230 VARIATION AND ITS RESULTS 



tarines sometimes are borne on one or more branches, of 

 a peach tree, and peaches may be borne on nectarine 

 trees; rnsset apples are sometimes borne on Greening ap- 

 ple trees; white roses are sometimes found on red-flowered 

 plants. 



;37r). Frequently a plant begins a new kind of varia- 

 tion long after birth, even after it has become well es- 

 tablished. It is on this fact that successful agriculture 

 depends, for the farmer makes his plants better by givdng 

 them more food and care: and betterment (like deterio- 

 ration) is only a variation as compared with the average 

 plant. Plants which start to all appearances equal may 

 end unequal: some may be tall and vigorous, others may 

 be weak, others may be dwarf : some will be worth har- 

 vesting and some will not. 



376. THE CAUSES OF VARIATIONS.— rr//vV///o».s ate due 

 to several aiul perhaps many causes. One class of causes 

 lies in the environment, and another lies in the tendencies 

 derived from parents. Of the environmental causes of 

 variation, the chief is food supply. Good agriculture 

 consists largely in increasing the food supply for plants 

 — by giving each plant abundant room, keeping out com- 

 peting plants, tilling the soil, adding plant-food. Fig. 

 383. Another strong environmental factor is climate 

 (Chapter XXVII). It is very difficult to determine the 

 exact causes of any variation. There is much difference 

 of opinion respecting the causes of variation in general. 

 The extent of variation due to food supply is well illus- 

 trated in Fig. 383. The two pigweeds grew only five 

 feet apart, one in hard soil by a walk, the other near a 

 compost pile. They were of similar age. One weighed 

 % oz.; the other 4% lbs., or 136 times as much. 



377. HEREDITY.— Marked variations tend to be per- 

 petuated. That is, offspring are likely to retain some 

 of the peculiarities ot their parents. This passing over 



