BREEDING AND HEREDITY 3 



created or destroyed. The primary object of cross- 

 ing is to combine within one strain two desirable 

 qualities existing in distinct strains. But an appar- 

 ent result of crossing is very often a much more rapid 

 production of something new than is brought about 

 by selection (see Plate II.). The novelty in such a 

 case, however, is an illusion, and due to the recombina- 

 tion of characters which have been long hidden and 

 long separated. Selection, then, creates something 

 new ; crossing merely recombines characters which 

 already exist. Very little — it may almost be said 

 that nothing — is Imown of the causes which determine 

 the origin of new characters, either in the domes- 

 ticated state or in a state of nature. At any rate, 

 there is not enough known to base a scientific prac- 

 tice of breeding upon. But the art of breeding by 

 the recombination of already existing characters — 

 and it is a question whether in its ultimate analysis 

 this may not be all that the breeder can do — has been 

 brought to a point of extreme precision by the dis- 

 covery made by Mendel in 1865. And the object 

 of this book is to make perfectly clear what that 

 discovery was, and how it affects the practice of 

 breeding. The manner of origin of new forms will 

 be dealt with because a knowledge of Prof, de Vries' 

 observations on this process is necessary to an appre- 

 ciation of the significance of Mendel's discovery. 



Ever since 1859, when Darwin effected the general 

 acceptance of the doctrine of evolution, it has been 

 believed that new forms have originated by the 

 accumulation of the almost imperceptible differ- 



