146 Mendel's Work 



the inheritance. What then did Professor T. H. 

 Morgan mean by saying that the problem of heredity 

 has been solved? He meant, we take it, two things: 

 (1) that the embryological discovery of the con- 

 tinuity of the germ-plasm, which is mainly due to the 

 work of Weismann, has explained the fact that like 

 tends to beget like; and (2) that the discovery of 

 the behavior of the chromosomes — which carry the 

 factors for many, at least, of the hereditary char- 

 acters — has enabled us to understand the precise way 

 in which certain characters of the parents are dis- 

 tributed among the offspring. 



As is well known, the rediscovery and development 

 of Mendel's work has made a change in the study of 

 heredity so radical that it must be called a meta- 

 morphosis. It has been shown that many organisms 

 consist, in part at least, of a great bundle of "unit 

 characters" which behave in inheritance as if they 

 were indivisible entities. They do not blend or inter- 

 grade ; they are present in a certain proportion of the 

 descendants ; typically, they are either there in their 

 entirety, or quite absent. But they may be in some de- 

 gree masked in their developmental expression by 

 other characters or by environmental conditions. 

 Their germinal representatives in the chromosomes 

 are called "factors," "determiners," or "genes," as we 

 have mentioned; and one factor may affect several 

 adult characters, while one adult character may be 

 the outcome of several factors. It seems, for instance, 

 that there are eight "factors" cooperating to produce 

 the normal coloration of a wild rabbit's fur. As has 



