Darwin’s book upon the subject was called The Origin of Species. It was a very modest and sufficient title. He did not even go to the length of calling it the origin of genera or orders or classes. He did not at first apply it to man.
This is the theory of the origin of species through Natural Selection. It was not pretended by either of these pioneers that Natural Selection was the sole way through which the differences of species came about. For example, Darwin devoted a considerable part of his working life to such collateral modes of differentiation as the hypothesis that Sexual Selection also had its share. Criticism has whittled down that share to practically negligible proportions, but I note the hypothesis here because it absolutely disposes of the assertion which Mr. Belloc hammers on the table, that the Theory of Natural Selection excludes any other modes of specific differentiation.
Testing the Theory
Very rapidly this conception of Natural Selection was extended by naturalists until it came to be regarded as the general process of life. They came to realise that all species, all genera, all classes of life, whatever else may be happening to them, are and always have been varying through the process of Natural Selection, some rapidly, some slowly; some so slowly as hardly to change at all through vast ages. I have stated the a priori case by which, given birth and death and individuality and changing conditions and sufficient time, it appears logically inevitable that the change and differentiation of species must occur, and must be now going on. If we had no material evidence at all it would still be possible to infer the evolution of species.
That a priori case has never been answered, and it seems to me unanswerable. But scientific men, with their obstinate preference for observation and experiment over mere logical gymnastics, rarely rest their convictions on a priori cases. A sustaining scepticism is a matter of conscience with them. To them an a priori case is merely a theory—that is to say, a generalisation under trial. For nearly three-quarters of a century, therefore, biologists have been examining whatever instances they could discover that seemed to contradict this assumption that the process of specific change under Natural Selection is the general condition of life. To this day this view is still called the Theory of Natural Selection, though to a great number it has come to have the substantial quality of an embracing fact.
It would have been amusing if Mr. Belloc had told us more of his ideas of the scientific world. Apparently he knows scarcely anything of museums or laboratories or the spirit and methods of research. And manifestly he has not the faintest suspicion of the way in which the whole world of vital phenomena has been ransacked and scrutinised to test, correct, supplement, amplify, or alter this great generalisation about life. He probably shares the delusion of most other men in the street, that scientific theories are scientific finalities, that they are supposed to be as ultimate as the dogmas of some infallible religion. He imagines them put over chiefly by asseveration, just as the assertions of a polemical journalist are put over. He has still to learn that theories are trial material, testing targets, directives for research. Shooting at established theories is the normal occupation of the scientific investigator. Mr. Belloc’s figure of the scientific investigator is probably a queer, frowsty, and often, alas! atheistical individual, poking about almost aimlessly among facts in the hope of hitting upon some “discovery” or “getting rid of a God.” He does not understand the tense relevance of the vast amount of work in progress. But for three-quarters of a century the thought and work of myriads of people round and about the world have borne directly or almost directly upon the probing, sounding, testing, of the theory of Natural Selection. It stands clarified and, it would seem, impregnable to-day.
Some Irrelevant Questions
Among questions bearing upon it but not directly attacking it has been the discussion of the individual difference. For example, are differences due to individual experiences ever inherited? Or are only inherent differences transmissible? What rôle is played by what one might call “normal,” relatively slight differences, and what by the “sports” and abnormal births in specific change? Do species under stress, and feeding on strange food or living in unaccustomed climates, betray any exceptional tendency to produce abnormality? Have there been, so to speak, storms and riots of variation in some cases? Can differences establish themselves while outer necessity remains neutral? Can variations amounting to specific differences in colour and form arise as a sort of play of the germ plasm and be tolerated rather than selected by nature? In what manner do normal differences arise? What happens to differences in cases of hybridisation? Here are sample questions that have been the seeds of splendid work and great arguments. Some of them were already under discussion in Darwin’s time; he was a pioneer in such explorations; many ideas of his have stood the test of time, and many suggestions he threw out have been disproved. When some casual “may be” of Darwin’s is examined and set aside, it is the custom of polemical journalists to rush about and proclaim to all who may be sufficiently ill-informed to listen that Darwin is “exploded.” Such explosions of Darwin are constantly recurring like gun-fire near a garrison town, and still he remains. None of these subsidiary questions affect the stability of this main generalisation of biology, the Theory of Natural Selection.
