These facts and reasonings, it is true, only go to prove, that animalcules, or beings of very small size, and low in the scale of animated existence, can be produced in this way by the inherent qualities of matter. No one will pretend, that a dog, a horse, or a man can thus be created. How can we account for the existence of these larger animals of a higher type, admitted to have been denizens of the earth only since the latest geological epochs, and therefore of comparatively recent origin? Here we come to another point in our author's theory,--the transmutation of species, or the successive _development_ of higher and higher orders of being out of the species immediately below them, through the accidental or natural fulfilment of certain conditions, in the course of a long period of years. Natural history teaches us, that there is quite a regular gradation among the several tribes of vegetables and animals; though we may not be able to range all the species, as constantly advancing in a single line, there is certainly the general appearance of a scale, beginning with the most simple, and going on to the most complex forms. While the external characteristics are very different, all are but variations of a single plan, which exists as the basis of all, and is varied in each individual only so as to accommodate it to the conditions under which the individual is to live. The germ of a higher animal--a mammifer, for instance--is the representative of a lower animal full-grown, like the _volvox globator_; the latter remaining in this initial stage, as an animalcule, through its whole existence; while the former is developed out of it, by successive stages, into a quadruped, or even into a man. Similar functions are performed in different animals by very different organs, the gills of fishes performing the same office as the lungs of the mammalia; and these different organs sometimes exist, at different periods, according to the degree of development, in the same animal. Thus, the tadpole, so long as it continues to be a fish, breathes by gills, which disappear and give place to lungs when it becomes a frog. Similar transformations of the insect tribe are familiar to all. Imperfect or rudimentary organs are found in certain animals, as the mammae of a man; a particular organ being here developed to a certain extent, though it is not needed; but being developed a little further, it becomes useful in the next set of animals in the scale. The same peculiarity is found among plants; the skilful gardener being able actually to develope these rudimentary organs by supplying the requisite conditions, and thus, as it were, to raise the plant one step in the scale.