In the latter part of the 17th century the doctrine of epigenesis thus advocated by Harvey was controverted on the ground of direct observation by M.
The weight of Malpighi's observations therefore fell into the scale of that doctrine which Harvey terms metamorphosis, in contradistinction to epigenesis.
In fact, while holding firmly by the former, Bonnet more or less modified the latter in his later writings, and, at length, he admits that a " germ " need not be an actual miniature of the organism, hut that it may be merely an " original preformation " capable of producing the latter.4 But, thus defined, the germ is neither more nor less than the "particula genitalis" of Aristotle, or the "primordium vegetale" or " ovum " of Harvey; and the " evolution " of such a germ would not be distinguishable from " epigenesis."
It is a striking example of the difficulty of getting people to use their own powers of investigation accurately, that this form of the doctrine of evolution should have held its ground so long; for it was thoroughly and completely exploded, not long after its enunciation, by Caspar Frederick Wolff, who in his Theoria generationis, published in 1759, placed the opposite theory of epigenesis upon the secure foundation of fact, from which it has never been displaced.
One of Harvey's prime objects is to defend and establish, on the basis of direct observation, the opinion already held by Aristotle, that, in the higher animals at any rate, the formation of the new organism by the process of generation takes place, not suddenly, by simultaneous accretion of rudiments of all or the most important of the organs of the adult, nor by sudden metamorphosis of a formative substance into a miniature of the whole, which subsequently grows, but by epigenesis, or successive differentiation of a relatively homogeneous rudiment into the parts and structures which are characteristic of the adult.