In Chapter II. we saw that many of the valuable and carefully reasoned results which Weismann had obtained are accompanied by much detailed speculation based upon an extremely limited fact basis. In his later writings especially, the reader who searches for the facts and reasoning which refer to such questions as the non-transmission of acquired characters will find everywhere an abundant commingling with speculations concerning the mechanical process of heredity. I have ventured in the appendix to sketch shortly this speculation of Weismann’s, and to give my own estimation of its value. Weismann was not the first to speculate or found a mechanical theory of heredity; Darwin himself published a theory of “pangenesis,” and this was, I believe, the only piece of speculation of which Darwin was ever guilty. That he might picture to himself, by means of material particles, the views that he held of heredity, Darwin supposed that during their lifetime every cell of the parent disengages small living particles--gemmules--which find their way to and are stored up in the generative cells, ready to develop in the next generation into cells similar to those from which they came. This theory was actually framed to support those cases where Darwin supposed that acquired characters are transmitted, for he says in one of his replies to Huxley’s letters:[33] “I do not doubt your judgment is perfectly just, and I will try to persuade myself not to publish. The whole affair is much too speculative; yet I think some such view will have to be adopted when I call to mind such facts as the inherited effects of use and disuse,” etc. In the theory of pangenesis the gemmules residing in the body or somatic cells will be subject to such influences as affect the cells, and will naturally transmit any change to which they have been subjected to the offspring that they eventually build up. Darwin’s view may be graphically represented by Fig. 2. Since Darwin’s time most biologists have come to doubt whether there is any evidence that acquired characters are transmitted at all, and incline rather to view racial change as altogether due to inborn variations, some of which variations have an advantage over others; Darwin himself, as Huxley pointed out,[34] laid less weight on the influence of acquired habits in his later than in his earlier writings. Modern biologists tend, therefore, to be more Darwinian than Darwin in respect to their thoroughgoing adherence to the action of selective influence; but of necessity they discard his theory of pangenesis, which was a pictorial expression of Darwin’s lingering Lamarckian tendencies. Let me give some idea of what the Neo-Darwinians picture to themselves as the “process of heredity.” It has been known now for many years that every cell in the body, including the sperm cells and ova, are descended from a fertilised ovum. Of these cells of the body all obviously die except those sperm cells and ova which