It is generally agreed to trace the Primates back to an active pioneer animal form which took to the trees, and which arose out of the widely-spread Insectivores. This deriva­tion will probably satisfy any reasonable genealogist. But, if we may use a parallel in human families, this active animal was as different from its congeners as Napoleon was from his four brothers who played a part in European history, and it is not necessary to say more as to the significance of this fact than that the relative importance of “chassis” and “body” is again a useful analogy. But we need to ask what those congeners did if we are to succeed in understanding the Napoleon-like course of him who became our Primate ancestor. From the original widely-spread and plastic raw material of the Insectivores allied forms took different lines, and their stories are written at great length in one small and the three other great orders of Bats, Carnivores, Ungulates, and Rodents. As it has been pointed out, Carnivores took to attacking larger prey, including their less fortunate relatives, and stepped into the arena as carnivorous animals; the Ungulates-to-be became herbivorous and developed into two great groups of hoofed animals, relying mainly on flight for safety; Rodents took to burrows for defence, ceased to trouble much about attack, and became gnawing animals; Bats adopted an aerial life—a poor form of it indeed like that of the aeroplane—and acquired a degraded fore-limb. Before leaving these great orders of animals, whom I do not desire to compare unfavourably with poor Louis, Jerome, Joseph or Lucien Bonaparte, it is convenient here to refer to a fact which comes to light immediately one looks into such a piece of classifica­tion as this of the orders arising out of the loins of the early Insectivores, and that is the functional concep­tion underlying it. Doubtless pure functional “characters” could never supply a whole system of classifica­tion in the light of the modern doctrine of descent with modifica­tion, and of zoological affinities. This is shown in a change from division of six orders of Birds-Running, Swimming, Wading, Climbing, Predatory and Perching Birds, to that of a few old-fashioned Ratite Birds, and all the rest, one which seems the best that can be offered at present.

Insects, Mollusca, Birds.

The grouping of animals by structural characters, and by affinities which are assumed, though based on almost undeniable evidence, whether into species, families, classes, phyla or sub-phyla, has its apotheosis in Mollusca and Insects. As to the second of these immense groups it has always seemed strange that their colourings and structural characters should have received such intensive study from Weismann to the exclusion of Mollusca, when he set out to prove his stupendous negative, and still more that of Vertebrates, among which his chief difficulty and desired triumph would seem to have lain. Mollusca though invertebrate are held by many to be in the line of ancestry of the highest forms of life, and at any rate insects are not. They are most fruitful fields indeed in which Nature has been able to show what she could do by her stern selective powers, but, from the point of view of descent with modifica­tion, may be fairly compared to a review of an army in time of peace, or the Kriegspiel of a German military staff. He who concerns himself with the fundamental difficulties of the problems at issue in evolution must make his notes of what experts tell him of such groups as those of Insects, Mollusca and Birds, and pass on to the higher forms in which on the one hand function becomes the predominant partner, and on the other individual experience becomes more and more important. He feels indeed at liberty to wish the entomologist and ornithologist all success, and to leave him at peace, in his siding, to pursue his delightful and interminable studies far from the dust and din of controversy.

Insectivores.

The critical territory of vertebrate, and still more of Mammalian forms, in which the genealogist pictures the five main groups of Insectivores, looking about them, if one may so speak, in the world around and pondering which of many paths they shall pursue, resembles certain centres that may be seen in towns where three, four, five, or seven different roads are open to the traveller, each with its incalculable effects on his ultimate career. If one may change here the metaphor it may be said that the Insectivores are the watershed of the Five Rivers of higher life. However much the wayfaring insect-feeders have diverged from this broad centre in structure, and however much the laws of genetics have widened this divergence, the facts of function stare one in the face when such descriptions of three of the four orders outside the Primate stock are pondered—Flesh-feeders, Herbivorous animals, Burrowers and Gnawers. These time-honoured names appealed strongly to older zoologists, and in them is implicit a large body of evidence for initiative in their evolution by pioneering work on the part of their ancestors. Though in these days Prototheria include Monotremes, One-vent animals, Metatheria, Marsupials or pouched animals, and Eutheria Insect-feeders, and though Mammals derive their indispensable name from the function by which they feed their young, the most severe of systematists cannot clear his mind from the old leaven of function in all these terms. They imply momentous potentialities prior to new structures, and the modern fails to ban entirely such functional names. I believe there is here no juggling with names and words on my part, but a stone in the founda­tion of the unambitious building which I am seeking to rear. It is ultimately connected with a directive power as well as the formation of sensori-motor arcs in the central nervous system.

