Volume I Part 11 (1/2)

This, then, was the earliest hypothesis touching the causes of organic evolution. But we may at once perceive that it is insufficient to explain all that stands to be explained. In the first place, it refers in chief part only to the higher animals, which are actuated to effort by intelligence. Its explanatory power in the case of most invertebrata--as well as in that of all plants--is extremely limited, inasmuch as these organisms can never be moved to a greater or less use of their several parts by any discriminating volition, such as that which leads to the continued straining of a giraffe's neck for the purpose of reaching foliage. In the second place, even among the higher animals there are numberless tissues and organs which unquestionably present a high degree of adaptive evolution, but which nevertheless cannot be supposed to have fallen within the influence of Lamarckian principles. Of such are the sh.e.l.ls of crustacea, tortoises, &c., which although undoubtedly of great use to the animals presenting them, cannot ever have been _used_ in the sense required by Lamarck's hypothesis, i.

e. actively exercised, so as to increase a flow of nutrition to the part. Lastly, in the third place, the validity of Lamarck's hypothesis in any case whatsoever has of late years become a matter of serious question, as will be fully shown and discussed in the next volume.

Meanwhile it is enough to observe that, on account of all these reasons, the theory of Lamarck, even if it be supposed to present any truth at all, is clearly insufficient as a full or complete theory of organic evolution.

In historical order the next theory that was arrived at was the theory of natural selection, simultaneously published by Darwin and Wallace on July 1st, 1858.

If we may estimate the importance of an idea by the change of thought which it effects, this idea of natural selection is unquestionably the most important idea that has ever been conceived by the mind of man.

Yet the wonder is that it should not have been hit upon long before. Or rather, I should say, the wonder is that its immense and immeasurable importance should not have been previously recognised. For, since the publication of this idea by Darwin and Wallace, it has been found that its main features had already occurred to at least two other minds--namely, Dr. Wells in 1813, and Mr. Patrick Matthew in 1831. But neither of these writers perceived that in the few scattered sentences which they had written upon the subject they had struck the key-note of organic nature, and resolved one of the princ.i.p.al chords of the universe. Still more remarkable is the fact that Mr. Herbert Spencer--notwithstanding his great powers of abstract thought and his great devotion of those powers to the theory of evolution, when as yet this theory was scorned by science--still more remarkable, I say, is the fact that Mr. Herbert Spencer should have missed what now appears so obvious an idea. But most remarkable of all is the fact that Dr.

Whewell, with all his stores of information on the history of the inductive sciences, and with all his ac.u.men on the matter of scientific method, should not only have conceived the idea of natural selection, but expressly stated it as a logically possible explanation of the origin of species, and yet have so stated it merely for the purpose of dismissing it with contempt[26]. This, I think, is most remarkable, because it serves to prove how very far men's minds at that time must have been from entertaining, as in any way antecedently probable, the doctrine of trans.m.u.tation. In order to show this I will here quote one pa.s.sage from the writings of Whewell, and another from a distinguished French naturalist referred to by him.

[26] For quotations, see Note A.

In 1846 Whewell wrote:--

Not only is the doctrine of the trans.m.u.tation of species in itself disproved by the best physiological reasonings, but the additional a.s.sumptions which are requisite to enable its advocates to apply it to the explanation of the geological and other phenomena of the earth, are altogether gratuitous and fantastical[27].

[27] whewell, _indications of the creator_, 2nd ed., 1846.

Then he quotes with approval the following opinion:--

Against this hypothesis, which, up to the present time, I regard as purely gratuitous, and likely to turn geologists out of the sound and excellent road in which they now are, I willingly raise my voice, with the most absolute conviction of being in the right[28].

[28] de blainville, _compte rendu_, 1837.

And, after displaying the proof rendered by Lyell of uniformitarianism in geology, and cordially subscribing thereto, Whewell adds:--

We are led by our reasonings to this view, that the present order of things was commenced by an act of creative power entirely different to any agency which has been exerted since. None of the influences which have modified the present races of animals and plants since they were placed in their habitations on the earth's surface can have had any efficacy in producing them at first. We are necessarily driven to a.s.sume, as the beginning of the present cycle of organic nature, an event not included in the course of nature[29].

[29] Whewell, _ibid._, p. 162.

