EVOLUTION AND HEREDITY 355 



transformation of food material, but changes in structures 

 as well. This has led scientific men to search for deter- 

 miners of one kind or another. Darwin believed that there 

 were particles (gemmules), of as many varieties as there are 

 categories of cells in an organism, which migrated to all 

 the cells during development and determined their specific 

 type. There were serious objections to such a view, but 

 other evolutionists have postulated somewhat similar deter- 

 miners (micellae, pangens, etc.). There is no definite 

 evidence, however, in favor of any such arrangement. 



The only positive evidence that indicates bearers of 

 hereditary qualities points chiefly to the chromosomes. 

 The fact that the chromosomes are divided with such care 

 during mitosis indicates that their accurate separation is 

 important. Biologists have long believed that this was an 

 indication that they were the carriers of hereditary char- 

 acteristics, and during the past few years Professor Morgan 

 believes that he has secured evidence by breeding fruit flies 

 (Drosophila ampelophila) which enables him to assign 

 particular characteristics to each of the chromosomes known 

 to be present in the germ cells. One chromosome, for exam- 

 ple, apparently carries the factors for bent wings and eye- 

 lessness; another, those for banded body, dwarfing, white 

 color of head, and at least twenty others; the two remain- 

 ing chromosomes are believed to carry twenty-eight and 

 forty-seven factors respectively. If chromosomes are the 

 sole bearers of hereditary qualities, those of the fruit fly 

 doubtless carry many more than have as yet been assigned 

 to them. But Professor Morgan speaks with enough assur- 

 ance concerning the behavior of those he has tested by 

 experiment to be able to assign characters to particular 

 positions on the chromosomes. 



CONCLUDING REMARKS ON EVOLUTION 



Morgan* says: "Today the belief that evolution takes 

 place by means of natural processes is generally accepted. 



* "A Critique of the Theory of Evolution," 1916. 



