One of the most important questions for every educated Catholic of today is: What is to be thought of the theory of evolution? Is it to be rejected as unfounded and inimical to Christianity, or is it to be accepted as an established theory altogether compatible with the principles of a Christian conception of the universe ?
We must carefully distinguish between the different meanings of the words theory of evolution in order to give a clear and correct answer to this question. We must distinguish (1) between the theory of evolution as a scientific hypothesis and as a philosophical speculation ; (2) between the theory of evolution as based on theistic principles and as based on a materialistic and atheistic foundation; (3) between the theory of evolution and Darwinism; (4) between the theory of evolution as applied to the vegetable and animal kingdoms and as applied to man.
As a scientific hypothesis, the theory of evolution seeks to determine the historical succession of the various species of plants and of animals on our earth, and, with the aid of palæ ontology and other sciences, such as comparative morphology, embryology, and bionomy, to show how in the course of the different geological epochs they gradually evolve from their beginnings by purely natural causes of specific development. The theory of evolution, then, as a scientific hypothesis, does not consider the present species of plants and of animals as forms directly created by God, but as the final result of an evolution from other species existing in former geological periods. Hence it is called "the theory of evolution", or "the theory of descent", since it implies the descent of the present from extinct species. This theory is opposed to the theory of constancy, which assumes the immutability of organic species. The scientific theory of evolution, therefore, does not concern itself with the origin of life. It merely inquires into the genetic relations of systematic species, genera, and families, and endeavours to arrange them according to natural series of descent (genetic trees).
How far is the theory of evolution based on observed facts? It is understood to be still only an hypothesis. The formation of new species is directly observed in but a few cases, and only with reference to such forms as are closely related to each other; for instance, the systematic species of the plant-genus Œnothera, and of the beetle-genus Dimarda. It is, however, not difficult to furnish an indirect proof of great probability for the genetic relation of many systematic species to each other and to fossil forms, as in the genetic development of the horse (Equidæ), of ammonites, and of many insects, especially of those that dwell as "guests" with ants and termites, and have adapted themselves in many ways to their hosts. Upon comparing the scientific proofs for the probability of the theory of evolution, we find that they grow the more numerous and weighty, the smaller the circle of forms under consideration, but become weaker and weaker, if we include a greater number of forms, such as are comprised in a class or in a sub-kingdom. There is, in fact, no evidence whatever for the common genetic descent of all plants and animals from a single primitive organism. Hence the greater number of botanists and zoologists regard a polygenetic (polyphyletic) evolution as much more acceptable than a monogenetic (monophyletic). At present, however, it is impossible to decide how many independent genetic series must be assumed in the animal and vegetable kingdoms. This is the gist of the theory of evolution as a scientific hypothesis. It is in perfect agreement with the Christian conception of the universe ; for Scripture does not tell us in what form the present species of plants and of animals were originally created by God. As early as 1877 Knabenbauer stated "that there is no objection, so far as faith is concerned, to assuming the descent of all plant and animal species from a few types" (Stimmen aus Maria Laach, XIII, p. 72).
Passing now to the theory of evolution as a philosophical speculation, the history of the plant and animal kingdoms upon our globe is but a small part of the history of the entire earth. Similarly, the geological development of our earth constitutes but a small part of the history of the solar system and of the universe. The theory of evolution as a philosophical conception considers the entire history of the cosmos as an harmonious development, brought about by natural laws. This conception is in agreement with the Christian view of the universe. God is the Creator of heaven and earth. If God produced the universe by a single creative act of His will, then its natural development by laws implanted in it by the Creator is to the greater glory of His Divine power and wisdom. St. Thomas says: "The potency of a cause is the greater, the more remote the effects to which it extends." (Summa c. Gent., III, c. lxxvi); and Francisco Suárez: " God does not interfere directly with the natural order, where secondary causes suffice to produce the intended effect" (De opere sex dierum, II, c. x, n. 13). In the light of this principle of the Christian interpretation of nature, the history of the animal and vegetable kingdoms on our planet is, as it were, a versicle in a volume of a million pages in which the natural development of the cosmos is described, and upon whose title-page is written: "In the beginning God created heaven and earth."
The theory of evolution just stated rests on a theistic foundation. In contradistinction to this is another theory resting on a materialistic and atheistic basis, the first principle of which is the denial of a personal Creator. This atheistic theory of evolution is ineffectual to account for the first beginning of the cosmos or for the law of its evolution, since it acknowledges neither creator nor lawgiver. Natural science, moreover, has proved that spontaneous generation–i.e. the independent genesis of a living being from non-living matter–contradicts the facts of observation. For this reason the theistic theory of evolution postulates an intervention on the part of the Creator in the production of the first organisms. When and how the first seeds of life were implanted in matter, we, indeed, do not know. The Christian theory of evolution also demands a creative act for the origin of the human soul, since the soul cannot have its origin in matter. The atheistic theory of evolution, on the contrary, rejects the assumption of a soul separate from matter, and thereby sinks into blank materialism.
Darwinism and the theory of evolution are by no means equivalent conceptions. The theory of evolution was propounded before Charles Darwin's time, by Lamarck (1809) and Geoffroy de Saint-Hilaire. Darwin, in 1859, gave it a new form by endeavouring to explain the origin of species by means of natural selection. According to this theory the breeding of new species depends on the survival of the fittest in the struggle for existence. The Darwinian theory of selection is Darwinism–adhering to the narrower, and accurate, sense of the word. As a theory, it is scientifically inadequate, since it does not account for the origin of attributes fitted to the purpose, which must be referred back to the interior, original causes of evolution. Haeckel, with other materialists, has enlarged this selection theory of Darwin's into a philosophical world-idea, by attempting to account for the whole evolution of the cosmos by means of the chance survival of the fittest. This theory is Darwinism in the secondary, and wider, sense of the word. It is that atheistical form of the theory of evolution which was shown above–under (2)–to be untenable. The third signification of the term Darwinism arose from the application of the theory of selection to man, which is likewise impossible of acceptance. In the fourth place, Darwinism frequently stands, in popular usage, for the theory of evolution in general. This use of the word rests on an evident confusion of ideas, and must therefore be set aside.
To what extent is the theory of evolution applicable to man ? That God should have made use of natural, evolutionary, original causes in the production of man's body, is per se not improbable, and was propounded by St. Augustine (see A UGUSTINE OF H IPPO, S AINT , under V. Augustinism in History ). The actual proofs of the descent of man's body from animals is, however, inadequate, especially in respect to paleontology. And the human soul could not have been derived through natural evolution from that of the brute, since it is of a spiritual nature ; for which reason we must refer its origin to a creative act on the part of God.
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