ORGANIC EVOLUTION 293 



in recent years by scientific plant breeders, DeVries among others. 

 It is impossible to give an adequate account of Mendelism in this con- 

 nection, for it has become so extensively developed that only the special 

 investigator can follow its ramifications. Some conception of it may be 

 obtained, however, from the simplest possible illustration. 



In the study of heredity the use of hybrids enables the investigator to 

 observe more distinctly the characters of each parent as they appear in 

 the progeny. Using a very simple hybrid, Mendel's law may be illus- 

 trated as follows, with the help of the accompanying diagram. 



Let A and B represent two species of plants that are crossed ; then one 

 of the hybrid plants that result may be represented by AB, which in- 

 dicates a mixture of the characters of the two parents. When A B pro- 

 duces progeny, the hybrid will " split " in the following ratio: one fourth 

 pure A , one fourth pure B, and one half the mixture A B. In the next 

 generation the A and B plants will produce only A and B plants, and so 

 on through successive generations; but the AB plants (hybrids) will 

 produce offspring that split the characters in the same ratio as before, 

 and so on. It is evident that this provides incidentally a test for hybrids, 

 and that in the case of a hybrid there is a splitting and the separation of 

 a certain proportion of the parent plants. It was this test that DeVries 

 applied to his evening primrose, and as it did not " split " in many 

 generations, he was convinced that he was dealing with a pure species. 



It can be understood that such simple hybrids as the one used in the 

 illustration do not represent the general situation in nature or under 

 culture ; but they serve to illustrate the fundamental feature of Mendel's 

 law, which is that a hybrid in the second generation splits up in some 

 definite ratio. It is not clear that all hybrids behave in this way; that is, 

 some of them may not be Mendelian hybrids; but the law is prevalent 

 enough to be used as the basis of very much scientific plant breeding and 

 of experimental work related to evolution. 



Heredity. — A phase of heredity has been presented under Mendel's 

 law, but the general subject should be considered briefly. Heredity 



