194* Life and Death, Heredity and Evolution 



acters follow the primary ones. If there are two races of 

 organisms, in each of which both members of each pair of 

 chromosomes are alike (so-called pure lines, or pure homozy- 

 gotes), we may call their two sets of chromosomes 



ABCD abed 



A B C D and abed 



Now, when the two sets are reduced in number by division 

 into two groups, with one member of each pair in each group, 

 evidently the only possible groups are ABCD from one 

 race, and abed from the other ; all the half nuclei of each 

 race will be alike. When two half nuclei from the different 

 races mate, all the resulting nuclei will show the combination 



ABCD 



abed 



That is, by the crossing of these two diverse races, progeny 

 are produced that are all alike. A new combination has 

 been produced, but only one combination from the original 

 two. So the progeny of the cross will be uniform, while the 

 parents were diverse. Variation has been greatly decreased. 



But now suppose that the progeny, showing this new and 

 uniform combination, mate among themselves. We have al- 

 ready for another purpose examined the results of this 

 (page 186); we found that a great number of different 

 hereditary combinations would be produced, such as 



AbcD , abCd , etc., etc. 

 AbCd aBCD 



Progeny of 81 different kinds of hereditary combinations 

 will result. By this mating of two parents that were just 

 alike variation has been greatly increased. 



Suppose that we compare this group of 81 diverse com- 

 binations with the two sets of grandparents. Has variation 



