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IMPROVEMENT THROUGH SEXUAL SELECTION.
breeding season, both with their teeth and claws, their hides being often covered with scars.
Very little is known about the courtship of quadrupeds in a state of nature, but it is certain that domesticated animals often show strong individual preferences and antipathies; and it seems probable that, in a state of nature, the females are allured or excited by particular males who possess certain characters in a higher degree than other males. Any such characters would thus have the advantage, being selected for perpetuation, either methodically or unconsciously, and in the course of many generations would tend to accumulate in particular directions; that is to say, would tend to become more marked or conspicuous.
In the same manner as man can give beauty, according to his standard of taste, to his male poultry,-can give to the Sebright bantam a new and elegant plumage, an erect and peculiar carriage, so it appears that in a state of nature female birds, by having long selected the more attractive males, have added to their beauty. In the same manner as man can improve the breed of his game-cocks by the selection of those birds which are victorious in the cockpit, so it appears that the strongest and most vigorous males, or those provided with the best weapons, have prevailed under nature, and have led to the improvement of the natural breed or species. Through repeated deadly contests a slight degree of variability, if it led to some advantage, however slight, would suffice for the work of sexual selection; and it is certain that secondary sexual characters are eminently variable.
The Accumulation of Variations by Natural Selection (or the Struggle for Existence).-The cases of selection
thus far noticed-or rather the classes of cases-are, after all, only the fewer and less important of all those that occur. Man's selection, conscious and unconscious, so far as we have yet considered it, operates only with regard to the domestic races; sexual selection, as seen in the choice of mates and the law of battle, is of more importance, but operates chiefly at particular seasons. But living things have also to maintain a more constant struggle, against other individuals of the same species who may be pugnacious; against other species which prey upon them or require the same kind of food; against the elements and the seasons, which may also deprive them of food, or hurt them by extremes of temperature, &c., &c. Those who survive in this struggle for existence do so because of some fitness in their organization or habits; and since any variations which render one individual more fit than another, in however slight a degree, are thus as it were selected, apart from man's interference, this process of nature is called Natural Selection. The expression of course is figurative, as it is not pretended that there is any choice exercised or volition shown in the inanimate elements, or that the animals concerned do their part in any more than a blind way; yet still it is sufficiently like that process which has been called man's unconscious selection, to justify the name which it has received, and is legitimate in the same way that it is legitimate for chemists to speak of elective affinity. Natural Selection, in a large sense, includes Sexual Selection, but it has been found convenient to treat of the two separately.
The Law of Multiplication.-All organized beings have enormous powers of multiplication. Even man, who increases more slowly than all other animals, could,
RAPIDITY OF MULTIPLICATION.
under the most favourable circumstances, double his numbers every fifteen years, or increase a hundred-fold in a century. Many animals could increase their numbers from ten to a thousand-fold every year; and if unchecked would soon require all the world to themselves. A calculation made by Professor Huxley1 shows that a single plant, having all the world before it with no rival, would, under favourable conditions, occupy every available spot of the globe before the end of the ninth year. Suppose all the dry land of the globe-which equals about 51,000,000 of square miles-to consist of the same kind of soil, and to have the same kind of climate, that the conditions are everywhere the same and exactly suited to the plant, and suppose that the plant produces annually fifty seeds (a moderate number), each requiring only one square foot of soil, the following figures will tell us the result:
1 x 50 in 1st year 50 X 50 2d 2,500 X 50 3d 125,000 × 50 4th 6,250,000 × 50 312,500,000 × 50 15,625,000,000 × 50 781,250,000,000 × 50 8th 39,062,500,000,000 × 50 9th = 1,953,125,000,000,000 51,000,000 sq. miles-the dry surface of the earth x 27,878,400the number of sq. ft. in 1 sq. mile Being 531,326,600,000,000 square feet less than would be required at the end of the ninth year. The Law of Limited Populations.-Notwithstanding this tendency to increase in geometrical progression, the
1 Causes of the Phenomena of Organic Nature.
number of living individuals of each species in any country, or over the whole globe, is practically stationary; so that the whole of this enormous increase must die off as fast as produced, excepting those individuals for whom room is made by the death of parents. As a simple but striking example, take an oak forest: every oak will drop annually thousands or millions of acorns, but till an old tree falls not one of these millions can grow up into an oak; they must die at various stages of growth. A single pair of birds, again, multiplying at the ordinary rate, would in fifteen years increase to more than 2000 millions; whereas we have no reason to believe that the number of birds in any country increases at all; so that every year as many must perish as are born. Which creatures shall die and which shall live depends upon circumstances now to be detailed.
The Struggle.-All carnivorous quadrupeds and other creatures of prey are furnished with suitable weapons of offence: the lion has his formidable teeth and claws, the eagle his claws and beak. On the other hand, every creature liable to be preyed upon possesses some means of defence: the tiger can oppose the lion with his own weapons, the hedgehog can roll itself into a ball, the tortoise is protected by a shelly carapace. Many beetles (of the family Curculionida) have the wing-cases and other external parts so excessively hard that you cannot put a pin through them without first drilling a hole, and this doubtless saves them from the attacks of birds, where butterflies and caterpillars fall a prey. An immense number of insects have stings, and some stingless ants (of the genus Polyrachis) are armed with long and sharp spines on the back, which must render them unpalatable to many of the smaller insectivorous birds.
STRUGGLE FOR EXISTENCE.
In the struggle for existence also, as in human warfare, something is owing to fertility of resource, and something to discretion.
"He that fights and runs away,
The horse, the zebra, the antelope, the hare, and many other animals, are protected by their swiftness; foxes have holes, and birds hide in their nests; great numbers of insects hide themselves among the petals of flowers, or in the cracks of bark and timber; and finally, extensive groups have a disgusting smell or taste, which they either possess permanently, or can emit at pleasure.
It will be evident that, owing to these opposing endowments, a process of selection must be continually going on. The lion with the strongest teeth and swiftest foot is more likely to catch his prey than one less well endowed in these respects, and therefore in seasons of scarcity would be selected to be preserved while another might starve. The swiftest hares and antelopes will escape their foes, and leave offspring who inherit their speed of foot; the animals with shorter or weaker legs must necessarily suffer more; the passenger pigeon with less powerful wings will sooner or later fail to procure a regular supply of food, and in consequence its numbers will diminish.
When the accustomed food of some animal becomes scarce, or totally fails, the creature can only exist by emigrating, or by becoming adapted to a new kind of food- -a food perhaps less nourishing and less digestible. Natural selection will now act upon the stomach and intestines, and all individual variations favourable to the new state of things will be taken advantage of to modify