may be put into a vat of nutrient fluid: there it lives and grows upon the nutriment supplied, so that a new particle may next be taken to impregnate another vat, and so on _ad infinitum_. Here the successive vats would represent successive generations of progeny; but, to make the metaphor complete, one would have to suppose that in each case the yeast-cell was required to begin by making its own vat of nutrient material, and that it was only the residual portion of the cell which was afterwards able to grow and multiply. But although the metaphor is thus necessarily a clumsy one, it may serve to emphasize the all-important feature of Weismann’s theory—namely, the almost absolute independence of the germ-plasm. For, just as the properties of the yeast-plant would be in no way affected by anything that might happen to the vat, short of its being broken up or having its malt impaired, so, according to Weismann, the properties of the germ-plasm cannot be affected by anything that may happen to its containing soma, short of the soma being destroyed or having its nutritive functions disordered. Such being the relations that are supposed to obtain between the soma and its germ-plasm, we have next to observe what is supposed to happen when, in the course of evolution, some modification of the ancestral form of the soma is required in order to adapt it to some change on the part of its environment. In other words, we have to consider Weismann’s views on the _modus operandi_ of adaptive development, with its result in the origination of new species.