tion. Of another class of entozoa, belonging to the sub- division Trematoda, there are five lands found in different organs of the human body — the liver and gall ducts, the portal vein, tlic intestine, the bladder, the eye. Then we have the Trichina spiralis^ which passes through one phase of its existence imbedded in the muscles and through another phase of its existence in the intestine; and which, by the induced disease Tiichiniasis, has lately committed such ra- vages in Germany, as to cause a panic. And to these we must add the Guinea-worm, which in some part of Africa and India, makes men miserable by burrowing in their legs. From this list of entozoa, which is by no means complete, let us pass to the epizoa. There are two kinds of Acariy one of them inhabiting the follicles of the skin, and the other producing the itch. There are other creatures that bury themselves beneath the skin, and lay their eggs there; and there are three species of lice which infest the surface of the body. Nor is this all: besides animal parasites, there are sundry vegetal parasites, which grow and multiply at our cost. The Sarcina ventricidi inhabits the stomach, and produces gastric disturbance. The LGptotlirix huccalis is extremely general in the mouth, and may have something to do with the decay of teeth. And besides these, there are microscopic fungi which produce ringworm, porrigo, pityri- asis, thrush, &c. Thus the human body is the habitat of parasites, internal and external, animal and ve- getal, numbering, if all were set down, some two or three dozen species; sundry of which are peculiar to man, and many of which produce in man great suffering and not un- frequently death. What interpretation is to be put on these facts by those who espouse the hypothesis of special crea- tions? According to this hypothesis, all these parasites were designed with a view to their respective modes of life. They were endowed with constitutions fitting them to live by absorbing the juices of the human body; they were fur- nished with appliances, often of a formidable kind, enabling them to root themselves in and upon the human body; and they were made prolific in an almost incredible degree, that their germs might have a sufiicient number of chances of 344 THE EVOLUTION OF LIFE.
finding tlieir way into the human body. In short, elaborate contrivances were combined to insure the continuance of their respective races; and to make it impossible for the suc- cessive generations of men to avoid being preyed upon by them. What shall we say to this arrangement? Shall we say that man, " the head and crown of things," was provided as a habitat for these parasites? Or shall we say that these degraded creatures, incapable of thought or enjojrment, were created that they might cause unhappiness to man? One or other of these alternatives must be chosen by those who contend that every kind of organism was separately devised by the Creator. Which do they prefer? With the concep- tion of two antagonistic powers, which severally work good and evil in the world, the facts are congruous enough. But with the conception of a supreme beneficence, this gratuitous infliction of misery on man, in common with all other terres- trial creatures capable of feeling, is absolutely incomj)atible.
^ 115. See then the results of our examination. The belief in special creations of organisms, is a belief that arose among men during the era of profoundest darkness; and it belongs to a family of beliefs which have nearly all died out as enlightenment has increased. It is without a solitary established fact on which to stand; and when the attempt is made to put it into definite shape in the mind, it turns out to be only a pseud-idea. This mere verbal hypothesis, which men idly accept as a real or thinkable hypothesis, is of the same nature as would be one, based on a day's observation of human life, that each man and woman was specially created — an hypothesis not suggested by evidence, but by lack of evidence — an hypothesis which formulates absolute ignorance into a semblance of positive knowledge. Further, we see that this hypothesis, wholly without support, essentially inconceiv- able, and thus failing to satisfy men's intellectual need of an interpretation, fails also to satisfy their moral sentiment. It is quite inconsistent with those conceptions of the divine nature wliicli tliey profess to entertain. If infinite power was to be demonstrated, then, either by the special creation of every individual, or by the production of species after a method akin to that in which individuals are produced, it would be better demonstrated than by the use of the two methods which the hypothesis assumes to be necessary. And if infinite goodness was to be demonstrated, then, not only do the provisions of organic structure, if they are especially devised, fail to demonstrate it; but there is an enormous mass of them which imply malevolence rather than bene- volence.
