Such, however, are the strongest arguments by which the advocates of the law hypothesis sustain their views of the origin of organism, life, and intellect. The next step in their reasoning is to show how animals and plants may be transmuted from one species, or genus, or family, to another; so that the existing vast variety can be traced to a few original germs. They maintain that these developments of the more from the less perfect have proceeded along certain parallel lines; one series of developments, for instance, taking the line of the fishes, another of the reptiles, another of the birds, another of quadrupeds, and so on. To prove these developments or transmutations, they appeal first to the physiological history of the mammalian embryo. In its earliest stages, it can hardly be distinguished, except in size, from the unborn polygastric infusoria. The brain of a human embryo appears at first like that of an invertebrate animal; next like that of a fish; then successively like that of a reptile, a bird, a rodent mammal, a ruminant, and a monkey. So the heart, at an early stage, looks like that of an insect; then it has two chambers, like that of a fish; then it becomes three chambered, like that of a reptile; and finally, four chambered, as in the mammalia. The inference which these theorists would draw from such facts is, that man actually begins his existence as an animalcule, and passes successively through the mould or condition of other animals, before he reaches the highest. And the reasons why he does become a man, rather than an echinoderm, or a fish, or a monkey, is only some slightly modifying circumstance, as, for instance, a longer gestation. It appears to me, however, that the inferences sound philosophy should derive from such facts are, first, that, while there is a seeming resemblance between the human embryo and that of lower animals, there is, in fact, a real and a wide diversity; so that the one infallibly becomes an inferior animal, and the other a man. Could a single example be produced in which a human embryo stopped at and became an insect, or a fish, or a monkey, there might be some plausibility in the supposition. But it is as certain to become a man as the sun is to rise and set; and, therefore, the human condition results from laws as fixed as those that regulate the movements of the heavenly bodies. That is a very superficial philosophy which infers identity of nature from mere external resemblance.