238 ESSENTIALS OF BIOLOGY 



fertilization of an ovum by a spermatozoon). They " come out " 

 from the crossing just as they went in. They are to be regarded 

 as discrete, atomistic, character-entities, just as the atoms of 

 chemical substances are discrete entities. The atoms enter into 

 various combinations with each other in the course of the reactions 

 of the '' parent-substances," yet they retain their individualities 

 throughout all the reactions. So the atomistic, Mendelian char- 

 acters retain their individualities although they may be assorted 

 and reassorted in the course of the matings of the parents and 

 among the progenies. 



(4) Most of these assortments and reassortments occur at 

 random : thus the disjunctions of the chromosomes that undergo 

 synapsis ; the reassortments of the genes in the reduction 

 divisions ; the combinations of different gene-complexes in the 

 fertilizations and the phenomena of crossing-over. In conceiving 

 this randomness we are moving away from the essential conception 

 of life — which is anti-randomness. 



(5) And, therefore, Mendelian speculation is forced to postulate 

 anti-randomness somewhere. We find this in the conception of 

 synapsis of homologous chromosomes — a very difficult problem. 

 Perhaps we find it also in the notion of linkages — so far as this is 

 not accounted for by the crossing-overs. 



In the following chapter we shall return to the subject of 

 Mendelian heredity, in so far as it touches upon the problems of 

 transformism. 



83. ON HEREDITY IN GENERAL 



There is no working hypothesis of heredity, for a hypothesis 

 of heredity is necessarily a hypothesis of development. We say 

 that the off"spring belongs to the same specific or racial category 

 as did its parent, and this is because the specific developmental 

 process by which an ovum became the parent is the same process 

 by which an ovum derived from the parent becomes the offspring. 

 We say that the specific developmental processes are '' the same," 

 neglecting those small, random deviations that we shall call 

 '* fluctuations " (Section 846). Now we can divide up the specific, 

 or racial category into sub-categories that we call Mendelian ones. 

 Each Mendelian category displays the specific or racial characters, 

 and, in addition to these, certain trivial characters, or combina- 



