Part 4

Similarly with the first two objections. If the degree of adaptation has not increased during evolution, then it is clear that progress does not consist in increase in adaptation. But it does not follow that progress does not exist; it may quite well consist in an increase of other qualities. So with complexity. Complexity has increased, but increase in complexity is not progress, say the objectors. Granted: but may there not be something else which has increased besides mere complexity?

No; the remedy for all our difficulties, and indeed the only way in which we can arrive at the possibility of saying whether biological progress exists or no, is to adopt the positive method.

Let us then begin our survey of biological evolution in the endeavour to find whether or no progress is visible there. To start with, we must be clear what are the sources of our knowledge on the subject.

Direct observation of progressive evolution has, of course, not yet been possible in the period--biologically negligible--in which man has directed his attention to the problem; and historical record is also absent. The best available evidence is that of paleontology: here the relative positions of the layers of the earth’s crust enable us to deduce their temporal sequence--and naturally, that of the organisms whose fossil remains they embalm--with a great deal of accuracy.

We can scarcely ever observe the direct transition from the forms of life in an older to those in a younger stratum, nor can we absolutely prove their genetic relationship. But in a vast number of cases it is abundantly clear that the later type of organization is descended from the former--that a group of forms in the younger stratum had its origin in one or more species of the group to which the forms in the older stratum belong. Sometimes, however, as in many groups of mammals, the gaps are few and small, the seriation almost complete. In any event we have here evidence which, so far as it goes, is perfectly admissible for the main lines and for many of the smaller branches of evolutionary descent.

Unfortunately, it does not go very far--or, we had better say, it is of restricted application. By the time we find well-preserved fossils in the rocks, the main groups of the animal kingdom and their chief subdivisions had been already differentiated, with the one important exception of the vertebrates; while time, heat, and pressure have so modified the earlier strata as to destroy the fossil forefathers of insects, molluscs, crustacea, and the rest, which they must have contained.

Within the vertebrate stock, then, we can learn a great deal from the semi-direct methods of paleontology: but for the history of the other groups and for their origin and interrelations, we are driven back upon comparative anatomy and embryology, into another field of more circumstantial evidence. When, for instance, we find that the fore-limbs of bat, bird, whale, horse, and man, although so different in function and in detail of structure, are yet built upon the same general plan, and upon a plan wholly different from that of the limbs, say, of a spider or an insect, we must either deny reason and say that this similarity means nothing; or assume that its cause is supernatural, outside the province of science, that it is the expression of some eternal Idea, or some plan of a personal creator (in which case, be it noted, the idea or the plan often appears to our intellect as unreasonable and indeed stupid); or finally that it implies community of origin with later divergence of development. When we are dealing with the smaller sub-divisions of some larger group, this method too gives us information of the same order of accuracy as does paleontology: but when we try to understand the relationships of these larger groups, then we are forced to renounce any claim to detailed knowledge. In broad outline, however, a great deal still remains, and this broad outline we can employ for our valuation of the whole sweep of biological progress, just as we can use the greater accuracy of vertebrate paleontology and comparative morphology to fill in the detail within a restricted field of its operation. From these various evidences, direct and indirect, we can paint for ourselves a picture of the evolution of life which, in spite of inevitable gaps and errors, is in its main features adequate and true.

Let us not be misled by the fact that disputes can and justifiably do arise over details: as Professor Bateson put it recently:--

“If the broad lines do not hold, then we must sink into irrationality or turn to flagrant supernaturalism.”

Let us then remind ourselves of some of these broad lines.

We know that there was a time when the earth, hot and fiery, could not have been the abode of life. Of the first origins of life we know nothing and guess little. What we can justifiably surmise is that the protoplasm of the original organisms was not yet differentiated into cytoplasm and nucleus, and that sexuality had not yet arisen. The bacteria, however specialized in other ways, are still in this primitive condition.

Later, we can with great probability infer that the independent units into which the stuff of life was subdivided reached a size which, though still minute, was at least not beyond or even close to the limits of microscopic vision; they were further provided with a nucleus, and occasionally underwent sexual fusion. In other words, they showed an organization which we call cellular; they were free-living cells. Such unicellular creatures must have been at one epoch sole inhabitants of the earth, and diverged into the most manifold types of structure and modes of life. Such of them as led an animal as opposed to a plant type of existence would be classified under the Protozoa or unicellular animals.

The colonial habit gives advantages of increased size and greater rapidity of motion, of which many Protozoa have availed themselves. A colonial existence once attained, division of labour, at first between the germinal and the somatic, later between different types of somatic units, will be a further advantage. Such organisms, of which we cannot say definitely whether they are compound aggregates or single wholes, would represent the most natural link between the unicellular Protozoan and the rest of the animal kingdom, the multicellular forms or Metazoa. And indeed such organisms exist at the present day--organisms such as Volvox, Zoothamnium, Proterospongia, and Myxidium--as adjuvant and confirmatory of our reasonable faith.

The multicellular organisms appear to have originated twice over, by divergent routes. There are the true Metazoa, to which belong all the higher types, and the Parazoa or sponges, which have never passed beyond a very primitive type of structure. Both start as simple sacs, whose walls are formed from two primary sheets or layers of cells. Leaving sponges out of account, the Hydroid polyps are the simplest representative of this grade of structure, while some of the Jelly-fish and Siphonophores have attained the utmost limit of its inherent possibilities.

