204 THE SCIENCE OF BIOLOGY 



of animals and plants, first by the practical man and later by 

 the investigator. Cytology has heretofore consisted almost 

 wholly of observational studies. But under the stimulus of 

 genetics a measure of experimental work is being undertaken. 

 Genetics and the cytology of germ-cells are now advancing 

 side by side. The present theory of sex-determination, 

 which is an outcome of investigations in both cytology and 

 genetics, illustrates the union of these two fields of study and 

 also the progress toward experimentation. 



The appearance of genetics as a full-fledged science is, 

 therefore, a recent development. Mendel's original publica- 

 tion appeared more than a half century, ago (1866). But 

 general knowledge of this great law of heredity dates from 

 the closing decade of the nineteenth century, when it was 

 independently rediscovered and when the original discovery 

 became generally known. With Mendel's law as a clue, an 

 amazing advance has been made. So much has been learned 

 regarding heredity and variation that Genetics has come into 

 being as a science. Already there are professorships and 

 research endowments within this newly created field. 

 Popular interest in heredity has stimulated the publication 

 of many books and articles in recent years. Mendelism is a 

 familiar topic, and the science of genetics is becoming almost 

 as well known to the public as bacteriology or pathology. 



Sex-Determination; The factors which determine sex 

 in man and the familiar animals have been the subject of 

 innumerable theories from ancient times until the present 

 day, all of which now appear to be groundless. Toward the 

 close of the nineteenth century the hypothesis most widely 

 accepted was that the sex of the individual was dependent 

 upon the amount or kind of food received during the earlier 

 period of development. This theory was believed to have 

 experimental evidence in its favor and hence obtained 

 recognition in biological circles. It was easily apprehended 

 and so gained wide acceptance in the popular mind. By the 

 terms of this theory, the sex was at first undetermined. As 



