8 PLANT-BREEDING 



neighboring types. Such differences must be assumed to 

 be produced each by a single mutation. By tliis means 

 the significance of the mutations may best be judged, and 

 whenever species differ from their nearest alhes in a higher 

 degree, the inference is allowed that they have been origin- 

 ated by more than one mutation. 



Since the publication of Darwin's theory, the probabil- 

 ity of such sudden changes playing an important part in the 

 evolution of species has always found some support. Of late, 

 the evidence has increased in this direction, especially under 

 the influence of Cope. Discontinuous evolution has been 

 defended among palaeontologists by Dollo, among zoolo- 

 gists by Bateson, and among botanists by Korshinsky. This 

 Russian author compiled the history of a large number of 

 varieties from the widely scattered horticultural literature 

 and showed that, in almost all cases where the history of 

 the origin of a variety was recorded, it originated suddenly. 

 Many other varieties, especially among trees and shrubs, 

 have been discovered as such in the field, and, although 

 their origin is not historically known, the constant absence 

 of intermediates pleads vigorously for the explanation of 

 tlieir differential qualities by mutation. 



The conception of mutations agrees with the old view 

 of the constancy of species. This theory assumes that a 

 species has its birth, its lifetime, and its death, even as an 

 individual, and that throughout its life it remains one and 

 the same. Thus it is only natural that wild species are 

 almost always observed to be constant, since by a mutation 

 they do not change themselves but simply produce a new 

 type. This is alhed to its ancestor as a branch is to a tree, 

 the stem continuing its own growth, no matter how many 

 branches it produces. Just so a species may produce quite 

 a number of new forms without being changed itself, in the 

 the least, thereby. Among palaeontologists Scott has given 



