51 8 EVOLUTION, HEREDITY, EUGENICS 



of the fact that Charles Darwin, like many others, ridiculed La- 

 marck's laws, particularly the one that suggested the inheritance of 

 acquired characters which had arisen through a new need, his pan- 

 genesis theory attempts to explain the inheritance of acquired 

 characters. 



As we have not yet determined the method by which somatic 

 characters may be transmitted to the germ-cells, we cannot accept 

 this theory. Our knowledge of the importance of biochemical 

 systems such as the horrnones may increase to a point where it will 

 in time be possible to accept a modified gemmule theory. 



A. Weismann (1834-19 14) in his book, The Germplasm, pub- 

 lished in 1893, denied the formation of the germplasm from the body 

 tissues of the individual, and maintained that its sole origin was 

 from the germplasm of the parent of the individual. The germplasm, 

 according to Weismann, is handed on from one individual to the 

 next descendant unchanged by environment. Since the body cells 

 are not inherited, there is no possibility of the inheritance of acquired 

 characters. 



His theory is that sexual reproduction ^ is a mechanism for dou- 

 bling the possible variations in the offspring. In such permutations 

 and combinations of the qualities of the uniting germplasm, chance 

 and the factor of natural selection may both become effective. 



He believed that the germplasm consists of hereditary units 

 called detey-minants^ and that still smaller units called biophors are 

 made up finally of molecules and atoms. As the determinants are 

 grouped together to form granules of chromatin {idants) making 

 up the chromosomes {ids), it is quite evident that the microscope 

 cannot be used to study these units. 



Unknown Units. — Other biologists have suggested ultramicro- 

 scopic units similar to the biophors and determinants of Weismann 

 and the gemmules of Darwin. De Vries called them pangenes; 

 Spencer named them physiological units; Galton, stirps; Hertwig, 

 idioblasts; Naegeli, micellar strands; and Weisner, plasomes. We 

 accept the fact that there are units which like chemical radicals are 

 always found united, yet preserve their individuality in different 

 combinations. How these ultramicroscopic units behave we are 

 still conjecturing. 



Francis Galton (1822-1911). — A law of heredity formulated by 



' Amphimixis is biparental parentage. (See page 125.) 



