Recent Developments in Heredity and Evolution 13 



tinguishing marks. But this same individuality is no less 

 true for all animals and plants. * In heredity, therefore, 

 there is transmitted not only a likeness to the parent, but 

 also an unlikeness, and this unlikeness constitutes indi- 

 viduality, a certain amount of variation from the parent. 



Darwin's conception was that nature selects from 

 among these varying individuals; that the means of selec- 

 tion is the competition that results from over-production; 

 that the better adapted individuals would naturally be 

 selected for survival; that their better adaptations, which 

 mean their individual peculiarities, would be transmitted 

 to their offspring; and that such selection, continued genera- 

 tion after generation, would so emphasize and increase the 

 favored variations that the old species boundary would be 

 crossed and a new species established. In other words, 

 small variations would be built up into larger ones, and 

 presently they would become too large to be included within 

 the boundary of the old species. 



Of course, objections have been raised to the theory of 

 natural selection as an adequate explanation of the origin 

 of species. There can be no doubt but that there is selec- 

 tion in nature, in the sense that not all the forms produced 

 survive; but many believe that this cannot change forms 

 enough to be regarded as new species; that any selection 

 thus made cannot be on the basis of any " lif e-and-death " 

 advantage of structure that one individual has over another; 

 and that the variations thus used are onh' the so-called 

 "fluctuating variations" which have nothing definite in 

 them as to direction or amount. 



4. Mutation. — We come now to the work of the last 

 decade, which is characterized by the rapid development 

 of the study of evolution by using experimental methods. 



