Part 34

We shall best understand the probable cause of Natural Selection by taking a country undergoing some physical change, as of climate for example. The proportional number of its inhabitants would almost immediately undergo a change, and some of its species might become extinct. From the complex and very intimate manner in which the inhabitants of each country are bound together, we may conclude that any change in the numerical proportion of some of its inhabitants, independently of the change of climate itself, would seriously affect the others. Were the country open on its borders, new forms would certainly immigrate, and this, too, would often seriously disturb the relations of some of its former inhabitants. In the case, however, of an island, or a country hemmed in by barriers, into which new and better-adapted forms could not readily enter, we would then meet with places in the economy of nature which would assuredly be better filled up, if some of the original occupants were in some manner modified, for had the area been open to immigration, these same places would have been seized by intruders. Thus, slight modifications, which any way favored the individuals of a species, would by better adapting them to changed conditions tend to become preserved, and Natural Selection would there have free scope for the work of improvement. Changes in the conditions of life cause or excite a tendency to vary. In the foregoing case the conditions are supposed to have changed, and this would manifestly be favorable, by giving a better chance of profitable variations occurring, to Natural Selection, for unless such do occur, Natural Selection can do nothing. As man, by adding up in any given direction individual differences, can certainly produce a great result with his domestic animals and plants, so could Natural Selection, but far more easily from having an incomparably longer time for its action. No great physical change, as of climate, nor any unusual degree of isolation to check immigration, is actually necessary, it would seem, to produce new and unoccupied places for Natural Selection to fill up by modifying and improving some of the varying inhabitants, for as all the inhabitants of a country are struggling together with nicely-balanced forces, extremely-slight modifications in the structure or habits of one species would often give it an advantage over others; and still further modifications, so long as the species continued under the same conditions of life and profited by similar means of subsistence and defence, would often still further augment the advantage. No country can be mentioned whose native inhabitants are now so perfectly adapted to each other and to their environment that none could be better adapted and improved, for in all countries the natives have been so far conquered by naturalized productions as to have allowed them to take firm possession of the land. And as foreigners have thus in every country beaten some of the natives, it may be safely concluded that the latter might have been modified with profit so as to have better resisted the intruders.

A man by his methodical and unconscious means of selection can produce and has produced great results. What may not Natural Selection effect? Man can only operate on external and visible characters, but nature cares nothing for appearances, except in so far as they are beneficial to any being. She can act on every internal organ, on every shade of constitutional difference and, in fine, on the entire machinery of life. Man selects exclusively for his own advantage, but nature solely for that of the being she tends, and under her judicious selection the slightest difference of structure or constitution may well turn the nicely-balanced scale in the Struggle for Existence, and thus be preserved. As fleeting as are the wishes and efforts of man, and as short as is his earthly career, so poor, therefore, must be the results which he accomplishes when compared with those accumulated by nature during whole geological periods. Is it a wonder, then, that her productions should be far _truer_ in character than man’s, and that they should be infinitely better adapted to the most complex conditions of life and should bear the stamp of far higher workmanship? Metaphorically speaking, Natural Selection may be said to be daily and hourly scrutinizing, throughout the world, the slightest variations, rejecting the bad, preserving and adding up the good, and silently and insensibly working, whenever and wherever opportunities occur, at the betterment of each organic being in relation to its organic and inorganic conditions of life. So slow is her work that we see nothing of the changes in progress, and only when the hand of time has marked the lapse of ages do we perceive that changes have been produced; but then so imperfect is our view into long-past geological periods, that we see only that the forms of life are now different from what they formerly were. That any great amount of modification in any point should be effected, a variety once formed must again, perhaps after a long interval of time, present individual differences of the same favorable character, and these must again be preserved, and so onward step by step. As individual differences of all kinds perpetually recur, this can hardly be considered as an unwarrantable assumption. Judged by the extent the hypothesis accords with and explains the general phenomena of nature, notwithstanding the ordinary belief that the amount of possible variation is a strictly-limited quantity, we are justified, it seems to us, in assuming that all this has actually taken place. But in looking at many small points of difference between species, which in our ignorance seem quite unimportant, we must not lose sight of the facts that climate, food and modes of life may have produced some direct effect, and also of the truth that, owing to the Law of Correlation, when one part varies, and the variations are accumulated through the Survival of the Fittest, other modifications often of the most unlooked-for nature will ensue.

