EVOLUTION TODAY 439 



gradual modification of the factors involved, which leads to the 

 second interpretation. 



2. It is infinitely more logical to suppose that the heritage in 

 any living organism is subject to change. It has a certain range 

 of power which enables it to respond within limits to different 

 stimuli, and when it has responded in one way it seems probable 

 that its future behaviour and development may be modified as a 

 result. An individual may enter one field of activity after being 

 trained for another, but his earlier training can hardly fail to 

 have an effect on his later work. 



This interpretation necessitates the wholly logical belief that 

 each change in an organism through response of its heritage to 

 conditions emanating from any part of the environment may make 

 possible other changes. It recognizes that departure from the 

 one-celled state for colonial or multicellular organization makes 

 possible other steps which the single-celled organism could not 

 directly attain, and that the attainment of triploblastic structure 

 makes possible other specializations impossible to diploblasts. 

 The primitive horses must have had a third digit strong enough to 

 support the weight of the body before the others could be lifted 

 from the ground, but once the latter ceased to bear weight, they 

 would have had a much better chance of reduction than before. In 

 all of these cases it is evident that the characters deal both with 

 the heritage and with environmental conditions. 



In spite of the logical aspects of this interpretation we must 

 still meet the old distinction between heritable and acquired 

 characters. The heritage is always expressed through the soma, 

 although it is perpetuated by the germ plasm. This is true of all 

 characters, even those which are admittedly inherited; neverthe- 

 less, if so-called acquired character's have any part in evolution, 

 there must ])e a point of transition where the heritage ceases to 

 find a stimulus from the external environment necessary for its 

 expression and responds to a condition which is normally certain 

 to be supplied by the developing body itself. 



Use and Disuse in the Chromosomes. The principle of use 

 and disuse recognizable in organic behaviour has been applied to 

 the chromosomes and th(Mr genes as an explanation of such cumu- 

 lative modification of the heritage through the generations. It 

 is now regarded as probable that the action of a given gene is not 

 limited to the cell in which it lies, but that the genes of a given 



