132 THE SCIENCE OF BIOLOGY 



and spermatozoon. The pairing of chromosomes in all the 

 cells of the body (Fig. 16) is thus explained by the bisexual 

 origin of the chromosomes. 



When we look at some part of an animal's body it some- 

 times seems to exhibit a mosaic of characters, seemingly 

 inherited from both parents. In the human hand, for ex- 

 ample, the shape of the fingers, the texture of skin, the 

 pigmentation are, let us say, those of the mother. The 

 nails, hairiness, and double- jointed thumb are those of the 

 father. The biological conclusion is that such mosaics of 

 features constituting adult bodies have been inherited in 

 approximately equal numbers from the two parents. This 

 is the external fact that we see in human beings and in the 

 animals within a breeding pen. There could hardly be a 

 more precise basis for such an equal inheritance of macro- 

 scopic features than the microscopic chromosomes, within 

 every cell of the body and descended equally from the two 

 parents. 



Evidence for the chromosome-theory is even more de- 

 tailed. In the fruit fly Drosophila, which has been the object 

 of more extended studies in heredity than any other animal, 

 there are four pairs of chromosomes (Fig. 16). The breeding 

 results show four groups of heritable qualities, which go 

 together in inheritance save for certain exceptions that do not 

 vitiate the chromosome-theory. These groupings or linkages 

 are shown in the accompanying table. It happens that the 

 size of the chromosomes in the four pairs (Fig. 16) roughly 

 parallels the size of the four groups of heritable characters as 

 thus far discovered. This may not be significant, but one 

 suspects that the small group of linked characters is borne by 

 the small pair and the large group by one of the large pairs of 

 chromosomes. In view of all the facts, the chromosomes 

 appear to be the most important factor in heredity. Through 

 them, rather than through the cytoplasm, the hereditary 

 constitution seems to be transferred from one generation to 

 the next. 



