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 Linnean (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 
Spectes. 
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! 
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. 
