564 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



of selection which are, instead, artificial. Thus plant-breeders select, 

 from among the variants they observe in nature or can produce 

 by hybridization or other genetical practices, the most desirable 

 tendencies which, when heritable, persist in subsequent generations. 

 Often two or more desirable characteristics exhibited by different 

 strains are combined in one strain by hybridization or other 

 techniques. By such means, through the ages, most of our domestic 

 races of vegetable, cereal, pulse, root, and other crops have come 

 into existence, and by modern methods are being improved all the 

 time in relation to modern conditions and needs. Thus strains of 

 many crops have been developed which are able to overcome 

 obstacles that previously prevented them from being grown success- 

 fully in whole regions. Examples of such major feats include the 

 pushing farther and farther north of the wheat belt in Siberia and 

 Canada, and cultivation of hardy strains of other cereals higher and 

 higher up in mountainous areas. Certain fruits, too, are now being 

 produced successfully in increasingly rigorous climes without the 

 aid of costly contrivances such as glasshouses. In addition there is 

 the chemical treatment by various ' plant growth substances ' which 

 is becoming increasingly important in many crop improvement 

 connections. Resistance to disease or frost or drought, promotion 

 of early flowering and rapid fruiting, and all manner of other adjust- 

 ments which can be made in plants by artificial selection and 

 enlightened breeding or chemical treatment, have greatly affected 

 the potential and often the actual ranges of the crops involved — ■ 

 which again strikes the distributional keynote of our subject. 



By such means does Man to a considerable degree mould plants 

 to his needs and extend the area as well as the productivity of his 

 crops. It should, however, be re-emphasized that these ' artificial ' 

 strains of plants are not only commonly incapable of withstanding 

 the competition of native vegetation if this is not kept back by Man, 

 but that Man has also often to modify the habitat even further for 

 success. This he accomplishes by such agricultural, horticultural, 

 or forestral practices as ploughing, fertilizing, mulching, irrigating, 

 draining, and so on. Thus such domesticated plants may, from 

 Nature's point of view, be considered doubly artificial — on one hand 

 in origin and, on the other, in their habitat. But as plant geography 

 deals in a practical way with plant distributions as we see them in 

 the world as a whole, even these ' artificial ' plants are important 

 objects of our study. Indeed Chapter VIII is largely concerned 

 with their modifications and distributions. 



