3^6 HOW CROPS GROW. 



Naturalists, acting under the older yiew, attempted to 

 draw specific cliaracters more finely than is now thought 

 practicable. Many plants formerly described as separate 

 species are now united together into a single species, 

 the various forms at first supposed to be specifically or 

 originally distinct having been shown to be of common 

 origin, either by producing them from each other or by 

 observing that they were connected through a series of 

 intermediate forms, insensibly grading into each other. 



Varieties — The individuals of any "species" differ. 

 In fact, no two individuals are quite alike. Circum- 

 stances of climate, soil, and situation increase these dif- 

 ferences, and varieties originate when such difEerences 

 are inherited and in the progeny assume a comparative 

 permanence. But as external conditions cause variation 

 away from any particular representative of a species, so 

 they may cause variation back again to the original type. 



Varieties most commonly originate in propagation by 

 seed, especially in case of the trees or plants commonly 

 cultivated for their fruit. Seedling grapes, apples, or 

 potatoes are very likely to differ from their parents. 

 Seed which has been imperfectly ripened or long kept is 

 said to be prone to yield new varieties. 



Less frequently variations arise in propagation by 

 cuttings, buds, gi'afts, or tubers. Pinks and Pelargo- 

 niums in the florist's hands are prolific of these "sports." 



The causes that produce varieties are probably numer- 

 ous, but in many cases their nature and their mode of 

 action is obscure or unknown. Scarcity or abundance 

 of nutriment, we can easily comprehend, may, on the one 

 hand, dwarf a plant, or, on the other, lead to the pro- 

 duction of a giant individual; but how, in some cases, 

 the peculiarities thus impressed upon individuals become 

 fixed, and are transmitted to subsequent generations, 

 while in others they disappear, is difficult to explain. 



Varieties may often be perpetuated for a long time by 



