Evolutionism, as a philosophical system, holds that the complexity of things is due to accumulated changes brought about by the activity of merely material things.¹³

Essential evolution (substantial change or origin of species, whose product is natural species) is when the essential perfections are gained or lost in a series of generations.¹⁴ Inter-racial evolution (accidental or small change, whose product is natural varieties) occurs when inherited non-essential traits are changed in a series of generations. Race is defined as a group of living things with relative stable and particular inherited characteristics, not differing essentially from other groups with different inherited characteristics. Klubertanz notes that inter-racial evolution does not even prove the possibility of essential evolution.

Philosophic species is the essence considered in its full determinatedness, and is predicated of individuals with the same essence, e.g., man is a rational animal.¹⁵ Biologists in taxonomy, on the other hand, classify living things, beginning with the most inclusive category, into kingdom, phyla (type), classes, order, families, genera, and species.¹⁶ Biologists give meanings to these classifications, so that species could be: living creatures that can mate and produce fertile offspring, e.g., Homo sapiens. Donat notes it is often difficult to tell which qualities are essential and which are accidental, so that it is sometimes difficult to determine if a biological taxonomic species is a natural species.¹⁷ Klubertanz notes that the source of confusion arose when biologists, who had studied philosophy, used terms which already had a technical meaning in the philosophy of nature and logic, like "species", assuming a univocal meaning.¹⁸

Monphyletic evolution asserts species arise from one primitive life form, or very few.

Polyphyletic evolution asserts contemporary species arose from lines of several species.¹⁹

Evolutionism here is distinguished from evolutionary atheism, which entirely excludes the Creator. Atheism is treated in another chapter of this dissertation, where this dissertation maintains some action by the Creator. Evolutionism here is also distinguished from just the biological fact, which may not have occurred.²⁰ Evolutionism here is distinguished from evolutionary finality, in which "more perfect organisms have their origin by progress from lesser origins," which is treated in the next chapter.²¹

Fixism, or the theory of permanence, denies all mutations of species.²² Fixism is allied with Creationism, but is not the same. Creation is production of a thing from nothing of self or subject (productio ex nihilo sui vel subjecti). Fixism was commonly held by all until the Renaissance.

Philosophically possible is distinguished from the biological fact. The fact of evolution has not yet been proven.²³ Philosophy treats the possibility in the abstract. Philosophy is the science of all things according to their ultimate causes as achieved by the light of natural reason.²⁴

Possibility is defined as the capacity for existence for the form of a definite possible thing: internally, that its constituent characteristics are not impossible, and perhaps additionally externally possible, if there is power to produce the thing.²⁵ Possible evolution can produce a new species.