But the doctrine of development not only enabled us to understand the facts already existing; it gave at the same time an impetus to the acquisition of unforeseen new ones. If at the present day we glance back at the development of the biological sciences within the last twenty years, we must be astonished both at the enormous array of new facts which have been evoked by the theory of development, and by the immense series of special investigations which have been called forth by this doctrine. But while the development theory for by far the greater majority of these investigations served as a light which more and more illuminated the darkness of ignorance, there appeared at the same time some other researches in which this doctrine itself became the object of investigation, and which were undertaken with a view to establish it more securely. To this latter class of work belong the “Studies” in the present volume. It will perhaps be objected that the theory of descent has already been sufficiently established by Darwin and Wallace. It is true that their newly-discovered principle of selection is of the very greatest importance, since it solves the riddle as to how that which is useful can arise in a purely mechanical way. Nor can the transforming influence of direct action, as upheld by Lamarck, be called in question, although its extent cannot as yet be estimated with any certainty. The _secondary_ modifications which Darwin regards as the consequence of a change in some other organ must also be conceded. But are these three factors actually competent to explain the complete transformation of one species into another? Can they transform more than mere single characters or groups of characters? Can we consider them as the sole causes of the regular phenomena of the development of the races of animals and plants? Is there not perhaps an unknown force underlying these numberless developmental series as the true motor power--a “developmental force” urging species to vary in certain directions and thus calling into existence the chief types and sub-types of the animal and vegetable kingdoms? At the time these “Studies” first appeared (1875) they had been preceded by a whole series of attempts to introduce into science such an unknown power. The botanists, Nägeli and Askenasy, had designated it the “perfecting principle” or the “fixed direction of variation;” Kolliker as the “law of creation;” the philosophers, Von Hartmann and Huber, as the “law of organic development,” and also “the universal principle of organic nature.” It was thus not entirely superfluous to test the capabilities of the known factors of transformation. We had here before us a question of the highest importance--a question which entered deeply into all our general notions, not only of the organic world, but of the universe as a whole.