"Those who cavalierly reject the theory of Lamarck and his followers as not adequately supported by facts," wrote Mr. Herbert Spencer,[331] "seem quite to forget that their own theory is supported by no facts at all"--inasmuch as no one pretends to have seen an act of direct creation. Mr. Spencer points out that, according to the best authorities, there are some 320,000 species of plants now existing, and about 2,000,000 species of animals, including insects, and that if the extinct forms which have successively appeared and disappeared be added to these, there cannot have existed in all less than some ten million species. "Which," asks Mr. Spencer, "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 modification _due to change of circumstances_, ten millions of varieties may have been produced as varieties are being produced still?" . . . . . . "Even could the supporters of the development hypothesis merely show that the production 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 great changes in all organisms, subject to modifying influences ... they can show that any existing species--animal or vegetable--when placed under conditions different from its previous ones, _immediately begins to undergo certain changes of structure_ fitting it for the new conditions. 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 and domesticated animals, and in the several races of men, these changes have uniformly 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 that it is a matter of dispute whether some of these modified forms _are_ varieties or modified species. 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