THE CARRIERS OF THE HERITAGE 229 



and, furthermore, when one passes in rapid review 

 the marvelous array of characteristics which make up 

 the sum total of what is obviously inherited in man, 

 the wonder grows that so small a bridge can stand 

 such an enormous traffic. A sharp-eyed patrol of 

 this bridge as the strategic focus of heredity is proving 

 to be one of the most effective points of attack in the 

 entire campaign of genetics. 



10. THE CHROMOSOME THEORY 



Certain investigators, who seek a morphological basis 

 for heredity, regard the chromosomes as the carriers 

 of the heritage ; in other words, as the source of the de- 

 terminers of ontogeny or the effective factors in the 

 process of differentiation. 



A few of the grounds for this theory are briefly 

 indicated below. 



First: In spite of the great relative difference in 

 size between the egg-cell and the sperm-cell, in heredity 

 the two are practically equivalent, as has been re- 

 peatedly shown by making reciprocal crosses between 

 the two sexes. The only features that are apparently 

 alike in both the germ-cells are the chromosomes. The 

 inference is, therefore, that they contain the determiners 

 which are the causal factors for the equivalence of 

 adult characters in heredity. The existence of an extra 

 chromosome in probable connection with the matter 

 of sex is, as will be pointed out later, an exception to 

 the exact chromosome equivalence of the two sexes, 

 which only goes to strengthen the supposition that the 



