A taxis is, then, an inevitable response by movement in a definite direction, to a directed stimulus. Including also tropisms it may be admitted that the movement is a purposeful, or at least, a useful one in some cases, as for instance the heliotropism and geotropism of the green plant. If we admit that Loeb’s description of the feeding of the caterpillar, as a tactic act, is true, we may also call this a useful act. But in the majority of cases tropisms and tactes are acts which appear to be of no use to the organism. The invasion of a part of the body which is irritated by a poison (as in inflammation) by leucocytes, is useful to the body itself, but we must regard the leucocytes as organisms, and their tactic motion leads to their destruction, and so also with other analogous acts. Just because of this we find difficulty in accounting for their origin in terms of natural selection. This does not matter so much, since it can hardly be maintained now that the tropistic or tactic act has any reality except in a very few cases--the motions of plants, galvano-taxis, the chemico-taxic movements of bacteria and leucocytes, and some other analogous cases, perhaps, are these exceptions. It can hardly be doubted that the extension of the concept to cover the motions of many invertebrates, and even some vertebrate actions, by Loeb and his school is a straining after generality which has not been justified. The hypothesis, as Loeb has stated it, is evidently almost certainly a logical one and was obviously elaborated as a protest against the anthropomorphism which saw in the flying of a moth into a flame the expression of an emotion; or in the movements of a caterpillar on a green shrub the expression of hunger and satiety and of the inherited experience of the animal; or in the avoidance by a _Paramœcium_ of a drop of acid the emotion of dislike of the feeling of pain. Well, let it be granted that this is so, and that the protest was a useful one, for it is obviously impossible that these notions as to the causes of the movements can be verified: does it improve matters to take refuge in an hypothesis which is just as purely physico-chemical dogmatism as the other is anthropomorphism? But the former hypothesis is at all events one which is susceptible of experimental verification and in this lies its usefulness, inasmuch as it has stimulated investigation. It is evident, however, that this verification has not yet been made. The differential afferent impulses set up by the energy-field; the increases or inhibition of muscular tone; the presence of photo-sensitive substances in the tissues of tactically acting lower animals; the change of velocity of chemical reaction, in these cases, which ought to follow stimulation--all these things _could_ be verified if they possess reality. Yet it is only indirect proofs, capable perhaps of other interpretations, and not direct experimental ones, which have so far