204 BRITISH PLANTS 
when its adaptation to the environment is most complete. 
Its extirpation means that competing races are better 
fitted for the new conditions than it is itself. Success in 
any environment is reached by different plants in different 
ways. Every habitat has a varied flora. Many species 
of plants, often differing widely in form, live together 
in the same society, under the same conditions. Each 
has solved the problem of adaptation on lines of its own. 
Having thus established that new species have arisen 
naturally from old, it remains to explain in what way a 
new specific character may be acquired. Two solutions 
to this problem have been suggested : 
1. The Theory of the Accumulation of Minute Differ- 
ences.—This was the theory which Darwin, after some 
hesitation, adopted. According to this view, a specific 
character—that is, a character distinctive of a species— 
has been produced by the accumulation, from generation 
to generation, of very small variations. When a varia- 
tion in a certain direction useful to the plant is initiated, 
the progress in each generation, or series of generations, 
is very small. It is the gradual accumulation of these 
minute differences, transmitted faithfully from one genera- 
tion to the next, that at last constitutes a new and dis- 
tinctive character, the sign of a new species. Unfor- 
tunately, Darwin had no proof of this, and several serious 
objections have been urged against it. One is that the 
intermediate forms linking up the new species with the 
old have never been observed. The missing links are not 
found in Nature. 
2. The Mutation Theory.—This theory, which has 
gained ground rapidly in late years, is associated with 
the name of De Vries. It was the outcome of Weismann’s 
researches on heredity and Mendel’s laws of heredity in 
hybrids. Darwin’s theory apparently received sanction 
from the old principle that Nature never makes a leap— 
Natura non facit saltum—but, according to the mutation 
theory, evolution proceeds, not by the gradual steps of 
minute differences, but by sudden leaps and bounds. 
Each leap, or mutation, constitutes a new species. There 
are no intermediate steps, and consequently no missing 
links. If this theory is true, a given species remains 
absolutely constant for long periods of time, displaying 
no appreciable variation from one generation to another. 
