Natural Selection.--Since natural selection is the main feature of Darwin's doctrine, we must devote more time to it. Darwin frequently complained that very few of his critics took the trouble to find out what he meant by the term natural selection. A few illustrations will make his meaning clear. Let us first think of artificial selection as it is applied by breeders of cattle, fanciers of pigeons and of other fowls, etc. It is well known that by selecting particular variations in animals and plants, even when the variations are slight, the breeder or the horticulturalist will be able in a short time to produce new races of organic forms. This artificial selection on the part of man has given rise to the various breeds of dogs, the 150 different kinds of pigeons, etc., all of which breed true. The critical question is, Have these all an individual ancestral form in nature? Observation shows that many different kinds--as pigeons--may be traced back to a single ancestral form, and thus the doctrine of the fixity of species is overthrown. Now, since it is demonstrated by observation that variations occur, if there be a selective principle at work in nature, effects similar to those caused by artificial selection will be produced. The selection by nature of the forms fittest to survive is what Darwin meant by natural selection. We can never understand the application, however, unless we take into account the fact that while animals tend to multiply in geometrical progression, as a matter of fact the number of any one kind remains practically constant. Although the face of nature seems undisturbed, there is nevertheless a struggle for existence among all animals. This is easily illustrated when we take into account the breeding of fishes. The trout, for illustration, lays from 60,000 to 100,000 eggs. If the majority of these arrived at maturity and gave rise to progeny, the next generation would represent a prodigious number, and the numbers in the succeeding generations would increase so rapidly that soon there would not be room in the fresh waters of the earth to contain their descendants. What becomes of the immense number of fishes that die? They fall a prey to others, or they are not able to get food in competition with other more hardy relatives, so that it is not a matter of chance that determines which ones shall survive; those which are the strongest, the better fitted to their surroundings, are the ones which will be perpetuated. The recognition of this struggle for existence in nature, and the consequent survival of the fittest, shows us more clearly what is meant by natural selection. Instead of man making the selection of those particular forms that are to survive, it is accomplished in the course of nature. This is natural selection.