No. 602] THE SELECTION PROBLEM 



85 



So far as the writer is aware the only important ex- 

 ceptions to the general rule exemplified by the above- 

 cited researches are those obtained by Jennings^'* in Dif- 

 ftugia, and by his students, Middleton^o with StylonycMa, 

 and Stockingsi with abnormalities in Paramecium. The 

 facts in the case of these exceptions are beyond question. 

 Just what their correct interpretation is does not seem to 

 be so clear. Jennings himself {loc. cit., p. 529) has ex- 

 pressed some doubt as to the significance of Miss Stock- 

 ing's results so far as concerns their relation to normal 

 reproduction. Morgan^^ has suggested that Jennings's 

 Difflugia results may possibly be due to a sorting out of 

 genetic diversities in the germ plasm, which came about 

 from earlier conjugations, the material thus not repre- 

 senting a strictly homozygotic pure line. If this sugges- 

 tion should prove to be valid the Difflugia work would 

 fall at once into the same category as the cases of sorting 

 out of pure lines from a mixed population by selection, 

 with which the studies of Johannsen, de Vries and Jen- 

 nings himself have made us familiar. 



There is another point in connection with this extremely 

 interesting and important investigation on Difflugia 

 which seems to the writer, in the light of his own experi- 

 ence in breeding, of really extraordinary significance. 

 One of the longest and most crucial selection experiments 

 in the whole series was that for diverse numbers of spines 

 in Family No. 326. During the first six periods after 

 selection was begun in this experiment (p. 488) "no 

 progress was made by selection." Then the basis of 

 selection was changed. This change is described by Jen- 

 nings in the following words (p. 489) : 



After this time selection was based to a considerable extent on past 

 performance. By this time many of the existent individuals had pro- 



*9 Jennings, H. S., Genetics, Vol. 1, pp. 407-534, 1916. 



"0 Middleton, A. R., Jour. Exp. Znnl.. Vol. 19. pp. 451-503, 1915. 



"Stocking, R., Jour. Exp. ZooL, Vol. 19, pp. 387-449. 1915. 



"Morgan, T. II., "A Critique of the Theory of Evolution." Princeton, 

 1916. 



