160 THE PATH OF SCIENCE 



brought him posthumous fame. He showed that the genes, 

 as we now call the units responsible for heredity, do not in- 

 terfere with one another when they come together at fertili- 

 zation. A hybrid inheriting genes for both tallness and 

 dwarfness does not have genes for medium size in its germ 

 cells: on the contrary, each of its offspring inherits from it 

 only tallness or dwarfness. When Mendel's paper was dis- 

 covered, it was quickly shown that his laws of inheritance, 

 as they came to be called, were not soinething peculiar to the 

 edible pea but were of universal application to plants and 

 animals, including man. 



The paper was discovered in 1900, and two years later a 

 fact of first-rate importance was pointed out by W. S. Sutton 

 of Columbia University. The way in which the chromo- 

 somes are distributed from parent to offspring was known. 

 Sutton pointed out that it was exactly the same as the way 

 in which the genes are distributed, according to Mendel's 

 findings. Mendel had died in 1894, a few years before van 

 Beneden had made his discoveries. Had he lived those few 

 years, Mendel might perhaps have forestalled Sutton. But 

 the last years of his life were so much occupied with the 

 financial affairs of his monastery that it is unlikely that he 

 kept in touch with chromosome research. 



It was already known in 1901 that the sexes differed slightly 

 in their chromosome complement, and it was not long before 

 people realized that chromosomes are not only the bearers of 

 the genes for ordinary characters, but also the determinants 

 of sex. A few years later an American biologist began study- 

 ing inheritance in a little fly rather similar to the housefly but 

 smaller, called Drosophila. This animal presents extraordi- 

 nary advantages for the study of heredity. It can easily be 

 kept in large numbers in the laboratory, the reproductive 

 cycle from one generation to the next is very short, and the 

 chromosomes are few. It has taught us more about heredity 

 than any other organism. A group of workers centered 

 around T. H. Morgan at Columbia University began to make 

 marvelous discoveries. It had been known for some time 



