Improvements Made in Plants by Man 433 



shown by the fact that variations become more active when the 

 external conditions are changed. This happens when plants are 

 taken to new countries; or are brought out of forest or field 

 into garden or greenhouse; or are subjected to high cultivation, 

 through which are provided better conditions for nourishment and 

 comparative freedom from natural enemies; or are given different 

 soils, fertilizers and exposures; or are crossed in reproduction, 

 a matter which we shall consider more fully in a moment. There 

 are, of course, limits to the change of conditions that plants can 

 endure, but within those limits all changes in external conditions 

 are followed by more rapid, diverse, and extreme variation. Va- 

 riation in organisms may be symbolized by the gentle trembling of 

 the surface of water held in a full dish at arm's length; if the 

 hands are deliberately shaken a little, the trembling increases, 

 though the shaking must be kept within limits, else the water is 

 spilled from the dish. The cultivators of plants, realizing well 

 that variation is the basis of the possibility of improving plants, 

 and observing that change in conditions promotes it, have from 

 the earliest times made use of these methods for rendering more 

 variable those forms which they wish to improve, but which 

 naturally exhibit little variation. This is precisely what they 

 mean by their expression " break the type." 



A third vital fact about variation is this : it is fortuitous, which 

 means that it takes place in any possible part, feature, or direc- 

 tion, indifferently, according to chance, and shows no tendency 

 to follow any particular lines, excepting in so far as structural 

 conditions may make it easier to vary one way than another. 

 Stems do not vary towards shortness alone nor longness alone, 

 but towards shortness, longness, thickness, thinness, roundness, 

 angularity, flexibility, stiffness, and any other peculiarities which 

 the construction of stems makes it possible for them to exhibit. 

 Moreover, these variations insist, so to speak, upon originating 

 and directing themselves, and all the ingenuity of man has not 

 yet enabled him either to originate or to determine the direction 



