If we assume, with Darwin and Lamarck, that life has been continuous, 

 then we have to answer the question How did different forms come to be? 

 We know that individuals differ from their parents, but will their offspring 

 differ still more from the grandparents? And will individuals in such a line 

 of descent ever differ enough from their ancestors to be a new species? 



The Germ Plasm The basic question is, of course, What connection is 

 there between an organism and the germ cells which it bears? or What con- 

 nection is there between a fertilized egg and the individual into which it 

 develops? These questions could not be effectively considered until after the 

 essential facts of fertilization had become known. According to the German 

 zoologist, August Weismann (1834-1914), each organism is what it is because 

 it developed from a certain germ plasm (see illustration, p. 508). 



It was Weismann's notion that the experience of an individual cannot in- 

 fluence the germ cells so as to make the offspring show the effects. The result 

 of exercise or of mutilations or of sickness, for example, should not appear in 

 the following generation. There is, in fact, no evidence whatever that modi- 

 fications produced in the course of an individual's lifetime ever appear in the 

 offspring, although many people firmly believe that such modifications are 

 actually passed on. 



In human beings and in other mammals, illness, alcoholism, or chemical 

 injury to the parent may bring about some effects in the offspring. But such 

 effects are not generally of the same kind in the child as in the parent. It is 

 easier to explain what happens in such cases as an injury that interferes with 

 the development of the fetus. 



It is, of course, impossible to prove a negative — that acquired characteris- 

 tics are not inherited (see page 342). The most that we can say about La- 

 marck's assumption is that no one has yet shown unmistakably that acquired 

 traits have been transmitted. But we have learned from countless experi- 

 ments since the time of Weismann that the chromosomes appear to be con- 

 stant and that the "genes" appear to be unchanged by the experience of the 

 body. 



What Kinds of Differences Are Inherited? When Weismann made 

 the distinction between germ plasm and soma, or body plasm, he anticipated 

 important later discoveries about the behavior of cell chromosomes (see 

 pages 368 and 386). We can now say with assurance that those qualities 

 which are determined by the germ substance or genes are inherited, whereas 

 the effects of experience or of external forces — which do not affect the germ 

 — are not inherited. 



We recognize, of course, that parents never actually hand over to their off- 

 spring particular features. Mother still has her curly hair; father still has his 

 round chin. Parents transmit a ctns^m germinal constitution. In order to de- 

 cide in any case how a particular organism came to be just as it is at the mo- 



507 



