The preceding account of some of the tropisms in animals and plants will serve to give an idea of how certain movements are direct responses to the environment. Some of the reactions appear to be necessary for the life of the individual, others seem to be of less importance, and a few of no use at all. Yet the latter appear to be as definite and well-marked as are the useful responses. I think the conviction will impress itself on any one who examines critically the facts, that we are not warranted in applying one explanation to those responses that are of use, and another to those that are of little or of no value. Inasmuch as the Darwinian theory fails to account for the origin of organs of little or of no value, it is doubtful if it is needed to explain the origin of the useful responses. If, on the other hand, we assume that the _origin_ of the responses has nothing to do with their value to the organism, we meet with no difficulty in those cases in which the response is of little or of no use to the organism. That great numbers of responses are of benefit to the organism that exhibits them can be accounted for on the grounds that those new species, that have appeared, that have useful responses, are more likely, in the long run, to survive, than are those that do not respond adaptively. We may now examine some of the more complicated responses and instincts, more especially those of the higher animals. Some of these are pure tropisms, _i.e._ definite responses or reactions to an external exciting agent; others may be, in part, the result of individual experience, involving memory; others, combinations of the two; and still others may depend on a more complex reaction in the central nervous system of the animal. These cases can be best understood by means of a few illustrations. As an example of a simple action may be cited a well-known reflex after cutting the nerve-cord of the frog, or after destroying the brain. If the frog is held up, and its side tickled, the leg is drawn up to rub the place touched. To accomplish this requires a beautifully adjusted system of movements, yet the act seems to be a direct reflex, involving only the spinal cord. An example of a somewhat more complex reflex is the biting off of the navel-string by the mother in rodents and other mammals; an act eminently useful to the young animal, although of no importance to the mother herself. The protection of the young by their parents from the attacks of other animals appears to be a somewhat complex instinct, and it is interesting to note that the protection is extended to the young only so long as they are in need of it, and as soon as they are able to shift for themselves the maternal protection is withdrawn.