S2 Selection Does Not Lead to Origin of Species. 



in our discussions on the origin of species, whilst the 

 latter has been relatively neglected. But it must not be 

 forgotten that the agricultural improved races do not 

 possess the constancy of true species •} whereas the vari- 

 eties and subspecies of the horticulturist can only be 

 distinguished from true species historically and systemat- 

 ically — not experimentally. 



In conclusion : we see that in estimating the value of 

 the experience of breeders for scientific purposes we have 

 to fix our attention on the simplest processes. Every- 

 thing that can be considered the direct or indirect result 

 of crossing must be excluded before we consider its bear- 

 ing on the theory of mutation or selection. Furthermore 

 one must sharply distinguish between the races that have 

 been produced by continued selection, and the constant 

 forms which owe their oriHn to a sudden fortuitous 



'fc.' 



change. 



In horticulture varieties arise by mutations, and vari- 

 eties are elementary species. In agriculture according to 

 the current view and excepting in the instances of the 

 unconscious isolation of elementary species, the highly 

 improved races arise gradually through selection, but they 

 never become species. 



§ 8. SELECTIVE BREEDING FOLLOWED BY VEGETATIVE 



PROPAGATION. 



We shall now proceed to deal with the scientific sig- 

 nificance of selection in those cases in which its products 

 are multiplied vegetatively. 



* Or if they do prove to be constant, they usually turn out to be 

 the result of the unconscious isolation of elementary species ; com- 

 pare Nilsson's results, described in § 12, pp. 114 ff and Archiv fiir 

 Rasscnbiologie, April i, 1906. (Note of 1908.) 



r 



