We may be encouraged to hope that such a result is possible from the words of one of those twin biologists who on the same night put forth their independently-arrived-at views as to what we are all agreed to regard as at least an important factor in the origin of species. No less a person than Mr. Wallace has written the following significant words:— “No thoughtful person can contemplate without amazement the phenomena presented by the development of animals. We see the most diverse forms—a mollusk, a frog, and a mammal—arising from apparently identical primitive cells, and progressing for a time by very similar initial changes, but thereafter each pursuing its highly complex and often circuitous course of development with unerring certainty, by means of laws and forces of which we are totally ignorant. It is surely a not improbable supposition that the unknown power which determines and regulates this marvellous process may also determine the initiation of these more important changes of structure, and those developments of new parts and organs which characterise the successive stages of the evolutions of animal forms.” These words advocate and confirm what I have elsewhere antecedently urged. Many influences doubtless may come into play in the origin of new species; but let us look a little narrowly at certain influences which _must_ come into play therein, and the action of which no man can deny. One of these influences (which no one has more richly illustrated than has the late Mr. Darwin) is that of heredity; but what _is_ heredity? In the first place it is obviously a property, not of new individuals, not of offspring, but of parental forms. As every one knows, it is the innate tendency which each organism possesses to reproduce its like. If any living creature, x, was self-impregnating and the outcome of a long line of self-impregnating predecessors, all existing in the midst of one uniform and continuously unvarying environment, then x would produce offspring completely like itself. This fundamental biological law of reproduction may be compared with the physical first law of motion, according to which _any body in motion will continue to move on uniformly at the same rate and in the same direction until some other force or motion is impressed upon it_.