The archetype or species itself is never manifest because it is supra-individual and thus supra-formal. It cannot be clothed in form. What we see in the many individuals of a species are the expression of its possibilities, which will be indefinite in number and, in a manner of speaking, inexhaustible in variety.
Species can be envisioned as an immutable and immaterial form: that is to say, no evolution is possible although individuals will necessarily manifest themselves according to all possible variations. These variations, however unique they may appear, are what they are–branches of a trunk–and cannot become detached from it.
Coming to Darwinism properly speaking, it is founded on the confusion of simple variation (within one and the same species) and specific differentiation (differences that separate one species from another): when simple variation occurs in its more eccentric forms, it is interpreted as the emergence of a new species.
By denying the reality of form, species becomes merely a descriptive tool for a set of material coincidences. In other words, species aren’t real–they are simply words we have chosen to describe groups of similar beings. Being only this an nothing more, no reason is seen why an exception should not be interpreted as the emergence of a new species from an existing one. On the other hand, from the point of view of traditional hylomorphism, this is strictly impossible since a being cannot possess two forms at once. (We say this for the purposes of this discussion because it is true with regard to substantial form, which is the idea intended here, although it is not true of the accidental forms, which are possessed in addition to the substantial.)
By allowing a kind of fluid impermanence to species, which is to say by reducing species to the individual level where it is constituted not by an essence but by nothing more than the individuals who are grouped under its name, and by adding to this the assumption that the development of life proceeds along purely material lines, it is easy to arrive at the conclusion that complex beings are the product of the coincidental transformation of species through advantageous variation and subsequent proliferation.