scientific concept; in Herbert Spencer's hands, it grew to be a probable and rational theory, based upon a serious array of confirmatory facts, and fulfilling all the conditions of a sound working hypothesis. If the reader will turn once more to Mr. Spencer's pronouncement, published seven years before The Origin of Species, he will see that there Mr. Spencer has brought together almost all the chief arguments which still weigh in favor of the theory of descent and modification. Mr. Clodd has collected a large number of passages from Mr. Spencer's early works—especially passages from scattered articles prior to the first public hint of Darwin's idea—which amply prove Mr. Spencer's claim to rank as an entirely independent author of the doctrine of organic evolution. The fact is, before Darwin's book appeared, the argument from variation, the arguments from plants and animals under domestication, the argument from embryology, the argument from geographical distribution, the argument from distribution in geological time, had all of them been brought forward, and some of them had been treated with great skill and effect, by Mr. Spencer. Indeed, it was above all von Baer's law of embryological development which led Mr. Spencer both to his first clear conception of the method of biological evolution, and to his first incomplete conception of evolution in general as fundamentally a progress from the homogeneous to the heterogeneous.
Why, then, if so many minds had already grasped the doctrine of descent with modification, did Darwin's immortal treatise produce so immediate and noteworthy a mental revolution? Why did the world which turned a deaf ear to Lamarck, and even to Spencer, listen gladly to Charles Darwin? Clearly, because Darwin had something new and important to add to the concept; and that "something new" was the theory of natural selection. This was Darwin's real contribution to the world's thought. He arrived at it at first as a stray aperçu; he followed it up, with Darwinian patience, with astonishing wealth of knowledge and instance, with single-hearted devotion to the particular subject, through the whole of his life; and he left it at the end as nearly certain as such a thesis can ever be made by human intelligence. The weak point in the hypothesis of organic evolution, before Darwin, was the difficulty of understanding the nature and cause of adaptation to the environment. That weak point, when supplemented by theological preconception, made many or most biologists hesitate to accept the nascent theory, in Lamarck's and Spencer's presentment. It is true, minds like Lamarck's and Spencer's could never for a moment, on the other hand, have accepted the crude and unthinkable dogma of separate creation; but the mass of biologists, incapable of high philosophic reasoning, held their judgment suspended, and waited for some other
