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STATEMENT OF WEISMANN'S SYSTEM UP TO THE YEAR 1886. SEEING that Professor Weismann's theory of heredity, besides being somewhat elaborate in itself, is presented in a series of disconnected essays, originally published at different times, it is a matter of no small difficulty to gather from the present collection of them a complete view of the system as a whole. Therefore I propose to give a brief sketch of his several cognate theories, arranged in a manner calculated to show their logical connexion one with another. And, in order also to show the relation in which his resulting theory of heredity stands to what has hitherto been the more usual way of regarding the facts, I will begin by furnishing a similarly condensed account of Mr. Darwin's theory upon the subject. It will be observed that these two theories constitute the logical extremes of explanatory thought; and therefore it may be said, in a general way, that all other modern theories of heredity-such as those of Spencer, Häckel, Elsberg, Galton, Nägeli, His, Brooks, Hertwig, and De Vries-occupy positions more or less intermediate between these two extremes. Therefore, also, we need not wait to consider these intermediate theories[2]. "When closely analyzed, Mr. Darwin's theory-or the "provisional hypothesis of Pangenesis"-will be found to embody altogether seven assumptions, namely: - 1. That all the component cells of a multicellular organism throw off inconceivably minute germs, or "gemmules," which are then dispersed throughout the whole system. 2. That these gemmules, when so dispersed and supplied with proper nutriment, multiply by self-division, and, under suitable conditions, are capable of developing into physiological cells like those from which they were originally and severally derived. 3. That, while still in this gemmular condition, these cell-seeds have for one another a mutual affinity, which leads to their being collected from all parts of the system by the reproductive glands of the organi