THEORIES OF EVOLUTION 133 



as the various rays which have been used experi- 

 mentally in their production. But we have no 

 reason to conclude that even an unstable unit will 

 change spontaneously. In some phase of the en- 

 vironment, internal or external, we must assume 

 that fluctuating conditions provide the stimulus 

 for the modification of the gene. Mutations, like 

 other organic phenomena, must involve a reaction 

 of the heritage to the environment. But in arriv- 

 ing at this conclusion we have approached the 

 other school of evolutionary thought, the La- 

 marckian idea of the potency of environmental 

 factors in the evolution of living things. 



Cuenot has taken up the matter of organic ad- 

 justment to new environments very logically in 

 his theory of preadaptation.^^ He points out that 

 a character present in an organism living under 

 certain conditions may, in a different environment, 

 serve their possessor in a different capacity. As the 

 organism moves from one region to another, or as 

 changed surroundings are forced upon it, new de- 

 mands are made upon its body which must be met 

 in the best possible way. Obviously a need cannot 

 immediately produce new structures in an organ- 

 ism; it must be met at once if the organism is to 

 survive, but it can be met only by the use of parts 

 already functional in the individual. In illustra- 

 tion of preadaptation many existing possibilities 



^ La genese des esjteces animales, 1917. 