The next great step was the intercalation of a third primary layer between the other two. The result of this, the so-called triploblastic type of organization, gives the ground-plan for all subsequent organizations; and later evolution consists mainly in the evolution of this ground-plan.

In other words, we can now pass from the consideration of the general plan of life’s architecture to that of its details. During the next great tract of time, that which was novel in life (for we must not be guilty of a petitio principii in yet speaking of “advance” or “progress”) was brought about in two main ways--by an increase in the size of organisms, and by an increase in the efficiency of their working.

The simplest Metazoa, such as the polyps, as well as the simplest three-layered forms, such as the free-living flat-worms, are all small, composed of an amount of material comparable with that contained in a single one of our hairs. In every group of Metazoa, increase of size is one of the main features that accompanies specialization, and the more specialized groups possess a higher average size than the less.

A jelly-fish against a polyp; a cuttle-fish against a primitive mollusc; a vertebrate against its chordate ancestor; the giant reptiles of the late secondary period against their forbears; a horse against Phenacodus; man against the earliest primates--over and over again does size increase with the march of time.

Not only this, but when there occurs aggregation of individuals to form units of a higher order, as in bees and ants and termites, and in man himself, there too increase of size in the new units thus produced is one of the most notable features. Is not human history in large measure the history of the increase in size of social units?

But size alone is not enough; there is also a definite improvement of the details of life’s mechanism--partly revealed as improvement in the efficiency of the parts themselves, partly in the adjustment of the parts to each other, and their subordination to the needs of the whole.

It is scarcely necessary to detail the improvements in efficiency of different organs during evolution: such are universally familiar. But a few examples will point my moral. The lowest three-layered forms have no circulatory system; this, rendered necessary later by increase of size, shows a gradual differentiation of parts in evolution. The exquisite machinery of our heart is directly descended from a minute pulsating ventral vessel such as that seen in Amphioxus. Protection and support are better cared for in insect than in worm, in mammal than in lamprey. But the most spectacular improvement of function, the most important of all the directional movements in evolution has been that affecting the nervous system and the sense-organs associated with it. Few people who have not gone carefully into the subject realize how imprisoned and windowless are the existences led by lower forms of life.

Even such physically well-organized creatures as Crustacea stand at an amazingly low mental level. The other day I was reading a careful account of experiments on the behaviour of crabs. The method by which the sexes recognize each other is so crude that I am not sure whether it deserves the term recognition at all. Before mating, which takes place immediately after a moult, the female is carried about for some time in the claws of the male. The mature males will attempt to lift up and carry off any members of the same species, male or female: but the only ones which will permit themselves to be thus carried about are females just before moulting. Hence by a general instinct to lift any members of the same species on the part of the males, and on the part of the females an instinct to allow themselves to be lifted when in the physiological condition which precedes moulting, the required end is brought about. But of any mental operation such as is involved in sex-recognition in man or any other mammal, there is no evidence.

Fish, to take another example, possess associative memory; they can learn. But they learn very slowly, and learn only the simplest things. The jump from their powers of memory to those of a dog, who can be trained comparatively quickly to carry out complicated tricks, is as great as the further jump from the powers of a dog to those of a man capable of learning a page of print by heart in two or three readings.

The first organs connected with mind to become elaborated are the organs of sense: but such receptor organs are useless to their possessor, however elaborate, unless put into relation with proper effector organs--organs for action, whether locomotor or secretory. So that the first steps are the elaboration of sense-organs, the increase of efficiency of muscles and glands, and, equally essential, the construction of an improved “adjustor system,” whereby the stimulus falling on the sense-organ may be translated into action and into the right kind of action. This adjustor mechanism is the central nervous system. Most of the further history of organisms may be summed up in one phrase--the evolution of adjustor mechanisms.

At first, it is chiefly of importance to be brought into relation with more and more of the happenings of the outer world, to be able to see and hear and feel and smell more and more delicately; and to react upon the outer world more and more efficiently and powerfully, to be able to move and to handle matter more quickly and with finer and finer adjustment.

But unless the adjustor mechanism be improved, this process soon tends to a limit. I may illustrate my meaning by a simple supposition. Suppose an organism capable of very little beyond reflexes and instincts and with but a scanty dose of associative power: of what conceivable use to it would be a telescope or a telephone? Man obtains a biological advantage from such accessory sense-organs in that, when thus apprised of events at a distance, he is enabled to plan out courses of action to meet the events which he imagines are going to overtake him: but both planning and imagination are entirely functions of an adjustor mechanism, and without such a mechanism, great enlargement of sensory power would only result in an organism reacting too often and unnecessarily to events in its environment.

There is, in fact, an obvious limit to the perfection which can be attained by receptor and effector organs. Striated muscles, the modelling of the skeleton and joints for speed in a horse or greyhound, the eye and ear of higher vertebrates, the mammalian sense of smell--no doubt it would be possible for life to have produced more perfect and more efficient mechanisms--but not, apparently, mechanisms much more perfect or much more efficient. They stand near the limit of biological efficiency.

There thus comes a time when it is impossible or extremely difficult to give an organism advantage in the struggle by improving its sense-organs or its locomotor system, or indeed any of its general physical construction, whereas it is still possible to confer the most important advantages upon it by means of improvements in the adjustor mechanism, improvements which involve and imply improvements of mind.