190 Life and Death, Heredity and Evolution 



that bear white chromosomes, half will have 'mates with black 

 chromosomes, half with white. Then all the different com- 

 binations so producible are shown at P 2, Figure 51 ; each 

 of these combinations occurs as frequently as any other. By 

 two of the four combinations we get offspring ( ex-con ju- 

 gants, F 2) with one chromosome white, one black; by one 

 we get offspring with both chromosomes white ; by one, off- 

 spring with both chromosomes black. Summing up the 

 eight offspring, we get the following proportions for the 

 offspring of the generation F 2 : 



2 white-white -(- 4 white-black -f- 2 black-black. 



Now this is exactly what is called Mendelian inheritance. 

 If we call white A and black a, the proportions give the fa- 

 miliar Mendelian formula 



AA + 2 Aa + aa 



Any other chromosome pair, or any character that de- 

 pends on a chromosome pair, will give the same result. 



We have found then, so far as knowledge has gone in this 

 direction, that mating produces the same kinds of results 

 in the Protozoa that it does in higher animals ; it gives bi- 

 parental inheritance, and also gives many diverse hereditary 

 stocks, and these results are produced in the same way in 

 the Protozoa that they are in higher organisms. 



The Effect of Mating on the Stock 



What effect has this on the entire species in which mat- 

 ing occurs? To answer this question, another fact as to 

 conjugation is of importance. 



Assortative Mating: When we place together in the same 

 vessel members of two different races of Paramecium, one 

 having large individuals, the other small ones, and then in- 

 duce conjugation, we observe a surprising fact. Members 



