412 OUTLINES OF EVOLUTIONARY BIOLOGY 



more conspicuous variations known as mutations. De Vries 

 points out that many of the so-called Linnean species, 

 such as Draba verna, are in reality made up of a large 

 number of "elementary species" which have arisen in this 

 manner, and certain results which have been obtained in experi- 

 ments upon the improvement of cereals appear at first sight to 

 afford considerable support to these views. 



It has long been known that an ordinary field of wheat con- 

 tains a larger or smaller number of " types," " mutations," or 

 " elementary species," which can be recognized by the experienced 

 eye, and it has been shown that if a single plant of one of these 

 types be isolated it will produce offspring like itself and continue 

 to breed true for an indefinite number of generations. Of course 

 it is necessary that there should be no crossing with other 

 types, but this is easily avoided, for, although accidental 

 crosses may occur, the cereals, with the exception of rye, are 

 usually self-fertilizing. Upon this knowledge is founded the 

 method of single as opposed to continuous selection, a single 

 selection of a suitable type being enough to establish the desired 

 strain. 



One of the first to make use of the method of single selections 

 was Patrick Shirreff : 



" His first discovery was made in the year 1819. He observed 

 a plant of wheat which surpassed its neighbors by its high 

 degree of branching. It yielded 63 ears with about 2500 

 kernels. He saved the seeds, sowed them on a separate field 

 and at considerable distances apart so as to induce in all the 

 plants the same rich branching. He contrived to multiply it so 

 rapidly that it took only two generations to get seed enough to 

 bring it advantageously into the trade. He gave it the name of 

 Mungoswell's wheat, and it soon became one of the most profit- 

 able varieties of Scotland. It has found its way into England 

 and into France, where it is still considered one of the best sorts 

 of wheat." 1 



The same method has been subjected to severe tests and 

 placed upon a thoroughly scientific footing at the Swedish 

 Agricultural Station of Svalof. 



We have seen, in Chapter XIV, that hybridization may 

 occasionally give rise to permanent races or strains exhibiting 

 new combinations of characters, and that this takes place in 



1 De Vries, op. fit., pp. 34, 35. 



