574 EVOLUTION 



According to this theory only those variations due to variations 

 in the germ-plasm are of any importance in the formation of new 

 species by natural selection. Also, in improving plants by select- 

 ing the desirable variants, nothing is gained unless the variations 

 have had their origin in the germ-plasm. 



Weismann's idea as to the two distinct kinds of protoplasm does 

 not apply well to plants, but otherwise his theory is in harmony 

 with experimental evidence. 



Mendelism and Evolution. As a result of Mendel's dis- 

 coveries we now know: (1) that the characters of the parents 

 distribute themselves according to a definite law in the offspring; 

 (2) that genes for certain characters do not assert themselves while 

 others do and consequently characters are commonly of two kinds, 

 dominant and recessive; (3) that characters behave as units; and 

 (4) that the characters of the parents may be combined in vari- 

 ous ways in future generations. 



Working according to the Mendelian laws, man has produced 

 new types of plants and animals by bringing about unusual 

 combinations of parental characters in the offspring. Through 

 careful and persistent work, it is possible to eliminate undesirable 

 features from races and also possible to combine the desirable 

 features of two races into one. Thus the rust-resistance of one 

 race of wheat and the desirable milling qualities of another have 

 been brought together in one race. Among Roses, Carnations, 

 and many other ornamental plants, the desirable features of two 

 races have been combined in one. In nature many variations 

 are no doubt due to the crossing of different types or races in 

 both plants and animals. Among insects, such as beetles and 

 butterflies, some forms so distinct in some cases as to be called 

 separate species have been found to be only hybrids between 

 other species. Among birds, such as the flickers, grackles, 

 and warblers, some types are apparently only hybrids between 

 other types. For example, the purple grackle is considered a 

 hybrid between the Florida and bronzed grackle. Among wild 

 plants, numerous variations are no doubt due to crossings between 

 different races. Among our cultivated plants, such as Clover, 

 Alfalfa, Rye, Timothy, etc., much variation is due to crossing by 

 insects, wind, gravity, etc. Such variations afford material 

 upon which natural selection can work and therefore may have 

 an important place in the formation of new species. 



