(see Carpenter); and if to these we add the numbers of animal and vegetable species that have become extinct, we may safely estimate the number of species that have existed, and are existing, on the earth, at not less than _ten millions_. Well, which is the most rational theory about these ten millions of species? Is it most likely that there have been ten millions of special creations? or is it most likely that by continual modifications, due to change of circumstances, ten millions of varieties have been produced, as varieties are being produced still?... Even could the supporters of the Development Hypothesis merely show that the origination of species by the process of modification is conceivable, they would be in a better position than their opponents. But they can do much more than this. They can show that the process of modification has effected, and is effecting, decided changes in all organisms subject to modifying influences.... They can show that in successive generations these changes continue, until ultimately the new conditions become the natural ones. They can show that in cultivated plants, domesticated animals, and in the several races of men, such alterations have taken place. They can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species are in other cases founded. They can show, too, that the changes daily taking place in ourselves--the facility that attends long practice, and the loss of aptitude that begins when practice ceases--the strengthening of passions habitually gratified, and the weakening of those habitually curbed--the development of every faculty, bodily, moral, or intellectual, according to the use made of it--are all explicable on this same principle. And thus they can show that throughout all organic nature there is at work a modifying influence of the kind they assign as the cause of these specific differences; an influence which, though slow in its action, does, in time, if the circumstances demand it, produce marked changes--an influence which, to all appearance, would produce in the millions of years, and under the great varieties of condition which geological records imply, any amount of change." This quotation shows, as perhaps no other reference might show, how, by the middle of the present century, science was trembling on the verge of discovery of that "modifying influence" of which Mr. Spencer speaks. That discovery made clear how all that had preceded it not only contributed thereto, but gained a significance and value which, apart from it, could not have been secured. When the relation of the several parts to the whole became manifest, each fell into its place like the pieces of a child's puzzle map. LEADING MEN OF SCIENCE. A. D. 800 TO A. D. 1800.