Since the publication of the Origin of Species, probably the most important contribution to biological theory is to be found in the researches of Dr. A. Weismann, and particularly in his large work, The Evolution Theory, of which a masterly English translation has recently appeared.[27] Weismann, on one side, represents an heroic attempt to bring back to the strictly mechanical principles of Darwinism the tide of biological speculation, which has been flowing more and more in the direction of recognizing an essential and not a merely fortuitous connexion between the goal of the evolution of natural forms and the means taken by nature to attain it. On another side he has brought the physiology of the cell into true relation with the natural history of the organism and of the species, and has become the author, or at least the first great expounder and systematizer, of a theory of heredity—the now famous Germ-Plasm theory—much of which seems a solid, permanent, and deeply important contribution to knowledge. But this theory seems to lead straight to a non-mechanical or psychic conception of the driving-force of evolution, and Weismann has therefore supplied the other part which, in the view of the present writer and of many others better qualified to judge, seems to be of the nature of a baseless and improbable hypothesis, devised to find a means of avoiding recourse to any non-mechanical conception of the ultimate nature of evolutionary processes. As we shall be much concerned with Weismann’s views, let us place at the head of our study of them a couple of passages in which his general attitude towards the phenomena of vital processes is expressed. “In our time,” he writes, “the great riddle has been solved—the riddle of the origin of what is best suited to its purpose without the co-operation of purposive forces.”[28] “We must certainly assume,” he declares, “that the mechanical theory of life is correct.”[29] A longer passage shows us what he understands by ‘mechanical’:— “The living machine differs essentially from other machines in the fact that it constructs itself; it arises by development from a cell, by going through numerous stages of development, but none of these stages is a dead thing, each in itself is a living organism whose chief function is to give rise to the next stage. Thus each stage of the development may be compared to a machine whose function consists in producing a similar but more complex machine. Each stage is thus composed, just like the complete organism, of a number of such ‘constellations’ of elementary substances and elementary forces, whose number in the beginnings is relatively small, but increases rapidly with each new stage.”[30]