Thus, however regarded, the hypothesis of special creations turns out to be worthless — worthless by its derivation; worthless in its intrinsic incoherence; worthless as absolutely without evidence; worthless as not supplying an intellectual need; worthless as not satisfying a moral want. We must therefore consider it as counting for nothing, in opposition to any other hypothesis respecting the origin of organic beings.
CHAPTER III.
GENERAL ASPECTS OF THE EVOLUTION-HYPOTHESIS.
§ 116. Just as the supposition tliat races of organisms have been specially created, is discredited by its origin; so, conversely, tbe supposition tliat races of organisms have been evolved, is credited by its origin. Instead of being a conception suggested and accepted wben mankind were profoundly ignorant, it is a conception born in times of com- parative enlightenment. Moreover, the belief that all organic forms have arisen in conformity with uniform laws, instead of through breaches of uniform laws, is a belief that has come into existence in the most-instructed class, living in these better-instructed times. ]^ot among those who have paid no attention to the order of IN^ature, has this idea made its appearance; but among those whose pursuits have famil- iarized them with the order of Nature. Thus the derivation of this modern hypothesis is as favourable as that of the ancient hypothesis is unfavourable.
§ 117. A kindred antithesis exists between the two fami- lies of beliefs, to which the beliefs we are comparing severally belong. While the one family has been dying out, the other family has been multiplying. Just as fast as men have ceased to regard different classes of phenomena as caused by special personal agents, acting irregularly; so fast have they come to regard these dijfferent classes of phe- nomena as caused by a general agency acting uniformly — the two changes being correlative. And as, on the one hand, the hypothesis that each species resulted from a supernatural act, having lost nearly all its kindred hypotheses, may be expected soon to become extinct; so, on the other hand, the hypothesis that each species resulted from the action of na- tural causes, being one of an ever-increasing family of hypo- theses, may be expected to survive and become established.
Still greater will the probability of its survival and estab- lishment appear, when we observe that it is one of a particu- lar genus of hypotheses that has been rapidly extending. The interpretation of phenomena as resulting from Evolution, has been independently showing itself in various fields of inquiry, quite remote from one another. The supposition that the Solar System has been gradually evolved out of dif- fused matter, is a supposition wholly astronomical in its origin and application. Geologists, without being led thereto by astronomical considerations, have been step by step ad- vancing towards the conviction, that the Earth has reached its present varied structure through a process of Evolution. The inquiries of biologists have proved the falsity of the once general belief, that the germ of each organism is a minute repetition of the mature organism, differing from it only in bulk; and they have shown, contrariwise, that every organ- ism, arising out of apparently-uniform matter, advances to its ultimate multiformity through insensible changes. Among philosophical politicians, there has been spreading the per- ception that the progress of society is an evolution: the truth that " constitutions are not made but grow,'' is a part of the more general truth that societies are not made but grow. It is now universally admitted by philologists, that languages, instead of being artificially or supernaturally formed, have been developed. And the histories of religion, of philosophy, of science, of the fine arts, and of the indus- trial arts, show that these have passed through stages as un- obtrusive as those through which the mind of a child passes on its way to maturity. If; then, the recognition of evoluo48 THE EVOLUTION OF LIFE.
tion as the law of many diverse orders of phenomena, has been spreading; may we not say that there thence arises the probability that evolution will presently be recognized as the law of the phenomena we are considering? Each further ad- vance of knowledge, confirms the belief in the unity of IsTature; and the discovery that evolution has gone on, or is going on, in so many departments of Nature, becomes a rea- son for believing that there is no department of Nature in which it does not go on.
§ 118. The hypotheses of Special Creation and Evolution, are no less contrasted in respect of their legitimacy as hy- potheses. ^Vliile, as we have seen, the one belongs to that order of symbolic conceptions which are proved to be illusive by the impossibility of realizing them in thought; the other is one of those symbolic conceptions which are more or less completely realizable in thought. The production of all organic forms by the slow accumulation of modifications upon modifications, and by the sloiy divergences resulting from the continual addition of differences to difierences, is mentally representable in outline, if not in detail. Various orders of our experiences enable us to conceive the process. Let us look at one of the simplest.
