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. 



