526 VARIATIONS 



animals are secured, and they are also the means by which desirable 

 types are lost after much time and labor has been expended in 

 securing them. A thorough understanding of the causes of 

 variations would be of much service not only in securing new 

 types but also in keeping desirable types stable. Our knowledge 

 of the causes of variations, especially of the internal ones, is quite 

 obscure. The causes of variations fall into two general classes. 

 They are either external, due to environment, or internal, due to 

 something within the organism. 



External causes. Environment affects both plants and 

 animals in all kinds of ways, and is directly responsible for most 

 of the differences among organisms. Differences in food, water, 

 light, altitude, influences of organisms upon each other, chemical 

 and physical nature of soils, and in all other environmental 

 factors cause variations. They cause the numerous fluctuating 

 variations previously discussed. The hardness and protein 

 content of the kernels of Wheat, height and yield of Corn, and 

 most all characters of crop plants are modified by regional 

 differences in climate and soil. If the differences in conditions 

 are permanent, the variations may be more or less permanent. 

 Varieties of crop plants that do well in one locality commonly do 

 not do well in all localities and in some localities continue to 

 give poor yields after being grown in the new locality for a 

 number of years. Plants transferred from a low to a high alti- 

 tude usually change in a number of ways and often the new feat- 

 ures are permanent as long as grown under the new conditions. 



Variations in parents often cause variations in offspring, as 

 stunted offspring from poorly nourished parents. It has been 

 demonstrated that the heaviest and largest seeds commonly 

 produce the best offspring. For this reason we are advised to 

 select seed from vigorous parents. Low temperatures retard the 

 development of plants, often resulting in immature seeds that 

 produce weak offspring. There are many ways in which we must 

 reckon with variations caused by environment. 



Internal causes. We know least about the internal causes of 

 variations. They are changes in the cells of the individual. 

 Usually they are changes in the sex cells of the parents of the 

 individual in which the variation appears. Sometimes these 

 changes are induced by external conditions, but often they seem 

 to have no relation to external conditions. Variations that are 



