PARTICULATE THEORIES OF HEREDITY 27 



gene produces definite effects on the developmental proc- 

 esses. It affects one or more of the characters that ap- 

 pear at some later stage in the individual. In this sense, 

 the theory of the gene is justified without attempting to 

 explain the nature of the causal processes that connect 

 the gene and the characters. Some needless criticism of 

 the theory has arisen from failure to clearly understand 

 this relation. 



It has been said, for example, that the assumption of 

 invisible units in the germ-materials really explains noth- 

 ing, since to these are ascribed the very properties that 

 the theory sets out to explain. In fact, however, the only 

 properties ascribed to the gene are those given in the 

 numerical data supplied by the individuals. This criti- 

 cism, like others of its kind, arises from confusing the 

 problems of genetics with those of development. 



Again, the theory has been unfairly criticised on the 

 grounds that the organism is a physico-chemical mechan- 

 ism, while the genetic theory fails to account for the 

 mechanism that is involved. But the only assumptions 

 made by the theory, the relative constancy of the gene, 

 its property of multiplying itself, the union of the genes 

 and their separation when the germ-cells mature, involve 

 no assumptions inconsistent with physical principles, and 

 while it is true the physical and chemical processes in- 

 volved in these events cannot be explicitly stated, they 

 relate at least to phenomena that we are familiar with in 

 living things. 



A part of the criticism of Mendel's theory arises from 

 a failure to appreciate the evidence on which the theory 

 rests, and also from a failure to realize that its proce- 

 dure is different from that which, in the past, has led to 

 the formulation of other particulate theories of heredity 

 and of development. There have been a good many of 

 these theories, and biologists have become, through ex- 



