Darwinian Evolution is repugnant to natural reason. One way to see this is to understand that it fundamentally rejects the principle of proportionate causality.
The principle of proportionate causality is often phrased in the saying: Nemo dat quod non habet. “Nothing gives what it does not have.” The phrase is often used in law, in the context of rights of ownership. Someone cannot give the right to ownership of something if he doesn’t have it. The saying, however, does not just hold in law, but also in nature, and can be reduced to a metaphysically and logically necessary truth. Just as the principle of causality demands that that which does not have existence of itself, but does exist, must have existence from another, and that elements of themselves diverse cannot be united without a unifying sufficient reason, the principle of proportionate causality demands that whatever agent causes an effect have the power to produce that effect, or, in other words, the agent’s causal power must be proportional to that effect.
In the language of the Scholastics (such as St. Thomas Aquinas and Bl. Duns Scotus), a causal agent can be a univocal agent or an equivocal agent. As an example of a univocal agent, think of fire setting something on fire, heat of something causing heat in another, or an animal in generation generating offspring of its kind. In such examples of causation, the agent has what it gives to the effect formally. The hot thing formally has heat, and its heat causes the same thing (heat) in another. (NB. that, contrary to ideas of motion following Descartes, the heat which a cause imparts to an effect is not numerically the same. This is because heat is an accidental form, not a subsisting thing which passes from here to there).
An equivocal agent is an agent of a different kind to the effect. In this case, it is not a question of the agent formally having that which it causes. It has it, as the Scholastics would say, virtually. To have an effect virtually means having the power to produce that effect (virtus is a Latin term for ‘power’ or ‘strength’). This implies that the causal agent is, as such, more perfect than the effect. The greater cannot come from the lesser. The more perfect cannot come from the less perfect. Being cannot come from non-being. The agent’s causal power must be proportionate to the effect that it causes. As an analogy, consider that, if I have only five dominoes in my hand, I cannot pass you six dominoes from my hand to your hand. If something of lesser being acts and something of greater being, in the same respect, is produced, the excess greater being demands explanation and sufficient reason for its existence, which cannot be found in the agent of lesser being because being can only come from being and a certain degree of being can only come from a proportionate degree of being.
St. Thomas Aquinas often makes the point that a univocal agent cannot be the cause of their species. For example, no human being can be the cause of humanity because then he would be the cause of himself, which is absurd. A univocal agent can only be a cause of this instantiation of the species in this particular matter. A dog that generates another dog is the cause of the substantial form common to them both being in the particular matter of his offspring. However, no dog can be the cause of there being dogs at all. Neither can the essence of what it means to be a dog necessitate that there be dogs. Quite clearly, it is possible for dogs not the have existed. Dogs are contingent beings. This is what is meant by the distinction in created things between essentia (‘essence’ - that by which something is the sort of thing it is) and esse (the infinitive ‘to be’ - ie. the act of being or that by which a thing is at all). The essence of dogness is metaphysically distinct from the actual esse any dogs might have when they exist in the real world. The essence of dogness does not necessitate the esse of dogs. If neither any particular dog or the essence of dogness as such can give us a sufficient reason for the existence of dogs in reality, then we have to look elsewhere for the cause of there being dogs in reality at all. It will turn out that this cause must be none other than God, the only one capable of ex nihilo creation.
One could suggest that dogs are not really an irreducible and independent essence but are just modifications of a broader essence. Therefore, development among such modifications of this essence does not seem repugnant. For example, if cats and dogs are really the same basic sort of thing which just have accidental differences. This seems highly unlikely given the fact that they cannot interbreed and produce any offspring at all. However, this is secondary to more fundamental points. There must be some basic kind of thing, some irreducible essence, no matter where you identify that. Furthermore, the basic essence, within which there are just accidental modifications, cannot be material bodies in general, since there is a fundamental difference in kind between animate and inanimate bodies, between life and non-life. Neither can the basic essence be living things in general, since there is a fundamental and irreducible difference between merely vegetative life and animal life. Neither can it be animal life in general because, among animals, there are certain fundamental and irreducible differences and levels of perfection. For example, Aristotle says that the basest animals have only the sense of touch (the basest and most material of the senses), while the highest and noblest animals have all five exterior senses including sight (the greatest, noblest, most spiritual, and most immaterial of the senses) and also the four interior senses (such as imagination and the synthesising faculty known as the ‘common sense’ to the Scholastics), and consequently higher degrees of locomotion. A clam is fundamentally different from a dog.
Sight is a perfection which dogs possess. It is also a perfection which is irreducible. You cannot add together heat and dryness or touch and taste, for examples, and get sight. You either have sight or you do not. You may have more accurate sight than another. But there is no middle ground between having sight and not having sight. It is also a perfection which is a necessary property of dogs, flowing from their essence. Even though you may have a blind dog, this is by way of privation. Something had to impede the dog’s natural ability to see. A sign of this is that when a dog goes blind, it’s eyes do not suddenly disappear. No, it’s eyes are for seeing, and it is only a damaging cause that has deprived the dog of his natural ability. Furthermore, a blind dog can still beget a dog that does have sight, because sight follows from the substantial form of the dog, which it imparts to its offspring, and is only prevented by something positively impeding it.
