52 THE PROBLEMS OF EVOLUTION 



According to the discoveries of genetics, the indi- 

 vidual components of the heritage are remarkably 

 constant, but we are indebted to the same science 

 for the discovery that a reassortment of these units 

 is possible up to the physical limitations of the 

 reproductive mechanism. This reassortment is 

 apparently due to chance. We can in no case say 

 that a given condition will bring about a certain 

 association of chromosomes during normal meiosis 

 and thus give a particular distribution of potenti- 

 alities in the resulting cells. Nor can we attribute 

 a controlling power to the activities of the cells 

 themselves. To this extent, then, the heritage 

 possesses a definite capacity for variation among 

 the individuals of a given species, and this vari- 

 ability is independent of environmental factors, 

 whether or not its expression in differentiated 

 characters of individuals may be equally inde- 

 pendent. 



The reassortment of individual determiners dur- 

 ing sexual reproduction is, however, of a nature to 

 guarantee the production of all possible combina- 

 tions, hence it cannot be a determining factor in 

 evolutionary change independently of other condi- 

 tions. It is important in this respect, but only as 

 one factor acting with others to attain a given 

 result. 



A more significant inquiry is whether or not the 

 heritage of an individual, a fixed association of 



