When we recall that in some organisms regeneration takes place in almost every part of the body, it does not seem possible that this power could have been acquired by selection. And when we find that many internal organs regenerate, that can rarely or never be injured without the animal perishing, it seems impossible that this can be ascribed to the principle of natural selection. It has also been found that if the first two cells of the egg of a number of animals, jellyfish, sea-urchins, salamanders, etc., be separated, each will produce an entire animal. In some of these cases it is inconceivable that the process could ever have been acquired through selection, because the cells themselves can be separated only by very special and artificial means. These, and other reasons, indicate with certainty that regeneration cannot be explained by the theory of natural selection. ------------------------------------------------------------------------ CHAPTER XI TROPISMS AND INSTINCTS AS ADAPTATIONS Of the different kinds of adaptation none are more remarkable than those connected with the immediate responses of organisms to external agents. These responses are usually thought of as associated with the nervous system; and while in the higher forms the nervous system plays an important role in the reaction, yet in many cases it is little more than the shortest path between the point stimulated and the muscles that contract; and in the lower animals, where we find just as definite responses, there may be no distinct nervous system, as in the protozoa, for instance. Many of the so-called instincts of animals have been shown in recent years to be little more than direct responses to external agents. Many of these instincts are for the good of the individual, and must be looked upon as adaptations. For example: if a frog is placed in a jar of water, and the temperature of the water lowered, the frog will remain at the top until the water reaches 8 degrees C., when it will dive down to the bottom of the jar; and, if the temperature is further lowered, it will remain there until the water becomes warmer again, when it will come to the surface again. It is clear that, under the ordinary conditions of life of the frog, this reaction is useful to it, since it leads the animal to go to the bottom of the pond on the approach of cold weather, and thus to avoid being frozen at the surface. Another illustration of an instinct that is a simple response to light is shown by the earthworm. During the day the worm remains in its burrow, but on dark nights it comes out of its hole, and lies stretched out on the surface of the ground. It procures its food at this time, and the union of the individuals takes place. In the early morning the worm retires into its burrow.