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 
fertilization 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, while 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 
