In briefly outlining the transmutations of living forms that took place during the Geologic Ages, we have said that changing conditions of life—through an ever-shifting environment—have compelled modifications in the form, structure, and habits of living creatures; and that those creatures which were unable to _adapt_ themselves, through _useful variations_, to the altering conditions of life have perished, while those that _did_ adapt themselves lived and progressed in organization. What, then, is the great agency through which some life forms have been _eliminated_ during the ages, while others have been _selected_ to continue through these ages? This agency is =Natural Selection=. The phrase “natural selection” is simply a convenient, condensed statement of observable and easily verifiable facts, viz.: that animals and plants are so situated in this world that they can only secure their food and mates by work, by effort, by struggles, whether consciously or unconsciously, and whether directly or indirectly; and that in these struggles those that are best equipped for their life duties are the ones that are most naturally successful in living and procreating their kind. The survival of those best adapted to their environment may be spoken of, in the language of Spencer, as the =Survival of the Fittest=. Darwin’s phrase—=Natural Selection=—has precisely the same significance and means that those creatures which are best fitted to their surroundings are the survivors. The working of Natural Selection may be made clearer by a brief reference to the =Artificial Selection by Man= of various animals and plants. Variations frequently take place in domesticated animals and plants. Some of these variations appeal to man as being of practical value, others as beautiful, and others again as curious or interesting. He selects those individuals whose variations he wishes to preserve, breeding only them together, and in this way accentuates those variations he desires to perpetuate. In course of time the accumulation of these differences becomes so marked as to make the animals differ greatly from the original stock from whence they came. A well-known illustration of this process is the pigeon. All our domestic pigeons, forming a large number of well-marked races, such as the fantail, the tumbler, the pouter, etc., have been produced from the ordinary wild rock pigeon of Europe. The bird fancier, noticing individual differences in the offspring of the wild rock pigeon, selected the peculiar individuals, and bred only them together. By this simple process of artificial selection and isolation of the chosen or selected individuals, all the races of pigeons have been produced. Plate I shows the wild rock pigeon _Columba livia_ and the domesticated pouter. The forms of these pigeons are very different, yet the wild rock pigeon has been transmuted into the pouter through the agency of Artificial Selection. The same is the case with