As under domestication these variations are known to appear at a particular period of life, and tend to reappear in the offspring at the same period, so, in a state of nature, it is reasonable to infer that Natural Selection will be enabled to act on and modify organic beings at any age, by the accumulation of variations useful at that age, and by their inheritance at a corresponding age. Thus, if it be profitable to a plant to have its seeds more and more widely disseminated by the wind, there can be no greater difficulty in conceiving this to be effected through Natural Selection than in conceiving the increasing and improving of the down in the pods on his cotton-trees by a wise selection upon the part of a cotton-planter. Natural Selection may modify and adapt the larva of an insect to a score of contingencies, wholly different from those which affect the mature insect, and these modifications through Correlation may work changes in the structure of the adult. On the other hand, modifications of the adult may affect the structure of the larva, but in all such cases Natural Selection will insure that these changes shall not be injurious, for, if they were so, the extinction of the species would be the inevitable result. Thousands of instances might be given to show the influence which Natural Selection, or Sexual Selection, which is only a less vigorous phase of the former, has had all through the ages in the adaptation of life to the places in nature which it was intended to occupy in pursuance of the plan formulated by the Great Originator and Designer of the Universe.

Despite the imperfection of the geological record, which has been urged as a serious objection to the theory of descent with modification, sensible, intelligent, educated men no longer doubt that species have all changed, and that they have changed in the way required, for they have changed slowly and in a graduated manner. This is clearly seen in the fossil remains from consecutive formations being invariably much more closely allied to each other than are those from widely-separated formations. It is true geological research does not yield those infinitely fine gradations between past and present species which the theory of Natural Selection requires, but when it is remembered that only a small portion of the world has been geologically explored; that only organic beings of certain classes, at least in any great number, can be preserved in a fossil condition; that many species when once formed never undergo any further change, but become extinct without leaving any modified descendants; that dominant and widely-ranging species vary the most and the most frequently, and that varieties are often at first only local, it is not at all surprising that the discovery of intermediate links to any considerable extent should not have been made. Local varieties, as is well known, will not diffuse themselves into other and distant localities until they have become very much modified and improved, and when they have thus diffused themselves, and are discovered in a geological formation, they will appear as if suddenly created there, and will simply be ranked as new species. Besides, formations have often been intermittent in their accumulation, and their duration has probably been shorter than the average duration of specific forms. And as successive formations in most cases are separated from each other by blank intervals of time of considerable length, and as fossiliferous formations thick enough to withstand future degradation can as a general rule be accumulated only where much sediment is laid down in the subsiding bed of the ocean, it follows that during the alternate periods of elevation and of stationary level the record will generally be blank or devoid of fossil remains. During these latter periods there will doubtless be more variability in the forms of life, and during the periods of subsidence a greater amount of extinction. Now, as geology plainly declares that each land has undergone great physical changes, we have a right to expect that organic beings have varied under nature in the same manner as they have varied under domestication, and such have scientific study and research found to be the case. And if there has been any variability under nature, such a fact would seem unaccountable unless Natural Selection, or the Survival of the Fittest, did not come into play. Upon the view that variations have occurred in nature and have been preserved and accumulated by Natural Selection, and not in the ordinary view of independent creation, we can understand why the specific characters, or those by which the species of the same genus differ from each other, should be more variable than the generic characters in which they all agree. Inexplicable as is the occasional appearance of stripes on the shoulders and legs of the different equine species and their hybrids on the theory of creation, yet how simply is the fact explained if we believe that they are all descended from a striped progenitor just as the different domestic breeds of pigeons are descended from the blue and barred rock-pigeons. Why, for example, should the color of a flower be more likely to vary in any one species of a genus, if the other species, supposed to have been created independently, have differently-colored flowers, than if all the species of the genus have the same colored flowers? On the theory that species are only well-marked varieties, of which the characters have become in a high degree permanent, the fact is intelligible, for they have already varied in certain characters since they branched off from a common progenitor, and by these characters they have come to be specifically distinct from each other. Therefore, these same characters would be more likely again to vary than the generic characters which have been inherited without change for an enormous period of time.

