If a horse has a star on its forehead like one of its ancestors, we say it is due to heredity. If an ox has color marks on its body like its parent, it is a case of heredity. If a human being has a disease which his ancestors had, very often he declares he inherited it from them, even if it be only a common catarrh. But this is a narrow view of the subject, and does not include all that a biologist means when he uses this word.
By heredity he understands the production from a fertilized ovum of an individual, with all the general characteristics of structure and function of body and brain of the species to which it belongs. It means that the offspring, however much they may vary in general characters, will always be of the same species as the parents. The offspring of dogs will be dogs; of wolves, wolves; of negroes, negroes, and of white men, white men. Anything less is not heredity in its full sense.
Darwin, whom we all love and honor, says: "The whole subject of inheritance is wonderful," and in this he but voices the universal sentiment of those who have given any serious consideration to it. Let me try to show you how wonderful it is by an illustration. From very ancient times the horse has been the constant companion of man. This animal, with his splendid muscular system, the most perfect, perhaps, of any creature, has for his food and shelter, and not always the best of these, rendered mankind almost infinite service. Now, every horse that has ever been born into the world began life as a minute ovum, which under the microscope presents no appearance of a horse, or any other animal, and, strange to say, this ovum is, to all appearance, like the ovum of other animals, and no amount of study, without knowing its origin, can decide whether it will develop as a dog, an ox, a horse or a man. After, however, it has gone through the process of gestation, this apparently simple egg becomes an animal of a very complex nature, with heart, lungs, brain, eyes, ears, mouth, stomach, and blood vessels, all where they should be and ready to perform their functions; with mental traits of a peculiar kind which adapt him to the service which man requires. Nay more: In the process of the evolution of the horse, little by little he has changed in various ways, and many, if not all of these changes in his bodily constitution and in his mental characteristics, which have been found useful or made him more serviceable to man, his greater docility, his increased size, his enormous strength and speed, his wonderful beauty, through a wise selection and the weeding out of the unfit on the part of the breeder, have been transmitted through heredity to his offspring, so that today only a paleontologist can tell us if he finds the remains of a primitive horse, that it belongs to the same class of animals as the horse of our time.
Theories.—Our theories of heredity will depend on the extent of our knowledge, and especially our knowledge of embryology. In the last century knowledge on this subject was very meagre, especially that part of embryology which could only be studied with the microscope; consequently the views of scientists and others of that time were exceedingly crude. The most important was that of Malphigi and Bonnet, who maintained that the miniature animal existed in the egg; that fertilization by the male element simply furnished it with food for growth, and that this was added to and stored up in its interstices. Cuvier, Haller and Leibnitz adopted substantially these views. The latter found them to support his opinion that everything was the result of growth from monads, and that there was no such thing in all nature as generation.
Such a theory was very simple, but it explained nothing except the bare production of offspring. It gave no clue to their endless variations, nor to the fact that they often resembled the father more than the mother. According to this theory the offspring should resemble the mother, as the complete individual is formed by her and should be in her image.
Leeuwenhock, one of the early microscopists, by the aid of his lenses, opened a new world to mankind, and discovered the sperm cells to be active, living, moving elements, and he gave a death-blow to the belief that the perfect organism exists in the ovum; but he went to the opposite extreme, and maintained that it exists in the male cell and that it is only fed and developed by the female. Even today we find in a vague way both these theories held by educated persons.
We are indebted to Harvey in the early part of the eighteenth century for advocating the view held by Aristotle, now known as Epigenesis, and combatting the view of growth from a miniature, but already perfectly formed animal, to a visible one. Epigenesis consists in the successive differentiation from the relatively homogeneous elements as found in the egg, to the complicated parts and structure as seen in the offspring.
According to Huxley, this work of Harvey alone would have entitled him to recognition as one of the founders of biological science, had he not immortalized himself as the discoverer of the circulation of the blood.
Not long after Harvey's publication, Casper Frederick Wolf established the theory of epigenesis upon a firm foundation, where it still remains.
The doctrine of epigenesis has very much complicated the whole question of heredity. No wonder even so great a mind as that of Darwin exclaimed, "The whole subject is wonderful." How can an egg, which in structure is comparatively simple, an aggregation of cells, not one of which bears the slightest resemblance to any organ in the body, develop into the perfect individual? How can this egg, formed in special organs, develop other organs than those like the ones in which it was formed? How can sexual cells develop brain cells, with their wonderful modes of action?