Page:The Effect of Research in Genetics on the Art of Breeding (1912).djvu/12

10 characters in their mutual effect on one another. It is, however, possible that new unit characters may result from the commingling of the different hereditary units which are to be considered as mutations rather than new combinations. As is well known, Weismann long ago advanced the hypothesis that valuable variations in evolution were due to the commingling of protoplasms from different parents having different hereditary tendencies, a process which he called “amphimixis.” He did not have in view, however, the formation of new unit characters as distinct from new combinations.

The most marked case known to the speaker, of the appearance of a new character which was apparently caused by the stimulation of hybridization, is the development of a marked spur or horn on the lip of a hybrid Calceolaria. This occurred among a series of hybrids between a herbaceous and a shrubby species made by Professor Atkinson and Mr. Shore, of the botanical department at Cornell University. One or two tapering horns about an inch in length and from 2 to 4 millimeters in diameter at the base, spring from the upper surface of the large corolla lip and grow erect to its surface. No such character, so far as can be learned, is known in the Calceolarias and it would seem to have been caused by the hybridization. It can not, apparently, be considered as a combination of any of the known characters of the species concerned.

Such apparently new characters appear rather commonly among large batches of hybrids, and while there is little evidence available on the subject, I am inclined to believe it will be found that hybridization may stimulate the production of new unit characters, which mendelize with the parental types.

While the evidence at our command regarding the artificial production of mutations is not yet sufficiently exact and trustworthy to enable us to draw definite conclusions and formulate recommendations for practical breeders, it may be stated that this is apparently one of the most profitable lines of experimentation for the immediate future.

Thus far I have only incidentally discussed hybridization and the advance of our knowledge in this direction. The scope of this address will not allow of an adequate treatment of this subject and it appeared wiser to discuss more in detail the problems of selection and variation. I can not, however, close this address without referring to this very important field of genetics.

No discovery in the field of breeding has had more effect or is more far reaching in its importance than the discovery of what have now come to be known as Mendel’s principles of heredity. While, as stated in the beginning of this address, breeders had long before the rediscovery of Mendel’s papers come to understand that there was a segregation of characters in the F2 generation and that it was possible to recombine in certain hybrids the desired characters from different parents, there was no definite understanding of the underlying principles, and no conception of the almost infinite possibilities of improvement which the field of hybridization opened to us.

The law of dominance, while not universal, has explained many cases of prepotency in one generation and failure of certain individuals to transmit the character in the next generation. It has explained many cases of latency of characters and may account for all such cases.

The law of segregation has shown us that the splitting of characters follows a definite method and that we can in general estimate the frequency of occurrence of a