For in the ever-repeated origination of highly organized individuals from an infinitesimal germ, the working-out of a prearranged plan of growth and development seems obvious. Thus the very idea of “development” which the facts of reproduction suggested stood in the way of applying to the living kingdom the same categories of genesis that were applied on mechanistic principles to reality at large. Indeed, the term “evolution” denoted originally just this phenomenon of individual genesis, and by no means the genesis of species. On the contrary, “evolution” in its literally sense presupposes the existence of the species, because it is precisely this which, in the person of parent individual, provides the prearranged plan to be “evolved” in every given case of generation. What evolves is not the model itself but its re-embodiment in each generation from germ to maturity: what evolves was involved in the germ, its potency there derived from its act in the progenitor. In terms of cause-effect relation, then, the parent accounts not only for its offspring’s existence, but also for its offspring’s form by its own possession of this selfsame form. This is a pattern very different from mechanistic chain of cause and effect and strongly suggest the operation of a causa formalis in addition to a causa effciens, or the existence of a substantial form, which were otherwise banned from the whole system of natural explanation. In short, the very concept of development was opposed to that of mechanics and still implied some version of other of classic ontology.

Hans Jonas