MODERN BIOLOGY 593 



Upon heredity, but it was really a scientist who had been trained in the 

 same school — namely, the above-mentioned Thomas Hunt Morgan 

 (born 1866, likewise a professor at Columbia University) — who discovered 

 both the aim of this research work and the means for carrying it out, thereby 

 providing the study of heredity with a wealth of material by way of detailed 

 discoveries of far-reaching theoretical application, such as had never been 

 found elsewhere. His subject for investigation has been a small parasite 

 fruit-fly, Drosoplnla melanogaster, of which it has been said that it has appar- 

 ently been created by God solely as an object of heredity research. It repro- 

 duces itself with incredible rapidity and profusion — in heat an individual 

 requires only twelve days to develop from the egg to sexual maturity — it 

 is extraordinarily hardy and can stand every possible kind of experimental 

 treatment; its cells contain only four pairs of chromosomes, all of a differ- 

 ent size and easily recognizable; and, finally, it has given rise under labora- 

 tory conditions to a profusion of mutations, the factors of which have been 

 found to be constant and well adapted to Mendelian investigations. Morgan 

 and his numerous pupils have examined millions of these creatures, in the 

 course of which he has built up a methodology of his own and a terminology, 

 which, owing to its refined subtlety, is extremely difficult for anyone but a 

 specialist to comprehend. Among the results of this research work we note, 

 first of all, a number of fresh principles, as hard and fast as Mendel's origi- 

 nal ones. As had already been assumed, the hereditary factors are localized 

 in the chromosomes, and it has been discovered that the factors in the same 

 chromosome are not free, but invariably follow one another upon cleavage-, 

 they are "linked," as it is called. Thus, all the factors in this animal are 

 grouped into four linkage-systems, one for each pair of chromosomes. Fur- 

 ther, after a series of ingenious experiments it has been possible largely to 

 determine the position of the factors in each chromosome, and, finally, in 

 certain cases to establish the absence of parts of chromosomes or entire 

 chromosomes as being the cause of various external modifications — ■ that 

 is, a set-back for the theory of the absolute constancy of the chromosomes 

 in the same species. Besides this, it has to a large extent been made possible 

 to ascertain the part played by the previously mentioned sex-chromosome 

 in the determination of sex and in heredity — a problem which, as a matter 

 of fact, observers not belonging to Morgan's school have also studied. In 

 this province likely explanations have been found for a number of hitherto 

 incomprehensible phenomena of heredity; among those that are generally 

 known may be mentioned the inheritability of certain diseases, such as hae- 

 mophilia and colour-blindness in man, and, further, a number of cases of 

 heredity in the sphere of mental diseases. And, finally, mention should be 

 made of the valuable studies in regard to the relation of the chromosomes in 

 species hybrids; in this sphere may be mentioned, of Scandinavian students. 



