at work, two parts of the same species might change in different directions, so as at last to become distinct and separate species. Darwin’s book upon the subject was called _The Origin of Species_. It was a very modest and sufficient title. He did not even go to the length of calling it the origin of genera or orders or classes. He did not at first apply it to man. This is the theory of the origin of species through Natural Selection. It was not pretended by either of these pioneers that Natural Selection was the sole way through which the differences of species came about. For example, Darwin devoted a considerable part of his working life to such collateral modes of differentiation as the hypothesis that Sexual Selection also had its share. Criticism has whittled down that share to practically negligible proportions, but I note the hypothesis here because it absolutely disposes of the assertion which Mr. Belloc hammers on the table, that the Theory of Natural Selection excludes any other modes of specific differentiation. TESTING THE THEORY Very rapidly this conception of Natural Selection was extended by naturalists until it came to be regarded as the general process of life. They came to realise that all species, all genera, all classes of life, whatever else may be happening to them, are and always have been varying through the process of Natural Selection, some rapidly, some slowly; some so slowly as hardly to change at all through vast ages. I have stated the _a priori_ case by which, given birth and death and individuality and changing conditions and sufficient time, it appears logically inevitable that the change and differentiation of species _must_ occur, and must be now going on. If we had no material evidence at all it would still be possible to infer the evolution of species. That _a priori_ case has never been answered, and it seems to me unanswerable. But scientific men, with their obstinate preference for observation and experiment over mere logical gymnastics, rarely rest their convictions on _a priori_ cases. A sustaining scepticism is a matter of conscience with them. To them an _a priori_ case is merely a theory--that is to say, a generalisation under trial. For nearly three-quarters of a century, therefore, biologists have been examining whatever instances they could discover that seemed to contradict this assumption that the process of specific change under Natural Selection is the general condition of life. To this day this view is still called the Theory of Natural Selection, though to a great number it has come to have the substantial quality of an embracing fact.