=Many Tropic Responses Apparently Purposeful.--=The query arises as to why if these responses are mechanical they are so often apparently purposive; that is, why do they so often subserve some useful end for the animal? While they do not always work out to the animal's benefit, as for instance in the case of the moth and the light or under many other conditions that can be devised experimentally, as a matter of fact under normal natural conditions they are on the whole useful to the organism, carrying it into suitable surroundings of food, lessened danger, temperature, and the like. The probabilities are that in their first origin the reactions were not purposive. However, if any proved harmful they would result in the extermination of their possessors and hence of that particular strain of individuals. Those types that happened to have useful reactions would be left and in course of time as the process of eliminating the others went on, would become the prevailing types. Any organism which the useful reaction had preserved would tend to hand it down to the succeeding generation where again it would be the conserver of those individuals which possessed it in sufficient degree. =Authorities Not Agreed on Details of Tropic Responses.--=Although all the foremost modern students of animal behavior accept as facts the more or less mechanical orienting effects of external stimuli, there is by no means unanimity of opinion regarding details. Some stress as the directive factor the continuous action of the stimulating agent on sensitive tissues symmetrically situated. Others would maintain that it is the time rate of change in the intensity of the stimulating agent, or that the factor is different in different cases. Some make much of an automatic sort of "trial and error" system by which certain organisms test out an inimical environment until the path of least irritation is hit upon as the way to safety. The field is a broad one and to get at the finer shades of distinction the reader will have to refer to the works of such authorities as Loeb, Jennings, Holmes and Mast. =Tropisms Grade Into Reflex Actions and Instincts.--=The tropisms in many cases become indistinguishable from _reflex actions_ and these in turn grade up into the _instincts_ of animals. The latter may be looked on as but subtler and more involved reactions made possible through a more intricate structural organization. As might be expected of instincts, the feature of utility is more in evidence than in simpler tropisms because they have become of proportionately greater magnitude, but the same fundamental mechanism is apparently at bottom of both. It has already been seen how the "instinct" of the blow-fly to lay its egg on meat is interpretable as a chemotropic response. Thus no elaborate psychic mechanism is necessary in such behavior.