There is no apparent similarity between a straight line and a circle. The one is a curve; the other is defined as without curvature. The one encloses a space; the other will not enclose a space though produced for ever. The one is finite; the other may be infinite. Yet, opposite as the two are in all their properties, they may be connected together by a series of lines no one of which difiers from the adjacent ones in any appreciable degree. Thus, if a cone be cut by a plane at right angles to its axis, we get a circle. If, instead of being perfectly at right angles, the plane subtends with the axis an angle of 89° 59', we have an ellipse wdiich no human eye, even when aided by an accurate pair of compasses, can distinguish from a circle. Decreasing the angle minute J by minute, the ellipse becomes first perceptibly eccentric, then manifestly so, and by and by acquires so immensely elongated a form, as to bear no recognizable resemblance to a circle. By continuing this process, the ellipse changes insensibly into a parabola. On still further diminishing the angle, the para- bola becomes an hyperbola. And finally, if the cone be made gradually more obtuse, the hyperbola passes into a straight line, as the angle of the cone approaches 180°. Now here we have five different species of line — circle, ellipse, parabola, hyperbola, and straight line — each having its pecu- liar properties and its separate equation, and the first and last of which are quite opposite in nature, connected together as members of one series, all producible by a single process of insensible modification.
But the experiences which most clearly illustrate to us the process of general evolution, are our experiences of special evolution, repeated in every plant and animal. Each organism exhibits, within a short space of time, a series of changes which, when supposed to occupy a period inde- finitely great, and to go on in various ways instead of one way, give us a tolerably clear conception of organic evo- lution in general. In an individual development, vfe have compressed into a comparatively infinitesimal space, a series of metamorphoses equally vast with those which the hypo- thesis of evolution asstmies to have taken place during those immeasurable epochs that the Earth^s crust tells us of. A tree difiers from a seed immeasurably in every respect — in bulk, in structure, in colour, in form, in specific gravity, in chemical composition: difiers so greatly that no visible resemblance of an}^ kind can be pointed out between them. Yet is the one changed in the course of a few years into the other: changed so gradually, that at no moment can it be said — Now the seed ceases to be, and the tree exists. What can be more mdely contrasted than a newly-born child and the small, semi-transparent, gelatinous spherule constituting the human ovum? The infant is so complex in structure 350 THE EVOLUTION OF LIFE.
that a cyclopaedia is needed to describe its constituent parts. The germinal vesicle is so simple that it may be defined in a line. Nevertheless, a few months suffice to develope the one out of the other; and that, too, by a series of modifica- tions so small, that were the embryo examined at successive minutes, even a microscope would with difficulty disclose any sensible changes. Aided by such facts, the conception of general evolution may be rendered as definite a concep- tion as any of our complex conceptions can be rendered. If instead of the successive minutes of a child's foetal life, we take successive generations of creatures — if we regard the suc- cessive generations as difiering from each other no more than the foetus did in successive minutes; our imaginations must indeed be feeble if we fail to realize in thought, the evolu- tion of the most complex organism out of the simplest. If a single cell, under appropriate conditions, becomes a man in the space of a few years; there can surely be no difficulty in understanding how, under appropriate conditions, a cell may, in the course of untold millions of years, give origin to the human race.
It is true that many minds are so unfurnished with those experiences of Nature out of which this conception is built, that they find difficulty in forming it. Habitually looking at things rather in their statical than in their dynamical aspects, they never realize the fact that, by small increments of modification, any amount of modification may in time be generated. That surprise which they feel on finding one whom they last saw as a boy, grown into a man, becomes incredulity when the degree of change is greater. To such, the hypothesis that by any series of changes a protozoon should ever give origin to a mammal, seems grotesque — as grotesque as did Galileo's assertion of the Earth's movement seem to the Aristotleans; or as grotesque as the assertion of the Earth's sphericity seems now to the New Zealanders. But those who accept a literally-unthinkable proposition as quite satisfactory, may not unnaturally be expected to make a converse mistake.
