66 INBEEEDING AND OUTBREEDING 



ditions because we know wliat has happened under sim- !> 

 ilar conditions. For example, it having been determined 

 that in the pomace fly; many characters are linked to chro- 

 mosomes whose distribution parallels that of sex, we 

 know it to be much more than a guess to say that the 

 color-blindness of man of which the hereditary distribu- 

 tion was described in Chapter III, is controlled by a factor 

 lying in the sex chromosome and recessive to the normal. 

 Though the whole mechanism in the higher plants and 

 animals can thus be pictured as one of sexual reproduc- 

 tion, in its details the results are still too complex to 

 analyze as concretely as the cases given for illustration. 

 Several thousand concrete differences between plants of 

 the various angiosperm families and between animals in 

 at least three different phyla have been followed through 

 pedigree cultures sufficiently carefully to make possible 

 a definite factorial analysis of their hereditary transmis- 

 sion. This has been possible, first, because variation has 

 taken place in these factors, enabling one to follow the 

 transmission of each member of an allelomorphic pair, 

 and, second, because this variation has been somewhat 

 qualitative in nature. Unfortunately for the peace of 

 mind of the biologist, however, the more numerous differ- 

 ences between animals and between plants are the quanti- 

 tative differences, the variations which make organs a 

 little larger or a little smaller. Now it is a great deal 

 easier to determine the transmission of the factor differ- 

 ences which determine that one flower shall be red and 

 another white than it is to trace the distribution of the 

 factors which determine that one flower shall be one inch 

 and another two inches long. Nevertheless, through the 

 efforts of numerous investigators it has been possible to 



