All the most marked groups, bats, pterodactyles, chelonians, ichthyosauria, anoura, &c., appear at once upon the scene.

Even the horse, the animal whose pedigree has been probably best preserved, affords no conclusive evidence of specific origin by infinitesimal, fortuitous variations; while some forms, as the labyrinthodonts and trilobites, which seemed to exhibit gradual change, are shown by further investigation to do nothing of the sort.

As regards the time required for evolution (whether estimated by the probably minimum{143} period required for organic change or for the deposition of strata which accompanied that change), reasons have been suggested why it is likely that the past history of the earth does not supply us with enough.

First, because of the prodigious increase in the importance and number of differences and modifications which we meet with as we traverse successively greater and more primary zoological groups; and, secondly, because of the vast series of strata necessarily deposited if the period since the Lower Silurian marks but a small fraction of the period of organic evolution.

Finally, the absence or rarity of fossils in the oldest rocks is a point at present inexplicable, and not to be forgotten or neglected.
Now all these difficulties are avoided if we admit that new forms of animal life of all degrees of complexity appear from time to time with comparative suddenness, being evolved according to laws in part depending on surrounding conditions, in part internal--similar to the way in which crystals (and, perhaps from recent researches, the lowest forms of life) build themselves up according to the internal laws of their component substance, and in harmony and correspondence with all environing influences and conditions.