At the same time it may be allowed that the fact that the mere contact of the male seminal fluid seems to awaken and call forth from the otherwise inanimate egg all those vital powers which are afterwards concerned in sustaining the life of the new being, is one of the most striking and simple examples of vital agency, and one less suited than most others to be observed or experimentally investigated. The theoretical physiologist, in contemplating this fact, is apt to conceive that here he has ar rived at one of the primitive causes or foun dations of animal life, and that he has here obtained the key to many of its hidden won dens be passes the limits which ought to bound his inquiries, and in most instances invents fanciful and curious speculations rather than makes sound generalizations of ascer tained facts.
2. 'Dowries of gencration.—The vast num ber of the theories of generation renders it impossible to mention even the more im portant in this place. Drelincourt, an author of the last century, brought together so many as two hundred and sixty-two " groundless hypotheses" concerning generation from the writings of his predecessors, " and nothing is more certain," quaintly remarks Blumenbach, " than that Drelincourt's own theory formed the two hundred and sixty-third." Of these theories two principal classes may be distinguished, according as they more di rectly relate, 1st, to the action of the parent organs, or 2d, to the changes in the egg belonging to the formation of the new animal. Of the first of these classes of theories Ilaller made three divisions, according as the offspring is supposed to proceed, 1st, exclusively from the organs of the male parent, 2d, entirely from those of the female, or 3d, from the union of the male and female products. The second class of these theories, that, viz. which relates more particularly to the formation of the new animal, may be arranged under two heads, according as the new animal is supposed, 1st, to be newly formed from amorphous materials at the time when it makes its appearance in the egg, or 2d, to have its parts rendered visible, by their being expanded, unfolded, or evolved from a previously existing though in visible condition in the germ.
The greater number of the older theories of generation may then be brought under one or other of the above-mentioned divisions, viz. the theory of the Ovists, of the Spermatists, that of Combination, Evolution or Epigenesis.
According to the first-mentioned of these hypotheses, or that of the Ovists, the female parent is held to afford all the materials neces sary for the formation of the offspring, the male doing no more than awakening the forma tive powers possessed by, and lying dormant in, the female product. This was the theory
of Pythagoras, adopted in a modified form by Aristotle ; and we shall afterwards see that it resembles most closely the prevailing opinion of more modern times. The terms, however, in which some of the older authors expressed this theory are very vague, as, for example, in the notion that the embryo or new product " is formed from the menstrual blood of the female, assisted by a sort of moisture descend ing from the brain during sexual union." According to the second theory, or that of the Spermatists, among the early supporters of which Galen may be reckoned, it was supposed that the male semen alone furnished all the vital parts of the new animal, the female organs merely affording the offspring a fit place and suitable materials for its nourishment.
Immediately upon the discovery of the seminal animalcules, these minute moving particles were regarded by some as the rudiments of the new animal. They were said to be miniature representations of men, and were styled ho munculi, one author going so far as to delineate in the seminal animalcule the body, limbs, features, and all the parts of the grown human body. The microscopic animalcules were held by others to be of different sexes, to copulate, and thus to engender male and female off spring; and the celebrated Leeuwenhoek, who was among the first to observe these animal cules, described minutely the mintier in whieh they gained the interior of the egg, and held that after their entrance they were retained there by a valvular apparatus.
The theory of Syngenesis or Combination seems to have been applied principally to the explanation of reproduction of quadrupeds and man,the existence and nature of the ova of which were involved in doubt. This hypothesis con sists in the supposition that male and female parents both furnish simultaneously some semen or product; that these products, after sexual union, combine with one another in the uterus, and thus give rise to the egg or structure from which the foetus is formed. In connexion with this theory we may also mention that of Meta morphosis, according to which a formative substance is held to exist, but is allowed to change its form in order to be converted into the new being; as also the notion of Bunn that organic molecules universally pervade plants and animals, that these are all endowed with productive powers, that a certain number are employed in the construction of the textures of organized bodies, and that in the process of generation the superabundant quantity of them proceeds to the sexual organs and there consti tutes the rudiments of the offspring.