Searching for the cause and meaning of everything which occurred, he at last achieved the discovery he was looking for, and so knew what is assuredly one of the greatest of spiritual delights. Largely by watching and studying the results of deliberate human selection with plants and animals, he was led to his great principle of natural selection, that is, assuming the tendency in all living beings to vary individually, that in the intense struggle for existence those variations which are beneficial and help the organism to live and prosper will be preserved and transmitted. The causes of variation, whether spontaneous or to be found in environment, Darwin never pretended entirely to explain. Of the principle of natural selection, or as Spencer phrased it in a form which Darwin admitted to be in some respects more satisfactory, ‘the survival of the fittest,’ perhaps its discoverer has given no better statement than the sentences in the second volume of the great work on ‘Plants and Animals under Domestication’: ‘To consider the subject under this point of view is enough to strike one dumb with amazement. But our amazement ought to be lessened when we reflect that beings almost infinite in number, during an almost infinite lapse of time, have often had their whole organization rendered in some degree plastic, and that each slight modification of structure which was in any way beneficial under excessively complex conditions of life has been preserved, whilst each which was in any way injurious has been rigorously destroyed. And the long-continued accumulation of beneficial variations will infallibly have led to structures as diversified, as beautifully adapted for various purposes, and as excellently coördinated, as we see in the animals around us. Hence I have spoken of selection as the paramount power, whether applied by man to the formation of domestic breeds, or by nature to the production of species.’[403] It is not to be supposed that the first grasp of evolution through variation and natural selection could have carried with it the full foresight of the enormous changes, mental, moral, and spiritual, that such a theory was likely to produce. Still, a mind so keen and active as Darwin’s was bound to catch some suggestion of those changes. For instance, the astonishing and deadly ease with which the old workings of Providential adaptation and design slipped under the new light into mere operations of mechanical law could not fail to foreshadow the philosophical and theological upheaval that was sure to follow. A spirit trained in the conventional atmosphere of the early English nineteenth century must necessarily have regarded such an upheaval with a certain amount of question, awe, and even dismay; and Darwin indisputably had something of these feelings mingled with the triumphant joy of discovery.