Is it possible or probable that the factors which led some group to the water alone, some to a life in water and on land at different parts of their lives, some to a crawling life on land and partly in water, some to the air and trees, some to nocturnal, some to hybernating, some to burrowing life, some to a diet of flesh, some to one of plants, some to the trees alone, some to the trees and land, some to the land by night and trees by day, and some for ever and wholly to the land—is it probable that any process of selection of suited structures with countless ages of trial and error, could have determined these changes of habit and habitat? At least one may claim that the balance of probabilities is heavily against that view, and that the forging of reflex-arcs, with all it means to the career of an individual, affords a more intelligible hypothesis, and that this is strongly supported by modern discoveries and doctrines arising from the work of physiologists, as will appear later.

The Place of the Nervous System in Evolution.

The constitu­tion of the nervous system is conditioned by conduc­tion, its fundamental and primary function. Its processes consist in the transmission of impulses from receptive fields to effective reactions through devious paths in a region which, even to-day, is a jungle, with many further secrets for physiology to reveal. From this point of view the nervous system may be looked at as a clearing-house and storehouse of impulses on their way in, on their way through, and on their way out. If so, the making of new reflex-arcs is a process which has gone on simultaneously with the formation of receptors in the skin, the higher sense-organs and such deep structures as muscles, and that of effectors of infinite variety—and these are called conveniently adaptations. When we hear from Professor Sherrington that the afferent fibres with their private paths which enter the spinal cord outnumber three times those which leave it, and that those of the cranial nerves should be added, so that the afferent fibres may be reckoned as five times more numerous than the efferent, we get a vivid idea of the fundamental importance of the formation and compounding of reflex-arcs into systems. Without that the most sensitive receptors and the widest range of structures and organs, small and great, would be as nothing and things of naught.

A neurone is the anatomical, as the reflex-arc is the functional unit of a central nervous system. Just as it is profitless to consider apart the engines and body of a motor car, as working machine, so is it to picture neurones and reflex-arcs separately in the living nervous system except for the purpose of an ideal construc­tion. In common with the organs and structures of higher animals they have to pass, as historical structures, through the stages of initia­tion, repeti­tion of rudimentary function, and selection by trial and error, till the “canalizing force of habit” issues in rudimentary and increasingly efficient effectors. It is in this final stage where the triumphs of selection have been won, and where their undeniable value and interest has led some exponents of the distributional laws of genetics to disregard, or accept as data, the early and formative stages. Theirs is a mental state which resembles that of Darwin, who, for once in a moment of haste, declared the question of the origin of life to be rubbish.

In the foregoing considera­tion of the formation of receptors of the skin it was assumed that certain common stimuli of the environment hammer out for themselves paths in the nerve-fibrils of the skin and by ceaseless repeti­tion lay down not only the receptor, which may be called the terminus a quo, but also the afferent fibres which ultimately find their way into the grey matter of the cord and brain. That this is the initial stage of the construc­tion of the higher nervous system can hardly be denied. But it carries the problem of the synthesis of the organism but a little way unless it be coincident with the construc­tion of new reflex-arcs and their co-ordina­tion into systems. Till this stage be reached in a rudimentary form the most cunning and exact adaptations and structures, or, as they may be broadly called effectors, will not advance the efficiency of the organism in the smallest degree. If the receptor be the terminus a quo the effector is the terminus ad quem. This is so obvious that it may be waved aside as a truism not worth the notice of a zoologist concerned with the major problems of biology. It may seem to challenge in a highly speculative region and manner the labours of the biometrician and Mendelian, but, if fairly met it no more encroaches on their territory than do the labours of the engineers who invented the first and crudest chassis of a motor car upon the elaborate and brilliant ingenuity, taste and skill of the coachbuilders who turn out the “body” of a sumptuous Rolls-Royce of 1920. But the latter would never have “arrived” if the former had not made his slow and arduous trials and errors and final success. So here, as in many other subjects, a truism has its use. If the biometrician and Mendelian will only abstain from erecting notice-boards to proclaim “No thoroughfare here,” we shall not be put down as trespassers or poachers on their ground and may range at large in certain fields of specula­tion.