ASPECTS OF KINETIC EVOLUTION 337 



and significant for biology as Dalton's law of definite propor- 

 tions for chemistry. Deductions from Mendelism followed in 

 rapid succession, such as the purity of germ-cells, inheritance 

 by character-units, and the localization of these in chromosomes. 



Mendelism as a phenomonon is both interesting and sugges- 

 tive, but it lacks warrant as a generalization, because the con- 

 ditions imposed by the experiments are as likely to be the 

 cause of the results as the general principles of heredity alleged 

 to have been revealed. There are, in fact, many reasons for 

 believing that the inbreeding which is deemed an essential pre- 

 liminary to experiments in Mendelism, induces the "disjunc- 

 tion " of the hybrids, instead of the purity of the germ-cells 

 or the antagonism of " dominant " and " recessive " character- 

 units. It is, perhaps, to be expected that Mendelism can be 

 found whenever the conditions of the experiment can be met, 

 but this does not prove that the phenomenon is a normal one. 

 Still less has it been shown that Mendelism has been a con- 

 tributing factor in evolution, since in Mendelian hybrids the 

 more recently derived characters are held not to be dominant, 

 but recessive, and would thus have the less chance of being 

 preserved under natural conditions of unrestricted crossing. 



Some writers have claimed for Mendelism a practical utility 

 as determining the methods of procedure in breeding, and many 

 plants and animals are being bred to learn which characters 

 are dominant and which recessive, it being taken for granted 

 that such facts have a fixed and definite value for each species 

 or variety, thus enabling the results of breeding combinations 

 to be known in advance. The utility of such knowledge is, 

 nevertheless, negative rather than positive ; it may keep the 

 breeder from attempting the impossible, but it seldom gives him 

 new leverage in attacking practical problems. The danger is 

 rather that the acceptance of erroneous theories of heredity 

 may delay his perception of facts and discourage his efforts. 



It seems to be agreed by several experimental evolution- 

 ists that white fur or feathers is a recessive character ; but 

 no attempt has been made to test the general basis of this 

 assumption by comparing interbred white mice with inbred 

 gray mice. Albinism is one of many mutations induced by 



