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 Svaléf 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 
