ioo HEREDITY AND VARIATION 



The experiments were made with consummate carefulness, but it is 

 difficult to accept the idea of the rigid fixity of the hereditary char- 

 acters in a pure line. It may be that in some cases, such as beans, 

 the viable limit of size has been reached. It may also be that the 

 variational steps that count do not occur often. Perhaps some time 

 must elapse before the organism takes another step. 



Prof. Castle asks : " Is it not possible that along with the striking 

 size differences due to nutrition there may occur also slight size 

 differences due to germinal variation within the pure line, that is 

 owing to variations in the potency of the same unit-character or com- 

 bination of unit-characters ? " And he points to Woltereck's success- 

 ful selecting-out of a variation in a parthenogenetic pure line of the 

 water-flea, Hyalodaphnia. He selected forms which showed the ex- 

 ceptional occurrence of a rudimentary eye, and definitely increased 

 the degree of development of that organ and the frequency of its 

 occurrence (up to 90 per cent.). 



In short, it is premature to abandon belief in the efficacy of 

 selection even in pure lines. 



8. Causes of Variation 



In regard to the causes of variation it is too soon to speak, 

 except in tentative whispers. What Darwin said must still 

 be said : " Our ignorance of the laws of variation is profound. 

 Not in one case out of a hundred can we pretend to assign any 

 reason why this or that part has varied." 



Yariability. The difficulty which every naturalist has felt in 

 trying to define the concepts of variability and variation is due to 

 the fact that living creatures are individualities in some degree, 

 personalities. In the ocean of matter and energy organisms are, 

 as it were, whirlpools, each one with a particular character of its 

 own. They are animate systems, each with a unity or individu- 

 ality which we cannot fully interpret. They have the power 

 again an ultimate prerogative of giving rise to other whirlpools, 

 to other animate systems, which tend to be like themselves. But 

 because each organism is a very complex whirlpool in a very com- 

 plex environment, and because a living individuality cannot 

 reproduce others without subtle molecular manoeuvres which we 



