though he himself, with all this light, did not attain to the truest conception of life, he nevertheless broke ground for those who afterward were to do so.
In more recent times biology has been enriched with an enormous amount of facts for which we are indebted to the labors of naturalists, or even of mere breeders, as also to the labors of anatomists and clinicians, but, above all, to researches in experimental physiology, wherein the application of physical-science methods to the discovery of the laws of vital phenomena has been attended with brilliant success. Amid the extreme complexity of these phenomena it was difficult to perceive the relations of succession which unite them, and to establish positive series. But when men of science refused any longer to content themselves with observing them as they occur spontaneously, and began to vary them by calling in the action of special agents, then modifications were produced, the true causes of which were easily recognized. As in the study of inorganic bodies we learned the laws of their actions and combinations by seeking to find out with the aid of reagents—which are, in fact, special modifiers—the way in which they behave under circumstances that are well known, being fixed beforehand by the observer; so, in the study of living bodies, the introduction of experimentation which alters, according to a plan determined beforehand, the conditions under which the functions of life are to be performed, has enabled us to perceive, with an exactitude previously unknown, the organic properties underlying these functions. Even in embryogeny, a science which once seemed to belong to the domain of simple observation, it has been possible, by way of experimentation, to gain results which shed some light upon teratology. The employment, in observation, of instruments of precision, and in particular of registering apparatus, and of all those processes which suppress causes of error resulting from the personal peculiarities of the observer, gives to the results of research a degree of certitude which renders indisputable facts properly so called, the only question that remains being as to whether these results have been rightly or wrongly interpreted. In addition to an immense amount of unquestionable facts, in addition to a knowledge of the elementary properties of organic tissues and an acquaintance with the special laws which represent the action of these tissues in presence of these modifiers, this general result has followed the conquests of biology, namely, that living bodies are now known to be subject to the self-same laws which govern inorganic bodies, and that, under the hand of the experimenter, the course of things within the tissues is precisely the same as without the tissues; that in the laboratory the elements of living bodies, like those of inanimate things, have their own way of affecting the mind that observes them—that is to say, they possess fixed essential properties which can be determined; and what remains yet to be known is, above all, the mode in which
