« PreviousContinue »
tion of the various forces opposed to life, in weeding out all those individuals which are least qualified to live, Mr. Darwin has given the well-known name of natural selection.1
Herbert Spencer, the distinguished English philosopher, calls the process “the survival of the fittest;" and either name well expresses the result of that action of natural forces on the living world which has been briefly summed up in the preceding part of this chapter. A few added words may help to make clear just what is this result.
In case many of the animals or plants of a region are destroyed, it is evident, that, in the great majority of cases (whatever the cause of the destruction), the killing-off will not be indiscriminate, but will first and mainly comprise those individuals which are least able to avoid the attack. For example, it has been found inexpedient in Belgium to raise white carrier-pigeons, because they are so much more likely than the darker sorts to be pounced upon by hawks. But, curiously enough, on the west coast of Ireland the sea-eagles pick out black hens from the flock; so that “the villagers, as much as possible, avoid rearing birds of that color.” 2
' It has been suggested that this term, natural selection, might well be restricted to the result of the competition between living organisms.
2 Darwin, Animals and Plants, etc., ii. p. 215.
And the same difference in liability to destruction from various causes is found among plants. Trees bearing purple plums are more exposed to the attacks of the black-knot than are those which bear green or yellow plums. Yellowfruited peach-trees are the ones which suffer most from the fungus producing the yellows on the leaves. And near Malaga, Spain, at the beginning of the attacks of the vine-disease, the green grapes were the most severely affected; " and red and black grapes, even when interwoven with the sick plants, suffered not at
And again : after some striking instances showing the very great difference which certain individuals of some species of bean show, when compared with others of the same species, in their power of resisting frost, Mr. Darwin says,
“It was impossible to behold these three plants, with their blackened, withered, and dead brethren all around, and not see at a glance that they differed widely in constitutional power of resisting frost.” ?
The dwarfish or misshapen sapling will be elbowed out of existence by its sturdier neighbor; the sick buffalo is the one that the wolves will drag down after the herd has passed; and of the drove of wild horses it is only the slowest and weakest that can be run down and caught. It has already been shown that variation, whether among animals or plants, is not the exception, but the rule; that not merely do slight variations occur, but sometimes very sudden and great ones, as in the instance of the axolotl and that of the brine-shrimps.
1 Darwin, Animals and 2 Ibid., ii. p. 300.
etc., ii. pp. 214, 215.
It is not pretended that natural selection is capable of giving rise to variations, but only that it acts in an extraordinarily powerful and certain way (from the immense scale on which it operates), in giving additional chances for life and reproduction to any desirable varieties that may be produced. Says Dr. Gray, in his “Darwiniana,"
“Natural selection is not the wind which propels the vessel, but the rudder, which by friction now on this side, and now on that - shapes the course. The rudder acts while the vessel is in motion, effects nothing when it is at rest. Variation answers to the wind.”
By the action of such causes as have been enumerated in the preceding chapter, countless variations must continually be produced, and many of these variations must be in favorable directions. Then, of course, those favored varieties which thus gain better means of offence and of defence; which can obtain food more readily, or live on less food; which reproduce more abundantly, or take better care of their young, - will be the fittest to survive, and will suurvive. One of two results will then follow: either the new species will displace some individuals of the parent-species, and so flourish side by side with the old form, or more commonly) the new species will at length entirely take the place of the old one. Only in very rare instances (that of the salamander, for example) can the new form live under such different conditions from its parent-species as not in any way to affect the numbers of the latter. And so, with much the same result as that which the farmer obtains by selecting his seedcorn, the gardener by thinning out his beds, or the cattle-raiser by selling off all his roughest calves for veal, Nature is at work, on an inconceivably great scale, thinning out the least perfect individuals of every species.
1 Mr. Darwin has been much criticised for personifying nature, and for speaking as though natural selection were a conscious matter. This he does not mean. No one who advocates his theory means thereby to imply the necessity of consciousness; and it is as allowable to speak of natural selection as to use the expression, “the lowling of the wind."
Now, just as business competition is most active in the largest cities, and as business methods are there brought to the highest pitch of perfection, so we find (and apparently from a similar cause) that in the great continents the animals are better fitted for the struggle for existence than they are in such a small continent as Australia, or than they are in any island far out at sea. Such wingless and defenceless birds as have been found in great numbers on some islands could have existed but a few days at most in the presence of such enemies as they would necessarily have encountered if placed on any continent. Birds like these, then, were not well adapted for varied conditions of life, and could never have existed over wide areas; but they form only an extreme instance of the general fact that the animals of islands or of small continents have not been so severely thinned out by all sorts of destructive causes as have those of the great land-masses. Perhaps the force of this argument will be more evident if it is illustrated by the well-known fact that improved breeds of cattle nearly always originate on farms where large herds are kept and bred; so that there is chance for a wide range of selection on the part of the stock-raiser. And the same is true of improved plants: they