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I have nothing, however, to do with Mr. Darwin's theory of pangenesis beyond avoiding the pretence that I understand either the theory itself or what Professor Weismann says about it; all I am concerned with is Professor Weismann's admission, made immediately afterwards, that the somatic cells may, and perhaps sometimes do, impart characteristics to the germ-cells.
Such self-multiplying gemmules without pangenesis would enable us to understand both the excessive weakness or non-existence of normal use-inheritance, and the excessive strength and abruptness of the effect of their partial destruction under special pathological conditions.
Scarcely less difficult to understand is the process of the stomach-carrying-off mode of metamorphosis before spoken of as existing in the Echinoderms. Next, as to certain patent and notorious facts: On the hypothesis of pangenesis, no creature can develop an organ unless it possesses the component gemmules which serve for its formation.
Pangenesis has probably also much truth in it, and has certainly afforded valuable and pregnant suggestions, but unaided and alone it seems inadequate to explain the evolution of the individual organism. Those three conceptions of the organic world which may be spoken of as the teleological, the typical, and the transmutationist, have often been regarded as mutually antagonistic and conflicting.
For myself I have based my field-researches and my testing of native plants on the hypothesis of unit-characters as deduced from Darwin's Pangenesis. This conception led to the expectation of two different kinds of variability, one slow and one sudden. The sudden ones known at the time were considered as sports, and seemed limited to retrograde changes, or to cases of minor importance.
He never pretended that there was the faintest corroborating evidence visible to the microscope in the organ, in the blood, or in the germ cell. It was not an accounting for what is, but for what it seemed possible to him might be. This theory of Pangenesis, in the shape in which Darwin promulgated it, has dropped out of consideration almost entirely.
Besides these objections to the sufficiency of "Natural Selection," others may be brought against the hypothesis of "Pangenesis," which, professing as it does to explain great difficulties, seems to do so by presenting others not less great almost to be the explanation of obscurum per obscurius. Mr. Darwin supposes that natural selection acts by slight variations. These must be useful at once.
"Leaving on one side, for the moment, these doubtful and insufficiently investigated cases, we may still maintain that the assumption that changes induced by external conditions in the organism as a whole are communicated to the germ-cells after the manner indicated in Darwin's hypothesis of pangenesis, is wholly unnecessary for the explanation of these phenomena.
Digby's general theory thus represents a strange mixture of epigenesis and pangenesis, and is not entirely devoid of "virtues."
Pangenesis, with its attraction of gemmules from all parts of the body into the germ-cells, and the free circulation of gemmules in the offspring till they hit upon or are attracted by the particular cell or cells, with which alone they can readily unite, seems a less feasible theory and less in conformity with the whole of the facts than an hypothesis of germ-continuity which supposes that the development of the germ-plasm and of the successive self-dividing cells of the body proceeds from within.
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