Two colts on the ranch may be full brothers,--from the same pedigreed stallion and the same pedigreed dam. At the age of two years these two young horses may be as alike as two peas in a pod. One of these promising young animals is chosen, because of some commendable peculiarity of temperament or action, to remain unmutilated, as a procreator of his kind upon the ranch. The other is subjected to the veterinarian's knife and ecraseur and deprived of the testes,--the male sexual glands. From the day of this operation these two animals (in every respect alike, except that one is unmutilated while the other is deprived of the glands mentioned above) develop along radically different lines. The stallion develops during his third year into the noble animal described above. This third year is his period of puberty and the changes which he undergoes physically and psychically are closely parallel to the changes which the human subject undergoes during his period of puberty. The gelding, on the other hand, develops into an animal that is in every respect a neuter. Physically this animal develops a body almost identical with that of the female of the same species. Temperamentally the gelding is a patient, plodding, beast of burden, and though under good grooming he may show considerable life, while under the control of his driver, he seldom shows any interest in other members of the horse family, either male or female, and in the pasture or on the ranch his neutral sex temperament is ever apparent. While he may contend mildly for a place at the feeding trough, he never essays the defense of any weaker members of the herd, and one stallion would put a hundred like him to flight. The thoughtful observer of this phenomenon cannot help wondering what has made this radical difference in the development of these two animals. The solution is not far to seek. From the beginning of puberty to the beginning of senile decay, the stallion derives from the testes what is referred to above as an internal secretion. Physiologists have endeavored to determine exactly what substance formed by the testes is reabsorbed into the lymph and blood. It may be a substance called _spermin_, but whatever the substance is, the physiologists agree that _the testes form some substance which is absorbed by the blood and lymph, is carried to the brain and spinal cord and there produces these profound effects indicated above_. So we have discovered the source of the stallion's strength and beauty. What is true of the horse is true of man. The young man at puberty begins to receive from his testes the internal secretion which leads to the development of his full manly powers. The sum total of the qualities peculiar to manhood has been called VIRILITY. For want of a better word, this term has been applied to the sum total of the male qualities of any animal whatsoever, so that the male qualities of the stallion are also compassed in the term virility.