378 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



measured and curves of variability made for each. It was found that a 

 different curve and mean resulted in each set. If the largest and the 

 smallest individual in any pure line is isolated and allowed to produce 

 a set of progeny, the mean and curve of variability will be the same, 

 because both the large and the small individual belong to the same 

 genotype, though varying phenotypically. 



In conclusion therefore we may say that, according to Johanssen, 

 organisms that appear to be alike, or are alike somatically, are identi- 

 cal phenotypically; but organisms, whether alike somatically or not, 

 that have the same determiners are genotypicatty identical or belong to 

 the same genotype. 



ARE DETERMINERS (GENES) CONSTANT OR VARIABLE? 



In our study of the causes of mutations we were forced to admit 

 that we are almost wholly ignorant of the causes of mutations. We 

 infer that in the majority of cases the change occurs within the germ 

 cell and hi the gene itself. In this pure-line work where the genes are 

 unmixed by intercrossing we should have a splendid opportunity of 

 testing the possibility of genes varying or becoming modified. In 

 none of these experiments hi pure lines was there any indication of 

 genes being modified, but some further work by Jennings seems to 

 imply that he has changed his position with reference to the modifi- 

 ability of genes. Using another protozoan, Difflugia, he found that 

 he did succeed in markedly shifting! the mean by selection, and thus 

 seemed to prove that genes were modifiable. This work is open to two 

 comments. First, protozoa are not suitable material for testing the 

 distinction between germinal and somatic changes, because the whole 

 individual is but a single cell; therefore any change that is passed on 

 may be merely a somatic change. Second, Jennings, in making his 

 selections, did so on the basis not so much of an individual itself as 

 upon the characters of its ancestors for some generations back. He 

 was, therefore, working more with genotypic than with phenotypic 

 considerations. If, as he claims, there was a progressive modification 

 of characters, such as numbers of spines, beyond the limit of vari- 

 ability in the original stock, the results would seem to warrant the 

 conclusion that genes are variable and that selection might be effective 

 in establishing new types within pure lines. 



Castle, as the result of a long and elaborate experiment with 

 "hooded rats," at first thought that the gene for the hooded pattern 

 was variable and could be enhanced by selection; later, however, he 



