220 THE PERPETUATION OF ADAPTATIONS 



round characteristic is dominant over the wrinkled. The Fi 

 plants when self-fertilized produced some yellow and round 

 peas, some yellow and wrinkled, some green and round peas, 

 and some green and wrinkled in the proportion of 9 13 13 :i. 

 The explanation is the same as for the simpler case of one pair 

 of characters. One parent produced germ cells containing the 

 factors for yellow (Y) and round (R), the other parent produced 

 germ cells containing the factors green (G) and wrinkled (W). 

 The fertilized eggs or Fi generation must therefore have con- 

 tained YRGW, the allelomorphs being YG and RW. These 

 allelomorphs are separated during maturation the germ cells 

 containing either YR, YW, GR or GW since these are the only 

 possible combinations. If the hybrids are self-fertilized there 

 would be four kinds of male and four similar kinds of female 

 gametes which would give sixteen possible combinations as 

 shown in Fig. 97. 



These experiments have been so often repeated and on so 

 many different plants and animals with many different charac- 

 teristics that the main conclusions of Mendel are now universally 

 accepted. Many characters, however, do not seem to segregate 

 or Mendelize, at least not in any simple way that can be pre- 

 dicted and these are the problems that modern experimentalists 

 are working on. 



C. HEREDITY OF SEX. Cytological evidence. Sex, with 

 many of the secondary characters which go with it is recognized 

 to-day as an aggregate of Mendelian characteristics. The 

 evidence on which this generalization is based is partly cytolog- 

 ical, partly experimental and while many perplexing problems 

 connected with sex are still unsolved the evidence in favor of 

 the heredity of sex is so strong that it may be accepted as the 

 most plausible working hypothesis at the present time. 



On p. 210 reference was made to divergent results in regard 

 to the number of chromosomes in development of the sperm 

 cells of certain animals. In a great many insects (belonging 

 to the orders hemiptera, diptera, homoptera, phylloxerans, 

 etc.), in nematode worms (Ascaris, Ancyrocanthus) and in 

 guinea pigs the process of spermatogenesis does not exactly 



