respecting animal breeding. True, Mendel's law has been found to apply with animals so far as the test has been applied, but it will be some time before much use can be made of it in that direction. It will take years to overcome the prevailing prejudices of animal breeders in favor of old-time theories that govern practice in that line.
Exceptions to the Law.
Millardet and others have given accounts of hybrids that immediately split up into the two parent forms, having no, or very few, hybrid progeny, and these have been cited as exceptions to Mendel's law. I have elsewhere[1] shown that Mendel's theory of the separation of parent characters offers a perfectly rational explanation of these cases. If a hybrid is obtained from two varieties each of which prefers its own pollen to that of the other, the resulting hybrid must split up at once into the two parent types, if it obeys the law in question, since each of the two kinds of ovules produced by the hybrid, being offered both kinds of pollen, is fertilized only by its own kind. Yet even in these cases, according to the laws of probability, it ought occasionally to happen that such a hybrid would produce a few hybrids, for it would occasionally happen that an ovule would be offered only one kind of pollen. And this is what actually occurred in hybrids of this class reported by De Vries. A few hybrids occurred along with an excess of both the parent types.
Likewise, when two varieties are crossed, each of which prefers the other's pollen, there will also be an apparent departure from the law; for in this case each of the two kinds of ovules on the hybrid will be fertilized only with the opposite kind of pollen, giving all heterozygotes. Such a hybrid will appear to be fixed in type at once. Such cases have been reported by many observers, including Darwin. In this case it may occasionally happen in later generations of the apparently stable hybrid, that an ovule will be offered only its own kind of pollen and we then get a reversion to one of the parent forms. We may yet find that many sports are to be explained in this manner.
Breeders frequently report entirely new characters in hybrids. If these actually occur we must look further than Mendel's theory for their cause. It must not be overlooked, however, that if the two parents of a hybrid are themselves heterozygote hybrids, Mendel's theory would call for characters unlike any of the visible characters of either parent. In this case, all latent characters in both parents would necessarily crop out in the second generation of the hybrids. We can not dismiss Mendel's theory in such cases until it has been demonstrated that the parents have no latent characters in them.
- ↑ Science, October 31, 1902.
