344 ELEMENTS OF BIOLOGY 



tion after generation, of so directing and controlling development 

 that a specific quality always appears as a consequence of its pres- 

 ence. It apparently has a fixed position on the chromosome so that 

 no matter how widely chromatin material may disperse when the 

 cell is not dividing, the genes always assume their proper position 

 when the chromatin becomes organized as chromosomes. On occa- 

 sion one of the members of an allelomorphic pair of genes under- 

 goes some sort of change, the result of which is a mutation. After 

 the mutation has occurred the changed gene may be introduced into 

 a generation and there be associated with its normal allelomorph 

 without being changed. The question that constantly comes to the 

 fore in any discussion of heredity is: Just what is the nature of a 

 gene.? No answer is possible at present. Two sets of facts are 

 known: Unities of some sort, for convenience termed genes, are 

 located on the chromosomes; as a result of their action the adult 

 animal or plant exhibits certain characters. Between these two sets 

 of facts there is a wide gap in our knowledge. Inserted between the 

 existence of genes in the nuclear makeup of the fertilized tgg and 

 the characters of the adult is a long series of intricate physical and 

 chemical events of development concerning which little is known, 

 and concerning how the chromosome makeup directs these events 

 nothing at all is known. Attempts have been made to identify the 

 gene as a molecule, but this does not help us much, for no molecule 

 now known to the chemist has the properties of self-reproduction 

 nor the other powers that have been ascribed to the gene. 



The term gene then represents an hypothesis; its nature and 

 mode of action are unknown, but the concept has been useful. 

 Based upon the theory of the gene, accurate prediction of offspring 

 characters can be made. Moreover, great progress has been attained 

 in practical animal and plant breeding and in the more important 

 abstract understanding of heredity. These facts alone justify the use 

 of the hypothesis. 



