Weismann, then, divides the cells which compose the bodies of the higher animals, including man, into two distinct kinds; the somatic, or body cells and the germ, or reproductive cells. These germ cells are, so to speak, batteries in which are stored a substance which Weismann calls germ-plasm. A minutely small portion of this germ-plasm from an individual of one sex, mixed with a similar portion from an individual of the other will produce a new individual. But--and here comes the keystone of Weismann’s arch--only a portion of the mixed germ-plasm is used up in the composition of the new individual; the rest is stored away in the germ-cells of the new individual for further reproduction when the time arrives. The only relation that this reserved germ-plasm has with the body cells of the new individual is that it is provided by them with room and board.
Thus, according to Weismann, from generation to generation, there is an unbroken stream of germ-plasm, and this constitutes his celebrated theory of “The Continuity of Germ-Plasm.” Granted this theory as a premise, and Weismann’s conclusions cannot be gainsaid. This germ-plasm being the sole “carrier of heredity,” nothing that happens to the somatic or body cells can be transmitted to the progeny.
Darwin had put forward a theory of heredity which he called “Pangenesis,” which made out a good case for the admission of the Lamarckian factor. According to this theory all the somatic or body cells give forth still smaller cells which he calls “gemmules.” These gemmules are collected, by some process not explained, in the reproductive organs. Here they are in packets, and these “packets of gemmules” are “the carriers of heredity.” One can easily see how by this process the effects of use and disuse would be transmissible for an organ shrunk by disuse would not be capably represented by an efficient delegation of gemmules at the reproductive headquarters.
Speaking of this theory, Grant Allen in his biography of Darwin says, “Let not the love of the biographer deceive us. Not to mince matters, it was his one conspicuous failure, and is now pretty universally admitted as such.” It must be remembered however, that Darwin was fully aware of its purely speculative character and with his usual caution entitled it the “Provisional Hypothesis of Pangenesis.”