534 EVOLUTION 
the environment may show no adaptive connection whatever 
with the external influence inducing the change. Another cause 
of changes among the units in the germ-plasm Weismann attrib- 
utes to interactions upon each other of different units brought 
together through fertilization. 
According to Weismann’s theory of the constitution of organ- 
isms, fluctuating variations are due to changes only in the 
somatoplasm, while mutations are due to shifts among the 
factors in the germ-plasm. Mutations arise as a result of changes 
within the individual, while fluctuating variations usually arise in 
response to influences external to the individual. Although the 
external influences causing fluctuating variations may at the same 
time so indirectly influence the germ-plasm as to cause mutations, 
the fluctuating variations themselves involve only the somato- 
plasm, and hence are not recorded in the germ-plasm. For 
example, a lack of moisture may cause such fluctuating variations 
as a reduction in size of leaves, fruit, and in number of flowers 
produced per plant, and at the same time induce such changes in 
the germ-plasm that a mutation in color of flowers, length of 
style, etc., may appear in the offspring, but the fluctuating varia- 
tions disappear with the vegetative structures involved. 
Weismann’s theories have not been sufficiently demonstrated, 
and there are some objections to them. Although the evidence 
seems to be against the inheritance of acquired characters, this 
question is not yet settled. 
