44 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



author of a large amount of fine work in genetics and will rank high 

 in the history of the subject. 



T. H. Morgan, our leading American geneticist, is best known for 

 his researches into the mechanism of Mendelian inheritance. Through 

 the statistical study of ratios and linkages of characters in the fruit fly 

 Drosophila, it has been possible to chart the localities of the deter- 

 miners or genes of at least 150 mutant characters. He has shown that 

 four linked groups of genes exist, corresponding to the four kinds of 

 chromosomes of the germ cells; one of these groups is sex-linked and 

 is therefore to be assigned to the X-chromosome of the mutant male. 

 Two other large groups are to be located in the two large autosomes, 

 and one very small group is assumed to be located in the microsome. 

 Not only have characters, or their determiners, been assigned to given 

 chromosomes, but they have been located in a linear series on a given 

 chromosome. So accurately have these loci been determined that 

 they may be used to predict unknown breeding ratios. It would 

 seem that when a theory serves so well that it may be used to predict 

 the results of experiments, such a theory must be founded on facts. 

 Morgan and his collaborators in genetics are now convinced that they 

 have discovered the actual mechanism of heredity in the behavior of 

 the chromosomes in maturation and fertilization and that it is unex- 

 pectedly simple. Their views have aroused considerable opposition, 

 but they have apparently met successfully all attacks up to the present. 

 If it be true that the actual machinery of variation and heredity has 

 been discovered, we are farther along in our understanding of the 

 causo-mechanical basis of evolution than we could have hoped to be 

 at so early a date. 



HEREDITY AND SEX 



Since Darwin's theory of sexual selection, sex has been a compli- 

 cating factor in evolutionary theories, and one of the chief advances 

 of the present century has been in connection with the factors con- 

 trolling sex determination and' sex differentiation. The evolution of 

 sex has also been a subject for considerable research. 



It now appears that sex is an inherited Mendelian character, the 

 determiner of which is carried in a definite chromosome or group 

 of chromosomes. Cytological examination of germ cells, under the 

 able leadership of E. B. Wilson, has now made it certain that sex, if 

 not directly the result of the presence or absence of specific chromo- 

 somes, at least is absolutely correlated with such chromosomes. It 

 appears, however, that the sex which is settled by the chromosome 



