20 PLANT-BREEDING 



character; because, if the characters were compound, they 

 would spHt, from time to time, and be divided into their con- 

 stituents. By this means a method is given of studying the 

 expressions which the same unit-character may assume in the 

 different organs of a plant. But this point will be more 

 closely studied in another chapter. 



The question now arises, whether it must be supposed 

 that species in nature ordinarily originate in the same way 

 as in the case of the evening primroses. Of course, the de- 

 tails of the process will be different in different cases. The 

 number of the new types and the frequency of the mutating 

 individuals in each will differ; sometimes they may, perhaps, 

 be more rare and, in other instances, more crowded. Other 

 differences there will be, also. The main point is, however, 

 that mutations occur suddenly and by leaps. One genera- 

 tion is sufficient to produce the whole new type. This is a 

 manifest contrast with the prevailing conception of slow and 

 almost invisible changes producing new species. It may 

 shorten the geological time rec|uired for the evolution of 

 the whole living world and bring it within the limits derived 

 from physical and astronomical evidence. Thus the theory 

 of mutation satisfies these demands. 



The cases observed in horticulture, the constancy of wild 

 species, the behavior of characters in crosses, the occurrence 

 of sharply defined small species within the ordinary species 

 of wild plants and even of agricultural crops, and many other 

 groups of facts, lead to the same conclusion. On the other 

 hand, the slow change of one species into another has not, as 

 yet, been proven in any distinct and clear case. Therefore, 

 we may assume that the mass of the present evidence points 

 to the conclusion that species originate laterally from other 

 species, by sudden leaps. These leaps we call mutations. 