§ 119. The hj^othesis of evolution is contrasted with, the hj^pothesis of special creations, in a further respect. It is not simply legitimate instead of illegitimate, because repre- sentable in thought instead of unrepresentable; but it has the support of some evidence, instead of being absolutely unsupported by evidence. Though the facts at present as- signable in direct proof that by progressive modifications, races of organisms that are apparently distinct may result from antecedent races, are not sufiicient; yet there are nu- merous facts of the order required. It has been shown beyond all question that unlikenesses of structure gradually arise among descendants from the same stock. We find that there is going on a modifying process of the kind alleged as the source of specific difierences: a process which, though slow in its action, does, in time, if the circumstances demand it, produce conspicuous changes — a process which, to all appearance, would produce in the millions of years, and under the great varieties of conditions which geological records imply, any amount of change.
In the chapters on "Heredity'' and '' Variation, " con- tained in the preceding Part, many such facts were given; and plenty more might be added. Although comparatively little attention has been paid to the matter until recent times, the evidence already collected shows that there take place in successive generations, alterations of structure quite as marked as those which, in successive short intervals, arise in a developing embryo — nay, often much more marked; since, besides difierences due to changes in the relative sizes of parts, there sometimes arise difierences due to additions and suppressions of parts. The structural modification proved to have taken place since organisms have been observed, is not less than the hypothesis demands — ^bears as great a ratio 352 THE EVOLUTION OP^ LIFE.
to this brief period, as the total amount of structural change seen in the evolution of a complex organism out of a simple germ, bears to that vast period during which living forms have existed on the Earth.
We have, indeed, much the same kind and quantity of direct evidence that all organic beings have gradually arisen through the actions of natural causes, which we have that all the structural complexities of the Earth's crust have arisen through the actions of natural causes. It may, I think, be fairly said, that between the known modifications undergone by organisms, and the totality of modifications displayed in their structures, there is no greater disproportion than between the geological changes which have been witnessed, and the to- tality of geological changes supposed to be similarly caused. Here and there are pointed out sedimentary deposits now slowly taking place. At this place, it is proved that a shore has been encroached on by the sea to a considerable extent within recorded times; and at another place, an estuary is known to have become shallower within the space of some generations. In one region a general upheaval is going on at the rate of a few feet in a century; while in another region occasional earthquakes are shown to cause slight variations of level. Appreciable amounts of denudation by water are visible in some localities; and in other localities glaciers are detected in the act of grinding down the rocky surfaces over which they glide. But the changes thus instanced, are infinitesimal compared with the aggregate of changes to which the Earth's crust testifies, even in its still extant sys- tems of strata. If, then, from the small changes now being wrought on the Earth's crust by natural agencies, we may legitimately conclude that by such natural agencies acting through vast epochs, all the structural complexities of the Earth's crust have been produced; may we not from the small known modifications produced in races of organisms by natural agencies, similarly infer that from natural agencles have slowly arisen all those structural complexities which we see in them?
The hypothesis of Evolution then, has direct support from facts which, though small in amount, are of the kind required; and the proportion which these facts bear to the conclusion drawn, seems as great as is the proportion between facts and conclusion which, in another case, produces acceptance of the conclusion.
§ 120. Let us put ourselves for a moment in the position of those who, from their experiences of himian modes of action, draw inferences respecting the mode of action of that ultimate power manifested to us through phenomena. We shall find the supposition that each kind of organism was separately designed and put together, to be much less consistent with their professed conception of this ultimate power, than is the supposition that all kinds of organisms have resulted from one unbroken process. Irregularity of method is a mark of weakness. Uniformity of method is a mark of strength. Con- tinual interposition to alter a pre-arranged set of actions, implies defective arrangement in those actions. The main- tenance of those actions, and the working out by them of the highest results, implies completeness of arrangement. If human workmen, whose machines as at first constructed require perpetual adjustment, show their increasing skill by making their machines self-adjusting; then, those who figure to themselves the production of the world and its inhabitants by a '^ Great Artificer," must admit that the achievement of this end by a persistent process, adapted to all contingencies, implies greater skill than its achievement by the process of meeting the contingencies as they severally arise.
