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Updated: May 22, 2025
However, this germ-plasm theory, with its attempt to explain heredity, is merely a "provisional molecular hypothesis"; it is one of those metaphysical speculations that attribute the evolutionary phenomena exclusively to internal causes, and regard the influence of the environment as insignificant.
These may wander through the organism and meet in the germ-plasm, and then, when a child-organism is produced, they “swarm,” so to speak, in it again “in some way or other,” and in some fashion control the development. This gemmule-theory was too obviously a quid pro quo to hold its ground for long.
If, on the contrary, it has some effect, this is likely to be due to a chemical change which it has induced in the germ-plasm. This chemical change might, by exception, bring about the original modification again in the organism which the germ is about to develop, but there are as many and more chances that it will do something else.
In his work on The Continuity of the Germ-plasm, and in his recent excellent Lectures on the Theory of Descent , he has with great success advanced the opinion that "only those characters can be transmitted to subsequent generations that were contained in rudimentary form in the embryo."
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.
The doctrine that the individual body was evolved by the forces of life, acted on and directed by natural selection, as guardian and transmitter of the germ-plasm, assumes a less paradoxical character when we perceive with what unfailing art Nature has constructed and devised the body and the mind for their function.
A logical result of the theory is the impossibility of the transmission of acquired characters, since the molecular structure of the germ-plasm is already determined within the embryo; and Weismann holds that there are no facts which really prove that acquired characters can be inherited, although their inheritance has, by most writers, been considered so probable as hardly to stand in need of direct proof.
The fundamental proposition of Weismann's theory, that heredity is due to continuity of germ-plasm, seems to contain important truth. But we need not therefore accept his theory of a germ-plasm so isolated and independent as to be beyond control or influence by the habits of the body.
To explain this Weismann went back to the unicellular protozoa. These animals are undoubtedly influenced by environment and vary under its stimuli. Here the variations were stamped upon the germ-plasm, and the commingling of these variously stamped germ-plasms has resulted in all the variations of higher animals. Of late Weismann has modified and greatly improved this portion of his theory.
One might represent the germ-plasm by the metaphor of a long, creeping rootstock from which plants arise at intervals, these latter representing the individuals of successive generations.
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