The theories of generation proposed before the commencement of the seventeenth century are either unsatisfactory or erroneous from the entire want of accurate knowledge prevailing before that time regarding the relation of the egg to reproduction. The conversion of one animal into another, constituting equivocal or spontaneous generations, was very generally believed in; and the process of the formation of the egg was equally ill understood in the lower and higher classes of animals. It was in the course of the seventeenth century that the labours, first of Harvey, and afterwards of Swammerdam, Redi, Malpighi, De Graaf, and Vallisneri, gave rise to greater precision of knowledge and opinions regarding this subject, and finally established the Ilarveyan dictum, " omne vivum ex ovo," which may be regarded as the starting - point or basis of modern researches.
The theories of generation seem after the period of Harvey to have changed somewhat their object, and to have been directed more exclusively to the explanation of the formative process, or the manner in which the parts of the fwtus are first formed in the egg and after wards attain their ultimate structure and con figuration.
It was then that the foundation was laid for the discussion between Epigenesis and Evolu tion, the two theories of generation which have more recently occupied the attention of men of science, and which, as has already been remarked, relate principally to the nature of the formative process. Harvey and Malpighi may be regarded as the first who endea voured, from the observation of facts, to establish the general law of Epigenesis as opposed to the older views of Preformation entertained by the Ovists or Spermatists; but it was not till near the middle of the last century that these opinions were opposed to one another in a decidedly controversial manner.
At that time Caspar Frederick Wolff* sup ported the system of Epigenesis by a reference to observations on the minute changes of the egg of the fowl during the early stages of formation of the chick, while Haller and Bonnet advocated the opposite opinion of Evolution.
Wolff and those who followed his system held that no appearance of the new animal is to be found in the perfect impregnated egg before the commencement of incubation, but that when the formative process is established by the influence of heat, air, and other circum stances necessary to induce it, the parts of the fcetus are gradually put together or built up by the apposition of their constituent molecules. lIallert referred both to his own observations on the chick and to a variety of collateral arguments in support of the system of Evolu tion, holding that when the fetus makes its appearance in the egg, it does so merely in consequence of the enlargement or evolution of its parts which pre-exist, though in an invisible condition, in the egg. Bonnetr carried
this theory further than any one else, but trusting mainly to the observations of Haller on the formation of the fcetus, he supported his overdrawn views on highly hypothetical reason ing. Bonnet, in what is termed the theory of Emboitement, held not only that the whole of the parts of the fcetus pre-exist in the egg before the time of their appearance, but also that the germs of all the animals which have been or are to be born pre-exist from the begin ning in the ovaries of the female ; that the genital organs of the first parents of any species, therefore, contain the germs of all their pos terity; that these germs lie dormant in their abode until one or more are aroused by the exciting influence of the male; and that con sequently there is not in nature the new forma tion of any animal.
We shall have occasion to shew in the article Ovum that the most recent researches concern ing the mode of formation of the fcetus in birds, quadrupeds, and other animals, and more particularly the microscopic observations of Meckel, Pander, Baer, and Ratlike,* have sliewn the theory of Epigenesis or super formation of parts to be much more consistent with what is known from observation than the theory of Evolution. In modern writings, however, the term Development is, without reference to theory, employed to denote the mode of growth of the fcetus more frequently than any other.
We would further remark in relation to our present subject that various names have at different times been given by authors gene ralizing the phenomena of development to the powers supposed to operate in the formation of the young ; as, for example, the Anima vege tativa, Nisus jimmativus, Vis plastiea, Vis essentialis, expansive, resisting, and vegeta tive forces. These terms can be considered as little else than general expressions of the fact that the fcetus is formed and grows in the egg, and are not more satisfactory expla nations of the cause of its formation than the hypothesis of organic affinity is of the process of assimilation in the adult animal. As the knowledge of minute anatomy and physiology has increased, and the accurate observation of the process of developement has been more extended, the number of such hypotheses has gradually diminished.