340 INTRODUCTION TO EVOLUTION 



biologists to conclude that mutations do not afford likely materials for con- 

 structive evolutionary change. We shall see, however, that the objection is 

 not insuperable. 



Are mutations always random, at all times and under all circumstances? 

 Or can such factors as antibodies direct the course of mutation (see above, 

 and pp. 345-346)? It would be worth much to be able to answer these 

 questions. 



A word of qualification is necessary in referring to the mutation process 

 as random. Mutations are random in the sense that they may occur when 

 they are not "needed," and may fail to occur when they are "needed," but 

 their randomness has limits. As emphasized by Blum (1955, Chap. 9), 

 the kinds of mutation which any one gene is capable of producing are 

 limited by the physical and chemical structure of the gene itself and by its 

 thermodynamic properties. Randomness occurs within definite limits. Such 

 limitations on the mutation process impose limitations on the course of evo- 

 lution itself, helping to determine what directions evolution can take and 

 what directions it cannot take. 



Inheritance of Acquired Characters 



The foregoing discussion has stressed the fact that mutations are inherit- 

 able changes arising in the germ plasm (genes) and that these germinal 

 changes later express themselves in altered body structure of subsequent 

 generations. The sequence of events is important: (1) change in the germ 

 plasm, (2) change in the body, of later generations. 



Exactly the reverse sequence of events has sometimes been postulated 

 as a factor in evolution: (1) change in the body, followed by (2) corre- 

 sponding change in the germ plasm. In fact, this idea represents a distinct 

 theory of evolution termed Lamarckism, after its originator, the French 

 biologist Lamarck, who lived from 1744 to 1829 and hence preceded 

 Darwin. 



The essence of Lamarckism is the idea that changes acquired or devel- 

 oped by individuals during their lifetimes are transmitted to their offspring; 

 this is the so-called "inheritance of acquired characters." We know that as 

 organs or parts of the body are used they develop and increase in strength 

 and size. Thus the college sprinter has at the peak of his training more 

 powerful muscles than he would have had if his most strenuous sport had 

 been bridge. Conversely, organs or parts of the body degenerate if unused. 

 The powerless leg muscles of a person bedridden for a protracted period 

 form a case in point. Lamarck's thesis was that bodily changes of this kind 



