278 Evolution and Adaptation 



or to the other parent. Thus while it may not be possible to 

 halve a single step {hence one-sided inheritance), yet when more 

 than one step has been taken the inheritance may be divided. 

 There is every evidence that most of the Linncean {wild) 

 species that Darwin refers to have diverged from the parent 

 form, and from each other, by a number of successive steps ; 

 hence on crossing, the hybrid often stands somewhere between 

 the two parent forms. On this basis not only can we meet 

 Darwin's objection, but the point of view gives an interesting 

 insight into the problem of inheritance and the formation of 

 species. 



The whole question of inheritance has assumed a new 

 aspect ; first on account of the work of De Vries in regard 

 to the appearance of discontinuous variation in plants ; and 

 secondly, on account of the remarkable discoveries of Gregor 

 Mendel as to the laws of inheritance of discontinuous varia- 

 tions. Mendel's work, although done in 1865, was long 

 neglected, and its importance has only been appreciated in 

 the last few years. We shall take up Mendel's work first, 

 and then that of De Vries. 



Mendel's Law 1 



The importance of Mendel's results and their wide applica- 

 tion is apparent from the results in recent years of De Vries, 

 Correns, Tschermak, Bateson, Castle, and others. Mendel 

 carried out his experiments on the pea, Pisum sativum. 

 Twenty-two varieties were used, which had been proven by 

 experiment to be pure breeds. When crossed they gave per- 

 fectly fertile offspring. Whether they all have the value of 

 varieties of a single species, or are different subspecies, or 

 even independent species, is of little consequence so far as 



1 Bateson, in his book on " Mendel's Principles of Heredity," has given an 

 admirable presentation of Mendel's results. I have relied largely on this in my 

 account. 



