76 EVOLUTION OF LIVING ORGANISMS. 



the presence and absence theory is correct, the character 

 of the heterozygote in the case of the blue Andalusian 

 fowl (p. 48) may be explained on the supposition that 

 the single dose of a factor in the heterozygote does not 

 produce the same result as the double dose in the homo- 

 zygote parent or offspring. 



Here may be mentioned the recent researches of 

 Jennings on the Protozoon Difflugia, of Castle on rats, 

 and of Morgan on Drosophila, all of which yield incon- 

 trovertible evidence of the occurrence of " continuity " in 

 mutation that is to say, of the possibility of isolating 

 strains differing from each other by quite small characters 

 or in the degree of development of some one character. 

 But while Morgan would interpret the gradation as due 

 to the cumulative effect of numerous modifying factors 

 brought to bear successively on the unit factor of the 

 character, Castle maintains that this unit factor itself 

 undergoes gradual change. 



Variations, then, may be continuous or discontinuous, 

 and there is no hard and fast distinction between the two 

 kinds (pp. 57, 93). An interesting question we have now 

 to consider is whether large or small variations are the 

 more important in evolution. Darwin was uncertain on 

 this point, but finally came to the conclusion that natural 

 selection dealt chiefly with the smaller. Certainly they 

 are the more numerous, and considering how severe may 

 be the struggle for existence, it can hardly be doubted 

 that they greatly influence the death-rate. The attempts 

 which are sometimes made to fix an arbitrary limit to the 

 " selection value " of a character are futile ; it all depends 

 on the intensity of the struggle at any particular time. 

 A character, useless during the greater part of the life of 

 an organism, may prove of vital importance on a par- 

 ticular occasion. The slightest difference in weight 

 between two seeds carried by the wind may decide that 

 the one will reach a favourable spot and not the other ; 

 the smallest inferiority in powers of resistance may cause 



