In some of the lowest forms of life, such as Paramecium, there are but one or two possible modes of reaction--reactions which it attempts in response to any one of the myriad changes that may occur in the outer world. As we ascend the scale, we find two chief types of alterations: in the first place an increase in the number of hereditarily-given modes of reaction, and in the second an increased power of “learning,” of altering behaviour in adjustment to experience. In the insects, the first is chiefly in evidence. Although many insects undoubtedly can profit by experience to a limited degree, yet most of their behaviour is instinctive, in the sense that it unrolls itself automatically and efficiently in the absence of previous experience or of any possible instruction. In the vertebrates, on the other hand, we see as we pass from the lower to the higher groups a definite, steady increase in the power of learning by experience, from the fish that takes weeks to associate a given colour with a given event such as feeding-time, to the dog or monkey capable of learning elaborate tricks after a couple of trials. But even in the most “intelligent” of birds or mammals, the power of image-formation is very probably absent,[21] and the power of concept-formation, of generalizing, certainly so. This fact (quite apart from the absence of tradition, although this too operates in the same direction) means that the associations of animals can only be arbitrary and individual: a rook in one country (to choose a somewhat far-fetched example) may happen to associate danger with fire-arms, one in another with bows and arrows. Life, for the animals, is a cinema, different for each individual, in which one event may be associated with another in the most diverse and haphazard ways. With the advent of the human type of brain, however, experience can be sorted out and properly docketed; the mere cinematographic record is converted into a drama full of significance, the diary into a card-index. By this means, and by tradition, it is possible for man to obtain a much more accurate and more complete grasp of the relationships of the objects that compose the outer world than is possible for any other animal. Through knowledge, as ever, comes power: and as a result, man has been enabled to invent tools and machinery, and so to enlarge enormously his control over his environment. Just as his “range,” in the zoogeographical sense, is extended to an unprecedented degree both in space and time, so tools represent, biologically speaking, an extension of himself as an operator. While man is using a tool, he and the tool together constitute but a single unit in the struggle for existence. As various writers have put it, tools and machines are temporary organs of man, which have the additional merit of being replaceable if lost or damaged.