Breeding of Cereals 83 



ably good yielders, every variety of a cereal consists of hundreds of 

 different types, which find the best conditions for success when 

 grown together, but which, after isolation, prove to be constant. 

 Their preference for mixed growth is so definite, that once isolated, 

 their claims on manure and treatment are found to be much higher 

 than those of the original mixed variety. Moreover, the greatest 

 care is necessary to enable them to retain their purity, and as soon as 

 they are left to themselves they begin to deteriorate through acci- 

 dental crosses and admixtures and rapidly return to the mixed 

 condition. 



Reverting now to Darwin's discussion of the variability of cereals, 

 we may conclude that subsequent investigation has proved it to be 

 exactly of the kind which he describes. The only difference is that 

 in reality it reaches a degree, quite unexpected by Darwin and his 

 contemporaries. But it is polymorphic variability in the strictest 

 sense of the word. How the single constituents of a variety originate 

 we do not see. We may assume, and there can hardly be a doubt 

 about the truth of the assumption, that a new character, once pro- 

 duced, will slowly but surely be combined through accidental crosses 

 with a large number of previously existing types, and so will tend to 

 double the number of the constituents of the variety. But whether 

 it first appears suddenly or whether it is only slowly evolved we 

 cannot determine. It would, of course, be impossible to observe either 

 process in such a mixture. Only cultures of pure races, of single- 

 parent races as we have called them, can afford an opportunity 

 for this kind of observation. In the fields of Svalbf new and un- 

 expected qualities have recently been seen, from time to time, to 

 appear suddenly. These characters are as distinct as the older ones 

 and appear to be constant from the moment of their origin. 



Darwin has repeatedly insisted that man does not cause variability. 

 He simply selects the variations given to him by the hand of nature. 

 He may repeat this process in order to accumulate different new 

 characters in the same family, thus producing varieties of a 

 higher order. This process of accumulation would, if continued for 

 a longer time, lead to the augmentation of the slight differences 

 characteristic of varieties into the greater differences characteristic 

 of species and genera. It is in this way that horticultural and 

 agricultural experience contribute to the problem of the conversion 

 of varieties into species, and to the explanation of the admirable 

 adaptations of each organism to its complex conditions of life. In 

 the long run new forms, distinguished from their allies by quite 

 a number of new characters, would, by the extermination of the 

 older intermediates, become distinct species. 



Thus we see that the theory of the origin of species by means of 



6—2 



