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 VITALISM : A BRIKF HISTORICAL AND CRITICAL REVIEW. 329 conditions, wherein the final effects are separated from the original causes by so many intermediate links that their con- nexion is not easily established ". Or else, this activity is something absolutely sui generis, meriting the dignity of the same isolation which distinguished electric or magnetic force a century ago. History shows how " as philosophy advances, life or activity in natural objects retires, and leaves them dead and inactive. Instead of moving voluntarily, we find them to be moved necessarily : instead of acting, we find them to be acted upon : and Nature appears as one great machine, where one wheel is turned by another, and that by a third : and how far this necessary succession may reach, the philosopher does not know" (28). Surely, then, he will hesitate before denying to vital phenomena the exten- sion of that unifying process which all progress bespeaks. The two sides of the same problem stand clearly displayed. On the one hand, Natural Science offers but a description of phenomena in arbitrarily-selected language. On the other hand, the human mind, dissatisfied with these narrow limits, tends always to exchange mechanism for a teleology where verification from experiment is no longer possible. Pursued objectively, causality seeks the relation between phenomena. Pursued subjectively, it concerns the origin of phenomena. From the universal standpoint of Natural Science, atten- tion has been directed to the interaction of external and internal conditions during development ; to the impossibility of a present description of growth, adaptation and heredity in mechanical language ; and to the lack of knowledge con- cerning the composition and structure of living substance. And it has been shown how necessary to the physiologist is the retention of the expression " vital force " ; whether by it be understood the resultant of ordinary physical forces of which he knows little, or something (distinct from and opposed to these forces) of which he knows nothing. Ee- garding the most recent rapid extension of knowledge in physics, a third meaning, more probable that either of the two former, may be entertained ; vital principle being held to express a combination of definite complex forces, which are mechanical in so far as they depend on phenomena that differ only in degree and not in kind from those of lifeless nature, which are vital in so far as they result from the activity of a substance, the conditions for the manufacture of which are quite unknown in the laboratory. In this way, neither mechanism nor vitalism, as now understood, can be preferred as the scientific theory of the future. The greater becomes the knowledge of the living and lifeless worlds, the