CHAPTER XI X 



THE PHYSICAL BASIS OF INHERITANCE 



When Mendel announced the results of his experiments in 1866, nearly 

 all biologists were engaged in the discussions and controversies that 

 followed the publication of Darwin's Origin of Species in 1859. The few 

 who knew of Mendel's work did not appreciate its importance, and it 

 was soon forgotten. By 1900, however, many biologists had come to 

 realize that a more precise evaluation of inheritance and variation was 

 essential for the understanding of evolutionary processes and had turned 

 to experimental studies. In that year, three botanists who were inde- 

 pendently investigating inheritance in plants came across Mendel's 

 paper, and, realizing its value, made it known throughout Europe and 

 America. Almost at once a number of botanists and zoologists repeated 

 Mendel's experiments, using a wide variety of plants and animals for 

 breeding stocks. Nearly all these experiments verified Mendel's findings 

 and soon established segregation-and-recombination and independent 

 assortment as general principles of biological inheritance. 



It must not be supposed, however, that modern genetics consists 

 merely in verifying Mendel's laws and discovering that they apply to 

 organisms in general. Mendel established two basic principles of in- 

 heritance and contributed an extremely useful experimental method. 

 Since his time an ever-increasing number of workers and the combination 

 of experimental breeding with other methods of research have carried 

 modern genetics far beyond the point reached by Mendel. 



In the interval between 1866 and 1900, tremendous advances had been 

 made in another field of biology that was eventually to become closely 

 knit with Mendelian breeding in the development of modern genetics. 

 This is the science of cell study, or cytology. In 1866, the "cell doctrine" 

 was in process of becoming established, but little was actually known 

 about the detailed structure or the lineage of cells, and comparatively 

 few biologists thought in terms of cells. Between 1866 and 1900, however, 

 special techniques had been devised for fixing and staining cells to permit 

 microscopic differentiation of their component parts; the microtome had 

 been invented for cutting microscopically thin slices of tissues; and 



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