32 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



theory," which forms the framework of nearly all of our modern 

 genetics. According to this view the germ plasm is immortal in 

 that it is perpetuated from generation to generation through the 

 instrumentality of mitotic cell division, each germ cell being the prod- 

 uct of the division of a previous germ cell back to the first germ cell 

 that arose at the dawn of life. Thus a germ cell cannot be a product 

 of the soma, but the soma is the product of germ cells. The soma loses 

 its generalized characters and specializes in various ways. Once 

 specialized, soma cells are believed to have lost their capacity to play 

 a germinal role. Specialization means mortality. Thus the relation- 

 ship between parent and offspring is not that the parent gives rise to 

 the offspring, but that the same germ plasm gives rise to both parent 

 and offspring. 



The logical conclusion to which this line of reasoning leads is that 

 the changes in the soma, no matter how produced, are helpless to 

 produce any effect upon the germ plasm, since germ cells come only 

 from germ cells and not from soma cells. Consequently Weismann 

 led the assault against Lamarckism and won the day so conclusively 

 that even in these modern times few biologists have the temerity to 

 express aloud any definite belief in the inheritance of acquired charac- 

 ters. Weismann's germ-plasm idea is the cornerstone of modern 

 genetics, though there are some forward-looking biologists who, looking 

 at things with a physiological bias, cannot make themselves believe in 

 the total independence of any tissue — even the sacred germ plasm. 



Weismann's influence was very great, especially during the last 

 decade of the nineteenth century, and his theories gave rise to an 

 immense amount of research, chiefly of a cytological and embryo- 

 logical character. 



ISOLATION THEORIES 



Among the theories subsidiary to natural selection as an aid to 

 species forming are the various isolation theories. One of the weak- 

 nesses inherent in natural selection had to do with the probable 

 swamping out of new types by promiscuous breeding with the more 

 numerous individuals of the older types. "Anything," says Metcalf, 

 ''which divides a species into groups, which do not freely interbreed, 

 is said to segregate (isolate) the members of the species into these sub- 

 divisions." 



Some American writers, especially Jordan and Kellogg, Gulick, and 

 Crampton, have dealt with the isolation factor in evolution and believe 