Upon the theory of Natural Selection, or the Survival of the Fittest, with its contingencies of extinction and divergence of character, we can see how it is that all past and present organic beings can be arranged within a few classes, in groups subordinate to groups, and with the extinct groups often falling in between the recent groups. We can see how it is that the mutual affinities of the forms within each class are so complex and diversified, and only adaptive characters, though of superior importance to the beings, are of scarcely any significance in classification, while those derived from rudimentary parts, though of no recognized service, are often of high classificatory value, and only embryological characters are frequently the most valuable of all. The real affinities of all organisms, in contradistinction to their adaptive likenesses, are due to inheritance or community of descent. Hence, a natural system of classification is a genealogical arrangement, with the acquired grades of difference, denoted by varieties, species, genera, families, etc., and their lines of descent have to be discovered by the most permanent characters, whatever they may be and how little of vital importance they may possess.

That species are immutable productions, which was until quite recently the current belief by laymen and naturalists, was almost unavoidable so long as the world was considered to be of short duration. But now that some idea has been acquired of the time that has elapsed since the beginning of earth-life, we are too apt to assume, without proof, that the geologic record is so complete, that it would have afforded us some plain evidence of the mutation of species, if they had undergone mutation. But the principal cause of our unwillingness to admit that one species has given birth to other and distinct species, is that we are always slow in admitting any great change of which we do not discern the intermediate steps. Just such a difficulty was felt by many geologists when Lyell first insisted that long lines of inland cliffs had been produced, and great valleys excavated, by the agencies which are still at work in the earth. No effort of mind can adequately grasp the meaning of even ten million of years, nor add up and perceive the full effects of the many slight variations to which species have been subjected during an almost infinite number of generations. The day, however, is not distant, when mankind will have become just as thoroughly convinced that species have been modified during a long course of descent, mainly through the Natural Selection of innumerous successive, slight and favorable variations as they are that the attraction of gravitation is an important element in the maintenance of the harmony that exists among the planetary spheres. That the law of the attraction of gravity, which is perhaps the greatest discovery ever made by man, is subversive of natural and revealed religion, which was at one time maintained by a no more distinguished person than Leibnitz, is now no longer objected to, even though its discoverer was unable to explain what is the essence of the principle he had discovered. No nobler conception of Deity could be entertained than that which attributes to Him the creation of a few original forms capable of self-development into other and needful forms, or the origination _de novo_ of these simple forms from inorganic nature. It places a higher estimate upon His Omnipotence than the belief that He required a fresh act of creation to supply the voids caused by the action of His laws. That science is as yet unable to throw any light on the far higher problem of the essence or origin of life, should constitute no valid objection to the theory of descent.

When all beings are looked upon not as special creations, but as the lineal descendants of some beings that existed long before the first bed of ancient Siluria was deposited, they seem to become ennobled. Judging from the past, we think it safe to conclude that no existing species will transmit its unaltered likeness to a distant futurity. Few, very few living species will transmit progeny of any kind, for the manner in which all organisms are grouped shows that the majority of species in each genus, and all the species in many genera, have left no descendants, but have become utterly extinct. It will only be the common and widely-spread species, belonging to the larger and dominant groups within each class, that will ultimately prevail and procreate new and dominant species. Since all the living forms of life are the lineal descendants of forms that lived long anterior to the Silurian epoch, it is reasonably certain that the ordinary succession by generation has never once been broken, and that no cataclysmic disaster has laid waste the entire world. Therefore, we may look into the future with some confidence of an equally secure and inappreciably enduring earth-life. And as Natural Selection operates solely by and for the good of each being, all corporeal and mental endowments will tend to progress toward perfection.

When we contemplate a tangled bank, with innumerable plants of diverse kinds, and many-voiced birds singing in concert, or waging destruction on manifold insects that are flitting about, or the long, slimy worm that has come up from its underground retreat, we are lost in wonder and admiration, and can only reflect that these elaborately constructed forms, so different from each other, and so strangely and intricately dependent on each other, have all been evolved by laws that act all around us. These are the laws of Growth with Reproduction; Inheritance, which is almost implied by reproduction; Variability from the action, direct and indirect, of the conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Existence, and as a consequence to Natural Selection, or Survival of the Fittest, entailing thereby Divergence of Character and the Extinction of less-improved forms. And thus, from the war of nature, and from famine and death, have arisen the higher mammalia, in which man, the _summa summarum_ of life, is included. He occupies the summit, toward which the efforts of millions of buried ages seem to have been tending. There is a grandeur in this view of life, with its several powers, originally breathed, by the operation of the natural laws, into one or a few forms of life, and that, while the earth, in obedience to the fixed principle of gravitation, has gone cycling on, endless forms, most beautiful and most wonderful, have been, and are being, evolved from so simple a beginning.