196 THE PHYSIOLOGY OF REPRODUCTION 



It has been supposed that the chromatin granules (which 

 Weismann identified with the ids) are the carriers of the 

 Mendelian allelomorphs, and that when these fuse together 

 during the conjugation of the chromosomes which precedes 

 the process of reduction (see p. 130), there is an exchange of 

 allelomorphs between the chromosomes. If this interpretation 

 is correct, it is simply a matter of chance whether an allelo- 

 morph remains in the chromosome which originally contained 

 it, or becomes transferred to the other chromosome of the con- 

 jugating pair. And since each of the two chromosomes passes 

 into a different product of cell division, the allelomorphs 

 would become distributed in precisely the kind of way that the 

 Mendehan theory postulates. ^ 



The Mendelian investigators have shown that by experimental 

 breeding it is apparently possible to superimpose certain char- 

 acters belonging originally to one kind of individual, upon 

 different characters belonging to another kind, thus creating 

 new combinations of characters. Thus it is claimed that by 

 starting with two individuals, each possessing two unit or 

 allelomorphic characters, which we may call A and X (associated 

 together in one individual) and B and Y (associated in the other), 

 it is possible in two generations to produce new individuals in 

 which the combinations are interchanged, A being associated 

 with Y, and B with X. It has been claimed also, that, in spite 

 of the new combinations, each of the original separate unit 

 characters can be preserved in a state of complete purity, and 

 without in any way affecting, or being affected by, the characters 

 upon which they have been superimposed. By resorting to such 

 methods, it has been thought possible to build up, little by 

 little, entirely fresh types of organisms, possessing new com- 

 binations of pure characters, which previously existed only in 

 different individuals. 



It remains to be considered how far this conception of an 

 organism as an individual capable of description in terms of 

 unit characters (each of which can be transmitted pure) is in 

 harmony with modern physiological theory, or justified by 

 experimental investigation. 



In the first place, it may be pointed out that the entire 

 ' Look, loc. cit. 