Modern day evolutionists think that life came from non-life, that animal life came from non-animal life, that animals with sight came from what did not have sight, and finally that rational animals with immaterial intellect and will (human beings) came from irrational animals. It should be plain by now why this is absurd and contrary to the principle of proportionate causality. The lesser cannot give rise to the greater. The more perfect cannot come from the less perfect. A univocal agent that by nature does not have a certain non-accidental irreducible perfection cannot give rise to an effect which does have that perfection by nature. It is impossible for a plant to see unless it be transformed from a plant to a seeing animal. But that is what is called a miracle, not a process of nature. For neither is it the case that a plant can be the equivocal causal agent in generating a seeing animal. The cause is not proportional to the effect. Plants do not have animality or sight virtually or eminently. They are an essentially lower grade of being.
We have seen that a plant itself does not have the power to generate an animal. Now to make some clarifications about substantial change. Substantial change is certainly a reality but not between any and very substance by any and every causal agent. The elements are the most basic formations of prime matter. A created generator with the requisite power could substantially change elementary bodies from one element to another. Furthermore, a created generator could reduce the potencies elements have for combining and producing new compounds into act. Thus, we have compounds. A created agent can chemically combine hydrogen and oxygen to form water. Furthermore, a living organism in generation can generate another of its own kind. Prime matter of itself is capable of being informed by any possible form that is capable of informing matter. But the active potency of the agent who causes the form to be in particular matter must be proportionate to the effect. Any living being is restricted to generating one of its own particular kind. It’s causal power in generation is proportional to that precise effect.
Whether the question is accidental or substantial change, the causal agent must have the requisite power (or ‘active potency’) to reduce the relevant potency to the relevant act. In accidental change, the substance remains the same and the potency the substance has for the accidental form is reduced to act, so that the substance actually has that accidental form. All created agents can do in accidental change is reduce potencies that are already in the substance to act. The substance has to be intrinsically capable of receiving that accidental form. In substantial change, the persisting subject of the change is prime matter which is of itself indeterminate to any substantial form. As we have seen, the causal agent that determines it to receive a substantial form has to be a proportionate cause of that substantial form informing the matter.
What no created agent whatsoever can do is create things ex nihilo. This is confirmed by common sense, philosophical reason, and Catholic dogma. For any particular dog to be generated, a generating dog is necessary as the proportionate cause. But for dogs omnino et totaliter to come into existence, only God, and God immediately, could cause that. (Unless we said that dogness is not an irreducible essence but just a modification of a broader irreducible essence. In which case, we just substitute that irreducible essence, with the understanding, as established above, that this essence contain all the irreducible perfections dogs have essentially, such as consciousness and sight). If dogs did not come into existence ex nihilo in time, then the only other alternative is that they existed sempiternally. But, all the same, God must be the cause of there being dogs at all. The fact there are always dogs remains always insufficient to explain why there are always dogs. In this case God eternally causes there to always be dogs.
Could a higher form of being such as a human or an angel change substantially change something into a dog? Manifestly, they cannot do so by generation. But neither can they mould a lesser form of being into the higher form of being that is a dog. Souls, which are the substantial forms of living beings, are not just basic modifications of matter like inanimate things, but are more ‘formal’ than, for example, water and air, and can only be made to inform matter by way of generation by a like generator (or by ex nihilo creation by the author of all being). What powers do humans or angels have that they can mould matter in such a way as to cause souls to inform bodies? Manifestly, this is ridiculous. This is said in order to exhaust hypothetical explanations. It is not of course the view of Evolutionists that humans or angels gave rise to other life forms, but that lower beings of themselves progressively gave rise to higher and higher forms of being.
Finally, could God aid the process of evolution in order to fill the gap, as some ‘theistic evolutionists’ try to suggest? This is, in fact, just to change the explanation. The whole point of Darwinian Evolution is that lower beings develop by a materialistic reshuffling of elements into higher beings. To enter God in every time would be to replace the Darwinian process with God’s direct act at every necessary point. If God were brought in to aid the transition from non-animal life to animal life, what would that entail? Of course, animal souls organise and inform elements and compounds as composite material parts to sustain the animal’s body. And, as we know, these are replenished by processes of nutrition. These are, therefore, taken up as certain material parts of the animal’s substance. There is no reason why God, in creating a certain type of animal, could not use already existing matter in disposing the body for the soul to inform. But, since this animal is the first of its kind, the matter cannot receive the form by way of generation, which is the natural way. The change, therefore, surpasses the order of nature and so can be caused by the Divine Power alone. For the same reason, only the Divine Power can raise the dead to life. So, this explanation would amount to a rejection of Darwinian Evolution, just as the explanation of ex nihilo creation of different living things within six days is.
Only a dog can beget a particular dog by generation and be the cause of that form being in that particular matter. And only God can create a dog ex nihilo and cause there to be dogs in existence at all. The same goes for any essential kind of living being.