ORIGIN OF MAN. 45
admit as species. Some special instances of variation are quite striking. The axolotl of Colorado breathes water throughout life. But Dumeril, Prof. Marsh and others have caused this animal to absorb its gills and breathe air. It thus becomes a salamander, not only of a different species, but a different genus and tribe. So, certain marine crustaceans, gradually habituated to water less and less salt, undergo a transformation to fresh water species. By reversing the process, the transformation is reversed. It thus appears that transitions from one specific form to another are in the order of nature, and we learn that the transition from one modification of the horse type to another, during the lapse of Tertiary time, is a solution of the problem of successional forms which may be legitimately contemplated. The genetic relation of one term to another is not in conflict with nature, but is now strongly suggested by the aptitude of organic forms to vary.
IV. Embryological evidence. Every embryo in its course of development passes, by continuity, through a succession of stages, beginning with a cell. My limited space forbids to particularize, but I may state that the human embryo exhibits not fewer than twenty-two stages pretty distinctly marked, each in succession more complicated than the preceding ones. Now the points to be made in view of embryonic stages are the following: I. The embryonic stages of all animals are, to a certain extent; identical. The simplest animals pass through but few of these stages ; higher animals more. 2. Embryonic stages furnish a parallelism with the gradations of existing animals. The lowest animals having gone through three or four stages are arrested and live as permanent representatives of the last stage reached. Animals successively higher pass through successively more of the stages, and, becoming arrested, stand permanently as pictures of the highest embryonic stages reached. So the gradation of animal forms in the world becomes a series parallel with the series exemplified by the embryonic history of every higher animal. 3. Embryonic stages furnish parallels with the geological succession. That is, the succession of types of animals in geological time presents the same characters and the same order as the embryonic history of any higher vertebrate.
Now the embryonic succession is a demonstrated continuity. Is