So much, then, for the state of the most enlightened and representative opinions on the question of evolution before the publication of Darwin's work; and so much, likewise, for the only reasonable suggestions as to the causes of evolution which up to that time had been put forward, even by those few individuals who entertained any belief in evolution as a fact. It was the theory of natural selection that changed all this, and created a revolution in the thought of our time, the magnitude of which in many of its far-reaching consequences we are not even yet in a position to appreciate; but the action of which has already wrought a transformation in general philosophy, as well as in the more special science of biology, that is without a parallel in the history of mankind.

Although every one is now more or less well acquainted with the theory of natural selection, it is necessary, for the sake of completeness, that I should state the theory; and I will do so in full detail.

It is a matter of observable fact that all plants and animals are perpetually engaged in what Darwin calls a ”struggle for existence.”

That is to say, in every generation of every species a great many more individuals are born than can possibly survive; so that there is in consequence a perpetual battle for life going on among all the const.i.tuent individuals of any given generation. Now, in this struggle for existence, which individuals will be victorious and live? a.s.suredly those which are best fitted to live, in whatever respect, or respects, their superiority of fitness may consist. Hence it follows that Nature, so to speak, _selects_ the best individuals out of each generation to live. And not only so; but as these favoured individuals transmit their favourable qualities to their offspring, according to the fixed laws of heredity, it further follows that the individuals composing each successive generation have a general tendency to be better suited to their surroundings than were their forefathers. And this follows, not merely because in every generation it is only the ”flower of the flock”

that is allowed to breed, but also because, if in any generation some new and beneficial qualities happen to arise as slight variations from the ancestral type, they will (other things permitting) be seized upon by natural selection, and, being transmitted by heredity to subsequent generations, will be added to the previously existing type. Thus the best idea of the whole process will be gained by comparing it with the closely a.n.a.logous process whereby gardeners, fanciers, and cattle-breeders create their wonderful productions; for just as these men, by always ”_selecting_” their best individuals to breed from, slowly but continuously improve their stock, so Nature, by a similar process of ”_selection_” slowly but continuously makes the various species of plants and animals better and better suited to the conditions of their life.

Now, if this process of continuously adapting organisms to their environment takes place in nature at all, there is no reason why we should set any limits on the extent to which it is able to go, up to the point at which a complete and perfect adaptation is achieved. Therefore we might suppose that all species would eventually reach this condition of perfect harmony with their environment, and then remain fixed. And so, according to the theory, they would, if the environment were itself unchanging. But forasmuch as the environment (i. e. the sum total of the external conditions of life) of almost every organic type alters more or less from century to century--whether from astronomical, geological, and geographical changes, or from the immigrations and emigrations of other species living on contiguous areas, and so on--it follows that the process of natural selection need never reach a terminal phase. And forasmuch as natural selection may thus continue, _ad infinitum_, slowly to alter a specific type in adaptation to a gradually changing environment, if in any case the alteration thus effected is sufficient in amount to lead naturalists to name the result as a distinct species, it follows that natural selection has trans.m.u.ted one specific type into another. Similarly, by a continuation of the process, specific types would become trans.m.u.ted into generic, generic into family types, and so on. Thus the process is supposed to go on throughout all the countless forms of life continuously and simultaneously--the world of organic types being thus regarded as in a state of perpetual, though gradual, flux.

Now, the first thing we have to notice about this theory is, that in all its main elements it is merely a statement of observable facts. It is an observable fact that in all species of plants and animals a very much larger number of individuals are born than can possibly survive. Thus, for example, it has been calculated that if the progeny of a single pair of elephants--which are the slowest breeding of animals--were all allowed to reach maturity and propagate, in 750 years there would be living 19,000,000 descendants. Again, in the case of vegetables, if a species of annual plant produces only two seeds a year, if these in successive years were all allowed to reproduce their kind, in twenty years there would be 11,000,000 plants from a single ancestor. Yet we know that nearly all animals and plants produce many more young at a time than in either of these two supposed cases. Indeed, as individuals of many kinds of plants, and not a few kinds of animals, produce every year several thousand young, we may make a rough estimate and say, that over organic nature as a whole probably not one in a thousand young are allowed to survive to the age of reproduction. How tremendous, therefore, must be the struggle for existence! It is thought a terrible thing in battle when one half the whole number of combatants perish. But what are we to think of a battle for life where only one in a thousand survives?