This tendency to increase at so tremendous a rate, and the fact that no two individuals are exactly alike, supply the material and the conditions for the great law of natural selection to operate. Each individual must of necessity compete so fiercely with other individuals of the same species, and of allied species, and with enemies, with climate and changing conditions that out of the struggling many only a few live to propagate. Those individuals that vary from the mass in the right direction live; the others die, leaving few descendants, or none. Though like produces like there are no two individuals exactly alike. If all of the same species were alike there would be no opportunity for the law of natural selection to operate. Nothing but chance would determine in the struggle for existence which individuals would live to propagate their species and which would die without descendants. Nor would it matter, so far as the species is concerned. But individuals do differ in their traits in some degree--Wallace has shown that variation usually amounts from ten to twenty, sometimes twenty-five, per cent of the varying part--and this variation, even if small, often spells the difference between an early death with no descendants and a long life with a numerous progeny. By the law of heredity the descendants are endowed in a greater or less degree with the same life-saving characteristics of their parents. Each generation being subjected to this weeding-out process, and only a few of the best fitted individuals being selected to preserve the species, we can easily see that, as the generations come and go, those essential, life-saving characters or traits are being developed to a greater and greater degree. And this means change, modification of species. With horn or tooth or claw or hoof or sting or poison or odor, among all the wild creatures that swim or crawl or run or climb or fly, the struggle for life and food and mate goes on today just as it has gone on for millions of years. Other factors in the struggle are alertness, agility, cunningness, sharpness of vision and hearing and smell, protective color of covering, fleetness of foot and wing, the degree of heat, the amount of moisture and the food supply. The traits that survive are of course those that are the most useful in a given environment. (By environment is meant surroundings, all outside influences, the not-me of each individual.) A change of environment, of the conditions of life, calls for a re-adjustment of the traits most vital to the individual in the new environment. In one environment one highly developed trait would be most useful while in another some other character or set of characters would be the saving factor, because a change of environment means a change of weather conditions, of the food supply and the means of getting it, of enemies, etc.