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THE AMERICAN NATURALIST [Vol. LI 



Illinois, in the work of Pearl and Snrface^^ with maize, 

 in MacDowell's^^ work on bristle number in Drosophila, 

 in the work of Zeleny and Mattoon^^ with the bar eye of 

 Drosophila, in the experiments of Castle and Phillips*'^ 

 with hooded rats, and in the work of some others. The 

 difficulty with all these experiments is not in the facts but 

 in their interpretation. 



Before taking up this question of interpretation, how- 

 ever, it seems desirable to point out certain objective 

 features which the results of these experiments have in 

 common. The first is that they all occur (with the ex- 

 ception of the Jennings cases already discussed) in sexu- 

 ally reproducing organisms, not certainly known to be 

 homozygotic with respect to all the factors which may be 

 concerned in the production of the selected characters, 

 and subject to a mixing of germ plasms in each genera- 

 tion. The second is that when any result at all follows 

 selection in most if not all of the cases it comes quickly 

 {i. e., in a few generations), is relatively large in amount, 

 and either no further change follows further selection or 

 if it does occur it is again sudden and large in amount. 

 This was true in the work with high producing lines of 

 hens, of all work with maize by the ear-row method, and 

 of MacDowell's with DrosopMa, and also, as MacDowelP^ 

 has so clearly demonstrated, in Castle's experiments with 

 rats. A third point which strikes one is that in many of 

 these successful cases of selection the basis of the selec- 

 tion has been fundamentally gametic, that of "progeny 

 performance," rather than solely somatic. Individuals 

 are selected for further multiplication which have demon- 

 strated their ability to produce progeny bearing 

 sired somatic qualities. This was certainly the 



Pearl, E., and Surface, F. M., Maine Agr. Expt. Stat. Ann. 

 1910, pp. 249-307. 



58 MacDowell, E. C, Jour. Exp. Zool, Vol. 19, 1915. 



50 Zeleny, C, and Mattoon, E. W., Jour. Exp, Zool., Vol. XIX, 

 529, 1915. 



80 Castle. W. E., and Phillips, J. C, Carnegie Institution, Publ. 

 61 MatDowell. E. C, Amer. Xat., Vol. 50, pp. 719-742, 1916. 



