174 Heredity and Environment 



and where the number of chromosomes is fairly large the number 

 of possible combinations of these chromosomes in the germ cells 

 becomes very great. In man, where there are probably 48 chro- 

 mosomes and, after synapsis, 24 pairs of maternal and paternal 

 ones, the possible number of permutations in the distribution 

 of these chromosomes to the different germ cells would be 2 24 , 

 or 16,777,216, and the possible number of different combinations 

 of fertilized eggs or oosperms which could be produced by a single 

 pair of parents would be (16,777,21 6) 2 , or approximately three 

 hundred thousand billions.f But probably other things than chro- 

 mosomes differ in different germ cells, and it is by no means cer- 

 tain that individual chromosomes are always composed of the same 

 chromomeres, or units of the next smaller order, and in view of 

 these possibilities it may well be that every human germ cell 

 differs morphologically and physiologically from every other 

 one, in short that every oosperm and every individual which de- 

 velops from it is absolutely unique. 



Significance of Sexual Reproduction. Indeed the production 

 of unique individuals seems to be the chief purpose and result of 

 sexual reproduction. In asexual reproduction the individual var- 

 iations which occur are chiefly if not entirely due to environment, 

 but in sexual reproduction they are also due to new combina- 

 tions of hereditary elements. The particular germinal organiza- 

 tion transmitted from one generation to the next depends upon 

 (a) the organization inherited from ancestors, (b) the particular 

 character of the cell divisions by which the germ cells are formed, 

 (c) the particular kinds of egg and sperm cells which combine in 

 fertilization. The inherited organization determines all the gen- 

 eral characteristics of race, species, genus, order, phylum. It de- 

 termines the possibilities and limitations of individual variations. 

 Given a certain inherited organization, the individual peculiarities 



t Excluding duplications there would be 3 24 different genotypes and 2 24 

 different phenotypes, assuming that chromosomes always preserve their 

 identity and that dominance is always complete. 