The actual attack and testing of the Theory of Natural Selection have yielded negative results. The statement of the theory may have been made finer and exacter, that is all. And yet the conditions of its survival have been very exacting. If the theory is to stand, the whole of plant and animal life in time and space must be arranged in a certain order. It must be possible to replace classification by a genealogical tree. Every form must fall without difficulty into its proper place in that tree. If it is true that birds are descended from reptiles or men from apes, then there must be no birds before the reptiles appear, and no men before apes. The geological record is manifestly a mere fragmentary history, still for the most part unread, but, however fragmentary it is, it must be consistent. One human skull in the coal measures blows the whole theory to atoms. The passage from form to form must be explicable by intermediate types capable of maintaining themselves; there may be gaps in the record, but there must be no miraculous leaps in the story. If an animal living in the air is to be considered as a lineal descendant of some animal living in the water, then the structure of the former bit by bit and step by step must be shown to be adapted, modified, changed about from that of the latter; it must have ears for water-hearing modified for air-hearing, and its heart and breathing arrangements must be shown to be similarly changed over, and so on for all its structure. All these requirements will follow naturally from the necessities of a process of Natural Selection. They follow logically upon no other hypothesis. They are not demanded, for example, by the idea of a Creator continually interfering with and rectifying some stately, unaccountable process of “Evolution,” which seems to be Mr. Belloc’s idea—so far as he ventures to display any idea of his own—in the matter. Such things as vestigial structures and a number of odd clumsinesses in living things—many still very imperfect adaptations to an erect position, for example—become grotesque in relation to such a view. A Creator who put needless or inconvenient fish structures into the anatomy of a land animal and made the whole fauna and flora of the land a patch-up of aquatic forms of life must be not so much a Divinity as a Pedant. But it is the burthen of the whole beautiful science of comparative anatomy that the structure of animals and plants, and their succession in time, fall exactly into the conditions defined by the Theory of Natural Selection. In the most lovely and intricate detail, in a vast multitude of examples, in plants and in animals alike, this theme of the adaptation of pre-existing structure is worked out.
We should in accordance with the Theory of Natural Selection expect to find traces of the ancestral form, not only in the lay-out of the adult animal, but in every phase of its life history, and that, in fact, is just what we do find. There is no more fascinating branch of comparative anatomy than embryology. Each life cycle we discuss tends to repeat the ancestral story, and only under the stress of necessity does it undergo modification at any point. There is little toleration in the life process for unnecessary divergencies. Economies are effected by short cuts and reductions, and special fœtal structures are granted reluctantly. So that even in man we find peeping through the adaptations imposed upon the human type by its viviparous necessities, and in spite of the advantage of every economy of force, memories, for example, of the gill slits, of the fish heart and kidney, of the reptilian skull, of the mammalian tail. I mention this fact in the Outline, and upon it Mr. Belloc comments in a manner that leaves one’s doubts poised between his honesty and his intelligence. He declares, which is totally untrue, that I “repeat the old Victorian tag”—I doubt if there ever was such a tag—that the embryo “climbs up the family tree.” He puts these words in inverted commas as though I have really adopted and used them, and for the life of me it is only by straining my charity to the utmost that I can accept that this was an accident. Of course every text-book of embryology for the last forty years has made it perfectly plain, as I have stated here, that the life cycle can be and is modified at any point, and that an embryo has much more serious work in hand than reciting its family history. It betrays its ancestral origins to analysis; but that is an altogether different matter. Mr. Belloc, however, is so densely ignorant himself upon these questions that he can imagine, or think it worth while to pretend to imagine and attempt to persuade his readers by the expedient of these inverted commas, that I entertain such a view. And then follow this, which I quote that the reader may the better understand a certain occasional acerbity in my allusions to Mr. Belloc:—