teachings of biology as not being “exact science,” yet we may boldly admit the truth of the assertion that we biologists are largely occupied with speculations, hypotheses, and other products of the imagination. All true science deals with speculation and hypothesis, and acknowledges as its most valued servant—its indispensable ally and help-meet—that which our German friends[2] call “Phantasie” and we “the Imagination.” Our science—biology—is not less exact; our conclusions are no less accurate because they are only _probably_ true. They are “probably true” with a degree of probability of which we are fully aware, and which is only _somewhat_ less than the probability attaching to the conclusions of other sciences which are commonly held to be “exact.” These remarks are addressed to an Association for the advancement of science—of science which flourishes and progresses by the aid of suppositions and the working of the imagination. The Association has been holding its annual sitting in various parts of the British Islands for more than thirty years, and yet it is still a very common and widely spread notion that science, that is to say, _true_ science according to those who hold the notion, does not countenance hypotheses, and sternly occupies itself with the exact record of fact. On the other hand, there are many persons who run to an opposite extreme, and call by the name of science any fanciful attempt to deal with or account for a certain class of phenomena. The words “science” and “scientific” are used so vaguely and variously that one might almost come to the conclusion that it would be well for our Association to plainly state what is that thing for the advancement of which its meetings are held. I cannot venture to speak in the name of my colleagues; and no doubt a review of the work done by the Association would most fitly explain what that body understands by the word “science.” At the same time it is permissible to take this opportunity of briefly stating what science is and what it is not, so far as I am able to judge of the fitting use of the word.