by domestication Laws governing the sterility of hybrids Sterility not a special endowment, but incidental on other differences, not accumulated by natural selection Causes of the sterility of first crosses and of hybrids Parallelism between the effects of changed conditions of life and of crossing Dimorphism and trimorphism Fertility of varieties when crossed and of their mongrel offspring not universal Hybrids and mongrels compared independently of their fertility Summary.
Again, with respect to the fertility in successive generations of the more fertile hybrid animals, I hardly know of an instance in which two families of the same hybrid have been raised at the same time from different parents, so as to avoid the ill effects of close interbreeding.
I do not doubt that this is usually the case, and that the fertility often suddenly decreases in the first few generations. Nevertheless I believe that in all these experiments the fertility has been diminished by an independent cause, namely, from close interbreeding.
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