532 EVOLUTION 



be larger than a plant that is shaded. Likewise, the amount of 

 soil moisture and mineral matter available, the amount of 

 transpiration to which the plant is exposed, the temperature of 

 the soil and atmosphere, and the velocity of winds, and the 

 amount of competition with other plants are also common en- 

 vironmental factors that cause variations. Variations which 

 are responses to variations in the environment are seldom, if 

 at all, inheritable and hence are fluctuating variations. Fluctu- 

 ating variations in individuals may be due also to fluctuating 

 variations in parents. Thus, if a parent plant is poorly nour- 

 ished, its seeds may be poorly developed and produce offspring 

 that vary from the ordinary type in size and vigor. 



Fluctuating variations also arise as a result of sex. Plants 

 produced by vegetative propagation are often less vigorous than 

 those grown from seed. The fusion of a sperm and an egg in 

 fertihzation often results in a rejuvenating effect which is mani- 

 fested in a more vigorous offspring. The relationship of the 

 sperm and egg involved in fertilization has an important bearing 

 upon the variations in the offspring. For example, in Corn the 

 offspring resulting from self-fertilization is not nearly so vigor- 

 ous as offspring resulting from a fertilization in which the sperm 

 and egg come from different parents, although the parents are 

 the same in type. In addition to fluctuating variations and 

 mutations, there are those numerous differences among indi- 

 viduals due to heredity, such as occur in the offspring when 

 parents differing in variety or species are crossed. In connec- 

 tion with these differences due to heredity, some fluctuating 

 variations, such as variations in size and vigor, commonly occur. 



Mutations are apparently caused by changes within the in- 

 dividual, and, although environmental factors may have much 

 to do with bringing them about, they are not direct responses 

 to variations in the environment. Usually they involve the en- 

 tire constitution of the individual, the gametes as well as vegeta- 

 tive structures, wliile fluctuating variations usually involve only 

 vegetative structures. 



Somatoplasm and Germ-plasm. — Weismann, a German bi- 

 ologist, and his followers hold the theory that a plant or animal 

 consists of two kinds of protoplasms, which act more or less 

 independently of each other. The protoplasm of which sperms 

 and eggs are formed they call germ-plasm, while all protoplasm 