So, too, it is with the contrast under its moral aspect. We saw that to the hypothesis of special creations, a difficulty is presented by the absence of high forms of life during those immeasurable epochs of the Earth's existence which geology 23 354 THE EVOLUTIOIs^ OF LIFE.
records. But to the hypothesis of evolution, this absence is no such obstacle. Suppose evolution, and this question is necessarily excluded. Suppose special creations, and this question, unavoidably raised, can have no satisfactory an- swer. Still more marked is this contrast between the two hypotheses, in presence of that vast amount of suf- fering entailed on all orders of sentient beings, by their imperfect adaptations to their conditions of life; and the further vast amount of suffering entailed on them by enemies and by parasites. We saw that if organisms were severally designed for their respective places in ]N^ature, the inevitable conclusion is, that these thousands of kinds of inferior organ- isms which prey upon superior organisms, were intended to inflict all the pain and mortality which results. But the hy- pothesis of evolution involves us in no such dilemma. Slowly, but surely, evolution brings about an increasing amount of happiness: all evils being but incidental. By its essen- tial nature, the process must everywhere produce greater fitness to the conditions of existence; be they what they may. Applying alike to the lowest and the highest forms of organ- ization, there is in all cases a progressive adaptation; and a survival of the most adapted. If, in the uniform worldng out of the process, there are evolved organisms of low types, which prey on those of higher types, the evils inflicted form but a deduction from the average benefits. The universal and necessary tendency towards supremacy and multiplica- tion of the best, apptying to the organic creation as a whole as well as to each species, is ever diminishing the damage done — tends ever to maintain those most superior organisms which, in one way or other, escape the invasions of the infe- rior, and so tends to produce a type less liable to the inva- sions of the inferior. Thus the evils accompanying evolu- tion are ever being self- eliminated. Though there may arise the question — Why could they not have been avoided? there does not arise the question — Why were they deliberately inflicted? Whatever may be tlioiiglit of them, it is clear that they do not imply gratuitous malevolence.
§ 121. In all respects, then, the hypothesis of evolution contrasts favourably with the hypothesis of special creation. It has arisen in comparatively-instructed times, and in the most cultivated class. It is one of those beliefs in the uni- form concurrence of phenomena, which are gradually sup- planting beliefs in their irregular and arbitrary concurrence; and it belongs to a genus of these beliefs which has of late been rapidly spreading. It is a definitely-conceivable hypo- thesis: being simply an extension to the organic world at large, of a conception built from our experiences of individual organisms; just as the hypothesis of universal gravitation, was an extension of the conception which our experiences of terrestrial gravitation had produced. This definitely- con- ceivable hypothesis, besides the support of numerous ana- logies, has the support of direct evidence: we have positive proof that there is going on a process of the kind alleged; and though the results of this process, as actually witnessed, are minute in comparison with the totality of results ascribed to it, yet they bear to such totality, a ratio as great as that by which an analogous hypothesis is justified. Lastly, that senti- ment which the doctrine of special creations is thought necessary to satisf}^, is much better satisfied by the doctrine of evolu- tion; since this doctrine raises no contradictory implications respecting the Unknown Cause, such as are raised by the antagonist doctrine.
And now, haidng observed how, under its most general aspects, the hypothesis of evolution commends itself to us, by its derivation, by its coherence, by its analogies, by its direct evidence, by its implications; let us go on to consider the several orders of facts which yield indirect support to it. We will begin by noting the harmonies that exist between it, and sundry of the inductions set forth in Part II.
CHAPTER lY.
THE ARGUMENTS FROM CLASSIFICATION, § 122. In § 103, we saw that the relations which exist among the species, genera, orders, and classes of organisms, are not interpretable as results of any such causes as have been usually assigned. We will here consider whether they are interpretable as the results of evolution. Let us first contemplate some familiar facts.