OTHER EXPERIMENTS ON HEREDITY 375 



generation in a particular pure line may fail to secure any result, 

 e.g. in the average size of the seed. Thus a ne plus ultra may 

 be attained by a line. Similarly, as regards certain characters 

 in guinea-pigs, Castle has repeatedly attempted without success 

 to bring about a change by selecting within an inbred race. 

 " Thus a very dark form of Himalayan albino, after a certain 

 amount of improvement by selection, could not be further dark- 

 ened to any appreciable extent " (Journ. Washington A cad. Sci., 

 vii., 1917, pp. 368-387). But what must be avoided is general- 

 ising from particular sets of facts. 



Thus it is certain that in many cases the principle of the pure 

 line does not apply. Certain characters of guinea-pigs, rabbits, 

 and rats have been found to respond readily to selection in a 

 particular direction. Moreover, even Johannsen has shown that 

 mutations may occur within pure lines, and these may afford 

 material for selection. 



Different sets of data suggest different conclusions, and these 

 need not be contradictory. " A study of albinism alone," Castle 

 says, " would lead one to believe in the fixity and constancy of 

 Mendelian genes, and the impossibility of modifying them by 

 selection. A study of white-spotting leaves one with the un- 

 shakable conviction that this form of gene is plastic and yields 

 readily to selection. Where only genes of the former sort are 

 involved, the principle of the pure line is applicable ; where genes 

 of the latter sort are involved, it is not applicable/' It is in- 

 advisable to stereotype conclusions amid the rapid progress of 

 a young science. 



Castle inclines to the view that in the smaller mammals, 

 which he has particularly studied, very few characters can 

 be safely referred to the agency of perfectly stable hereditary 

 factors or genes. Very important is his conclusion that, 

 " aside from colour, there are very few valued economic 

 characters in our domestic animals which are not inherited 

 after the manner of blends " (see Journ. Washington A cad. 

 Sci., vii., 1917). 



As was to be expected, the discoveries of the Mendelian 

 experimenters have raised problems in solving others, and 

 various elaborations have been found necessary in order to 



