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, ...
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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, H???ckel, Elsberg, Galton, N???geli, 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 organism; and that, when so collected, they go to constitute the essential material of the sexual elements-ova and spermatozoa being thus aggregated packets of gemmules, which have emanated from all the cells of all the tissues of the organism. 4. That the development of a new organism, out of the fusion of two such packets of gemmules, is due to a summation of all the developments of some of the gemmules which these two packets contain. 5. That a large proportional number of the gemmules in each packet, however, fail to develop, and are then transmitted in a dormant state to future generations, in any of which they may be developed subsequently-thus giving rise to the phenomena of reversion or atavism. 6. That in all cases the development of gemmules into the form of their parent cells depends on their suitable union with other partially developed gemmules, which precede them in the regular course of growth. 7. That gemmules are thrown off by all physiological cells, not only during the adult state of the organism, but during all stages of its development. Or, in other words, that the production of these cell-seeds depends upon the adult condition of parent cells: not upon that of the multicellular organism as a whole.
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