Semi-Centennial Volume. 95 



Do Use and Disuse Modify Heredity? 



By LvMAN C. WoosTKR, Sttite Normal .School, Kini)oria. 



In considering this basic problem of evolution, it is evidently neces- 

 sary to study the dynamics of heredity. The fact that all forms of 

 life inherit the tendencies and powers of their ancestors is unques- 

 tioned. Fish beget fish, birds beget birds, and mammals beget mammals, 

 each after its own kind. But how these tendencies and powers are 

 transmitted through the egg, a one-celled body, is not so easy to com- 

 prehend. 



The microscope shows that the egg consists of a cell wall, cytoplasm, 

 concerned principally with nutrition, and nucleus, the part controlling 

 cell division and bearing the hereditary qualities of the organism. All 

 the parts named, the wall, the cytoplasm and the nucleus, are composed 

 of protoplasm, the physical basis of life according to Huxley. Asso- 

 ciated with the protoplasm of the egg are various hydrocarbons and 

 carbohydrates, which yield energy for the cell activities when oxidized, 

 and certain other food materials not yet assimilated. 



Naturally the evolutionist is interested chiefly in protoplasm. The 

 chemist tells him that it consists of an unknown number of kinds of 

 protein, many of which are of unknown chemical composition. The 

 biologist with the microscope can distinguish several varieties of pro- 

 toplasm, chiefly from the fact that they absorb various aniline dyes in 

 different degrees. One of these, which he has named chromatin, ab- 

 sorbs more of the dye stuff than any other, and is the most active kind 

 of protoplasm in the cell in cell division, and hence is most interesting. 

 The other kinds have special work to do in nutrition and in assisting 

 the chromatin, but are not thought to have much to do in heredity. 

 The centrosomes and linin threads, however, merit watching. The cen- 

 trosomes initiate cell division in most animal cells and in the cells of 

 the lower plants. The linin threads manage the details of cell division. 



All the cells of all the tissues are derived from the fertilized egg 

 cell through the process of cell division and growth of each cell from 

 absorbed nourishment. All the daughter cells become specialized in a 

 remarkable way as soma cells. Every cell of the body can do seven 

 things, but the soma cells become specialized so they can do one of the 

 seven things better than they can do the other six. The cells lining the 

 alimentary canal are adept in the absorption of nourishment. The gland 

 cells are wond^>rful chemists. Other cells are chiefly interested in the 

 excretion of waste. The egg and sperm cells continue the species. The 

 cells of the special senses receive sensations. The muscle cells can con- 

 tract so as to bring about movements of the parts of the body. The 

 ganglion cells of the nervous system can do or not do many things for 

 the good of the animal. 



When the fertilized egg cell divides, each resulting cell has the 

 contract for forming half of the body of all symmetrical animals, or, 

 should the cells separate, the daughter cells may produce identical 

 twins as in man. As the daughter cells increase in number by cell 

 division, and in size through the absorption of nourishment, they as- 

 semble in tissues and organs, change their shape and structure, all as 



