Page:The Effect of Research in Genetics on the Art of Breeding (1912).djvu/6

4 his conception of pure lines, biotypes, genotypes and phenotypes. His experiments in the selection of pure lines of beans in an attempt to produce large and small seeded types, have led him to conclude that selection within a pure line is ineffective in producing changes. He did, however, secure new types from pure lines through mutations.

Tower’s experiments with the potato beetle in attempting to create by selection, large and small races, albinic and melanic races, and races with changed color-pattern, although conducted carefully for from ten to twelve generations, failed to give any evidence of producing permanently changed types. While strains of plus and minus variates gave populations with a range of variation apparently markedly restricted to their respective sides of the normal variation range, still these selected strains did not greatly exceed the normal range of variation in either direction, and when the selection was discontinued, in two or three generations, again produced populations exhibiting the normal range of variation. Clearly no new unit characters had been added by the selection. Tower, however, found that by subjecting the beetles, during the process of the formation of gametes, to certain abnormal conditions, he was likely to obtain mutations in the progeny that would immediately form the beginnings of new races.

Jennings in a series of selection experiments conducted with paramecium, which were continued for over twenty generations, obtained no evidence of a permanent modification of the type.

Pearl has conducted an extended experiment in the selection of chickens in the attempt to produce a breed of high egg-laying capacity. His results have led him to the conclusion that selection alone has no effect in producing a permanent improvement or a change of type.

Up to the present time these are the principal contributions to the subject, that discredit the effectiveness of selection as an active agency.

On the opposite side of the controversy we have the very careful and extensive researches of Castle and MacCurdy in the selection of Irish rats to increase the black-colored dorsal band on the one hand and to decrease or obliterate it on the other. Castle appears to have obtained very positive results favoring the gradual cumulative action of the selection, as he succeeded in markedly increasing the amount of black in one strain until the rats were almost wholly black, and in the other strain almost wholly obliterating the black. The speaker is not informed whether the inheritance in hybridization of these apparently new characters has been tested. If a new character has been added it should maintain itself and segregate after hybridization.

The experiments conducted by Dr. Smith and others at the Illinois Experiment Station on selecting high and low strains of corn with reference to oil and protein content, have resulted in markedly distinct strains possessing these qualities, which are inherited apparently as long as the selection is continued. It seems certain that the oil and protein content has been increased considerably beyond the maximum which existed in the original race. The writer is informed by Dr. Smith that experiments have been made in cultivating these varieties without selection and that the new characters have been maintained for several years without marked regression. We must apparently conclude then that new heritable characters have been acquired in the course of the selection, but it will probably be difficult to determine