The biological literature about evolution amounts to thousands of books and papers published every year. There is also a significantly smaller number of publications by Thomists who try to show how evolutionary theory may be reconciled with Thomistic philosophy. But the common problem of both Thomistic and non-Thomistic literature is a lack of clear and consistent definitions of terms. Let’s look at just a couple of examples found in the Thomistic literature.
One of the books says: “The task of evolutionary biology is to explain new species as arising out of earlier ones in the same way that mountains and lakes are explained as having arisen out of earlier geological formations.”1
Here biological evolution is compared to geological processes such as elevation of mountains, erosion of rocks, etc. But the first part defines evolution as new species arising from the earlier species. We already have confusion, because a new species is definitely something entirely new compared to an old species. For example, the common evolutionary story says that birds evolved from reptiles; thus, initially there were only reptiles (mostly robust and heavy quadrupeds walking firm on the earth) and finally we have small, gentle, and quite agile creatures that differ in almost all internal and external organs and systems from the reptiles. Thus, in biological evolution, over the course of time, we would see the emergence of some total novelties, such as wings, pneumatic skeletons or the “four-cycle” respiratory system. We do not see such creativity in any geological processes. What we see instead is mountains changing into different mountains or into valleys or sea beds which change into mountains or valleys again. Nothing new is created in geological processes.
So, what does the definition tell us? Does it mean that those new “species” that evolve from the old ones are just minor variants emerging due to environmental adaptations? We know that if we move a population of foxes from dry and hot conditions to cold and humid they may survive and adapt by developing thicker fur, thicker blood and perhaps other features. But we do not see an emergence of a new species from the earlier species. In fact we know quite well that a polar fox is as much a fox as a desert one.
Moreover, when we check the fossil record, we do not see dramatic change in the animal forms that have inhabited the earth over long ages. A hundred-million-year-old crocodile fossil is identical to the currently living crocodile of the same species. It is true that some species appear and others disappear in the fossil record, but we do not have evidence that those “newer” species evolved from the older ones. On the contrary, forms of all species remain unchanged over millions of years, and this is the rule rather than an exception. So, how should we understand the quoted definition? It seems to be at least incoherent. It claims that entirely new species of living beings may arise owing to evolution, but at the same time it compares evolution to geological processes which do not create anything new.
Moreover, this definition assumes the existence of species—new species evolve from the earlier ones. But where did those earlier species come from? According to this definition evolution does not explain the origin of species. At the end of the day, we do not know if the definition speaks about macro- or microevolution. Does it mean that evolution creates species, or just minor variants within species? Are all species sharing common ancestry, or there were many beginnings of life? These and many other questions remain unanswered. But the answer to these questions is crucial, if we are to know what we are talking about when discussing evolution and Aquinas’s philosophy.
Another definition found in Thomistic literature defines evolution as: “the scientific claim that all the living organisms on our planet have a common biological origin and that these diverse organisms arose through a process of natural selection acting on genetic diversity.”
This definition ends up requiring circular reasoning. If natural selection acts on genetic diversity, the genetic diversity must exist prior to natural selection. But genetic diversity is carried by the diversity of organisms which, according to the definition, are produced by natural selection. The diversity of life must exist so that natural selection can create the… diversity of life.
One could also wonder how much science there is in the “scientific claim” that “all the living organisms on our planet have a common biological origin.” Such a general statement about all life on earth sounds more like a philosophical postulate rather than a conclusion from any scientific research.
Given such ambiguities and even contradictions in the definitions provided by Thomistic evolutionists, we need to resort to clear and consistent definitions and—by doing so—establish the problem facing those who would reconcile Aquinas with evolution.
 G.M. Verschuuren, Aquinas and Modern Science, Angelico Press 2016, p. 153.
 N.P.G. Austriaco, J. Brent, Th. Davenport, J.B. Ku, Thomistic Evolution: A Catholic Approach to Understanding Evolution in the Light of Faith, Tacoma, WA: Cluny Media 2016, p. I