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. 



