If a piece is broken from a crystal, and the crystal is suspended in a saturated solution of the same substance, new material is deposited over its whole surface, and, as it grows larger, the broken side is completed and the crystal assumes its characteristic form. But of what advantage is it to the crystal whether it is complete or incomplete? In the case of an animal it is of some importance to be able to complete itself after injury, because it can then better obtain the food necessary to keep it alive, or it can better escape its enemies; but this is not the case with the crystal. In conclusion, therefore, it is obvious that the adaptations of organisms are something peculiar to living things, and their obvious purpose is to maintain the integrity of the individual, or that of the species to which the individual belongs. We are, therefore, confronted with the question as to how this peculiarity has come to be associated with the material out of which living things are made. In subsequent chapters this will be fully discussed, but before we take up this topic, it will be necessary to reach some understanding in regard to the theory of evolution, for the whole subsequent issue will turn upon the question of the origin of the forms of animals and plants living at the present time. ------------------------------------------------------------------------ CHAPTER II THE THEORY OF EVOLUTION One of the most important considerations in connection with the problem of adaptation is that in all animals and plants the individuals sooner or later perish and new generations take their places. Each new individual is formed, in most cases, by the union of two germ-cells derived one from each parent. As a result of this process of intermixing, carried on from generation to generation, all the individuals would tend to become alike, unless something else should come in to affect the result. So far as our actual experience reaches, we find that the succeeding generations of individuals resemble each other. It is true that no two individuals are absolutely alike, but if a sufficiently large number are examined at a given time, they will show about the same variations in about the same proportionate numbers. Such a group of similar forms, repeating itself in each generation, is the unit of the systematists, and is called a species.