In the present chapter we may first consider, from the point of view of discontinuous variations as contrasted with the theory of the selection of individual variations, the structural adaptations of animals and plants, _i.e._ those cases in which the organism has a definite form that adapts it to live in a particular environment. In the second place, we may consider those adaptations that are the result of the adjustment of each individual to its surroundings. In subsequent chapters the adaptations connected with the responses of the nervous system and with the process of sexual reproduction will be considered. It should be stated here, at the outset, that the term _mutation_ will be used in the following chapters in a very general way, and it is not intended that the word shall convey only the idea which De Vries attaches to it; it is used rather as synonymous with _discontinuous and also definite variation_ of all kinds. The term will be used to include “the single variations” of Darwin, “sports,” and even orthogenic variation, if this has been definite or discontinuous. Form and Symmetry Almost without exception, animals and plants have definite and characteristic forms. In other words, they are not amorphous masses of substance. The members of each species conform, more or less, to a sort of ideal type. Our first problem is to examine in what sense the form itself may be looked upon as an adaptation to the surroundings. It is a well-recognized fact that the forms of many animals appear to stand in a definite relation to the environment. For instance, animals that move in definite directions in relation to their structure have the anterior and the posterior ends quite different, and it is evident that these ends stand in quite different relations to surrounding objects; while, on the other hand, the two sides of the body which are, as a rule, subjected to the same influences are nearly exactly alike. The dorsal and the ventral surfaces of the body are generally exposed to very different external conditions, and are quite different in structure. The relation is so obvious in most cases that it might lead one quite readily to conclude that the form of the animal had been moulded by its surroundings. Yet this first impression probably gives an entirely wrong conception of how such a relation has been acquired. Before we attempt to discuss this question, let us examine some typical examples.