– Secondary Sexual Organs as 372 Adaptations – Individual Adjustments as 375 Adaptations – Color Changes as Individual 375 Adaptations – Increase of Organs through Use 376 and Decrease through Disuse – Reactions of the Organism to 377 Poisons, etc. – Regeneration 379 CHAPTER XI Tropisms and Instincts as 382 Adaptations CHAPTER XII Sex as an Adaptation 414 – The Different Kinds of Sexual 414 Individuals – The Determination of Sex 422 – Sex as a Phenomenon of Adaptation 439 CHAPTER XIII Summary and General Conclusions 452 INDEX 465 ------------------------------------------------------------------------ EVOLUTION AND ADAPTATION CHAPTER I THE PROBLEM OF ADAPTATION Between an organism and its environment there takes place a constant interchange of energy and of material. This is, in general, also true for all bodies whether living or lifeless; but in the living organism this relation is a peculiar one; first, because the plant or the animal is so constructed that it is suited to a particular set of physical conditions, and, second, because it may so respond to a change in the outer world that it further adjusts itself to changing conditions, _i.e._ the response may be of such a kind that it better insures the existence of the individual, or of the race. The two ideas contained in the foregoing statement cover, in a general way, what we mean by the adaptation of living things. The following examples will serve to illustrate some of the very diverse phenomena that are generally included under this head. Structural Adaptations