206 OUTLINES OF E VOLUTION AEY BIOLOGY 



insight into the real nature of heredity. We may regard the 

 existence of " unit characters " as being conclusively demonstrated 

 by these experiments, and the fact that we are able to interchange 

 these characters and to add and subtract particular characters to 

 and from the organism seems to indicate very clearly that the 

 germ plasm must contain material primordia or " factors," as they 

 are often called, which are responsible for the development of 

 these characters. 



In so far as they demonstrate the existence of such factors 

 which evidently correspond very closely with Weismann's hypo- 

 thetical determinants the Mendelian experiments may be taken 

 as affording confirmation of Weismann's theory of the constitu- 

 tion of the germ plasm. Their results also harmonize very well 

 with those of recent cytological investigations. We have seen 

 that there is strong evidence for regarding the chromatin 

 substance of the nucleus as the material basis of heredity. We 

 have further seen that this chromatin is very complex in 

 structure and that the chromosomes, or units of the highest order 

 of which it is composed, can in many cases be optically resolved 

 into chromomeres or units of the next lower order. According 

 to Weismann's theory these chromomeres or ids are in their 

 turn made up of the determinants. Moreover the associa- 

 tions and redistributions of chromosomes which take place 

 in the process of " reduction " and in the conjugation of the 

 germ cells seem to afford ample opportunity for those permuta- 

 tions and combinations of factors the occurrence of which is demon- 

 strated by the Mendelian experiments. As Professor Farmer 

 observes, " The facts of meiosis 3 are seen to fall completely into 

 line with conclusions drawn from experiments on breeding as far 

 as the numerical distribution of characters is concerned." 2 



The doctrine of the "purity of the gametes" teaches us that 

 any given mature germ cell contains only one member of any 

 given pair of alternative factors or primordia (allelomorphs). 

 This may be accounted for by the pairing of the chromosomes 

 and the subsequent halving of their number which take place at 

 some period or other prior to the formation of the gametes. 

 There is good reason to believe that each member of such a pair 

 of chromosomes is homologous with or morphologically equivalent 



1 I.e. the phenomena accompanying the reduction in the number of chromosomes 

 which takes place periodically in all typical organisms (tide Chapter X.). 



2 Croonian Lecture. Proceedings of the Royal Society, B, Vol. 7'J, l'J07. 



