A modern version of this old belief was the idea advanced by Harvey that the ovum consisted of fluid in which the embryo appeared by spontaneous generation. Loeuwenhoek's development of the microscope in the 17th century led immediately to the discovery of the spermatozoon by one of his students. At the time, the "preformation theory" was probably the most widely accepted--i.e., that the adult form exists in miniature in the egg or germ, development being merely an unfolding of these preformed parts. With the discovery of the spermatozoon the preformationists were divided into two schools, one (the ovists) holding that the ovum was the container of the miniature individual, the other (animalculists) according this function to the spermatozoon. According to the ovists, the ovum needed merely the stimulation of the spermatozoon to cause its contained individual to undergo development, while the animalculists looked upon the spermatozoon as the essential embryo container, the ovum serving merely as a suitable food supply or growing place. This nine-lived notion of male supremacy in inheritance was rather reinforced than removed by the breeding of domestic animals in the still more recent past. Attention has been focused on a few great males. For example, the breed of American trotting horses all goes back to one sire--Hambletonian 10. The great Orloff Stud Book, registering over a million individuals, is in the beginning founded on a single horse--a male. It is not strange that we still find among some breeders vestiges of the ancient belief that the male predominates in inheritance. A superior male can impress his characters in a single year upon 100 times as _many_ colts as a female of equal quality could produce in her lifetime. So slight an incident in his life is this reproductive process for each individual that he could if he devoted his life solely to reproduction stamp his characters upon a thousand times as many colts as could a female. Thus under artificial breeding conditions, the good males do have a tremendously disproportionate share in improving the whole breed of horses, though each single horse gets his qualities equally from his male and female parents. Though Mendel knew an astonishing amount about inheritance a half-century ago, it is worth noting that the foundation upon which rests our present knowledge of sex has been discovered less than twenty years before--the reference is, of course, to the chromosomes as the carriers of inheritance. While from the standpoint of biology the opinions of two decades ago about sex literally belong to a different age, some of them have been so persistent in sociological thought and writings that they must be briefly reviewed in order that the reader may be on his guard against them. Books which still have a wide circulation deal with the sex problem in terms of a biology now no more tenable than the flatness of the earth.