This general rule, that the longer the paths of embryonic development of two animals keep identical the more nearly the two animals are related, when Huxley wrote, was founded on a much smaller number of facts than now are known. Since 1860 an enormous bulk of embryological investigation has been published, and the total result has been to confirm Huxley's position in the fullest possible way. A certain number of exceptions have been found, but these exceptions are so obviously special adaptations to special circumstances that their existence only makes the general truth of the proposition more clear. The most common kind of exception occurs when two closely related animals live under very different conditions. For instance, many marine animals have close allies that in comparatively recent times have taken to live in fresh water. The conditions of life in fresh water are very different, especially for delicate creatures susceptible to rapid changes of temperature, or unable to withstand strong currents. Thus most of the allies of the fresh-water crayfish, which live in the sea, lay eggs from which there are soon hatched minute, almost transparent larvæ, exceedingly unlike the adult. In the comparatively equable temperature of sea-water, and in the usual absence of strong currents, these small larvæ, as Huxley shewed later in his volume on the _Crayfish_, live a free life, obtaining their own food, and by a series of slow transformations gradually acquire the adult form. In fresh water, however, the delicate larvæ would be unable to live, and the mode of development is different. The series of slow transformations is condensed, and takes place almost entirely inside the egg-shell; so that, when hatching occurs, the young crayfish is exceedingly like the adult. Apart from such special cases, it is true that the study of development affords a clear test of closeness of structural affinity. Huxley then proceeds to discuss the development of man. "Is he something apart? Does he originate in a totally different way from dog, bird, frog, and fish, thus justifying those who assert him to have no place in nature, and no real affinity with the lower world of animal life? Or does he originate in a similar germ, pass through the same slow and gradually progressive modifications, depend on the same contrivances for protection and nutrition, and finally enter the world by the help of the same mechanism? The reply is not doubtful for a moment, and has not been doubtful any time these thirty years. Without question, the mode of origin, and the early stages of the development of man are identical with those of animals immediately below him in the scale; without doubt, in these respects, he is far nearer the apes than the apes are to the dog."