174 Life and Death, Heredity and Evolution 



The offspring are finally produced by the mating of two 

 of these half cells (or germ cells), each containing a half 

 nucleus. A nucleus with any combination of the chromo- 

 somal packages may unite with any other. Thus we shall 

 get in the case imagined such combinations as 



ABcD aBcd 



a B C d or a B C d and the like. 



The total number of different combinations produced when 

 these were originally four pairs of different chromosomal 

 packets is 81. 3 



If the number of chromosomal pairs is greater, the number 

 of combinations produced by mating is greatly increased. 

 For each additional pair of chromosomes the number of 

 possible combinations is multiplied by 3. Where there 

 are 4 pairs of chromosomes, as apparently in man, the 

 number of possible diverse combinations mounts far up into 

 the trillions. 



Each one of these combinations of chromosomal packets 

 gives a different result in heredity; a different set of 

 hereditary characters. The result is that the progeny pro- 

 duced by the different germ cells differ from each other, and 

 it becomes impossible to predict from the characteristics of 

 the parent what will be the characteristics of particular off- 

 spring, for many diverse kinds of offspring can be produced 

 by the same pair of parents. 



These processes are best known in higher organisms. But 

 study shows that the same things occur in the Protozoa. 

 These matters are extremely difficult to work out in these 

 minute creatures, and an immense amount of work remains 

 to be done before we shall know with full details what happens 



If there were n pairs of chromosomes present in the original nuclei, 

 then by the formation and mating of germ cells in the way described, 

 the number of different combinations producible is 3. 



