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THE AMERICAN NATURALIST [Vol. XLIX 



the variation polygon indicates that the change is permanent, so 

 far as ordinary racial characters have permanency. 



But why, it may be asked, has selection not achieved more in 

 this case ? "Why should the descendants of, say, a 25 per cent, beet 

 not score better than this? There are probably several reasons 

 why. (1) Physiological reasons probably offer obstacles. A beet 

 can not be formed which is all sugar. There has to be present 

 in the beet a machinery for manufacturing the sugar. Perhaps 

 25 per cent, is an impossibly high average for a race of beets. 

 (2) Perhaps the exceptional 25-per-cent. beet owes its extra 

 sweetness in part to environmental causes which are not per- 

 manent. In that case the extra sweetness is ' ' somatic rather 

 than germinal," as we should say in the case of an animal. 



(3) Finally the discovery that beets are never self -fertilized, 

 but in every generation are cross fertilized, explains why im- 

 provement of the beet through selection is so slow and tedious a 

 process. "What progress could the animal breeder expect to make 

 if he were able to select only the dams, but never the sires, for his 

 flocks ? This is the condition which confronts the plant breeder 

 in attempting to improve the sugar beet. The animal breeder 

 is often chided with the small numbers which his experiments 

 yield as compared with the enormous numbers which an ex- 

 periment with plants may produce, but the animal breeder has 

 at least this satisfaction that when the animals are securely 

 penned there need be no uncertainty about pedigrees. 



The careful observations of Shaw show that thrips, so common 

 in the blossoms of plants and yet so minute as easily to escape 

 notice and to penetrate within silk nets and under paper bags, 

 may be a cause of unsuspected cross-pollination and unaccount- 

 able " mutation " in the breeding of cereals and other plants. 



W. E. Castle 



October 24, 3914 



A NOTE ON MULTIPLE ALLELOMORPHS IN MICE 



Professor T. H. Morgan has recently published in this jour- 

 nal the results of some of his experiments on color inheritance in 

 mice. In this paper he offers material which he considers "evi- 

 dence establishing" a series of multiple allelomorphs. His series 

 consist at present of four forms, "yellow, gray white-belly, gray 



