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 met successfully all attacks up to the present. If it be 
true that the actual machinery of variation and heredity has been dis- 
covered, 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 
