Again, the question of types seems to be pointed to in the curious fact of what I may call the double development of birds from reptiles. Mr. Mivart says, "If one set of birds sprang from one set of reptiles and another set from another set of reptiles, the two sets could never by 'natural selection' only have grown into such perfect similarity." Yet we can trace the _Struthious_ birds (those that, like ostriches, do not fly) through the Dinosaurs and _Dinornis_, and the flying Carinate birds though pterodactyles, _Archaeopteryx_, and _Icthyornis_, &c. It might well be added to this part of the subject, that granted that developmental changes were often small, that progress was attained little by little, this does not appear to have been always the case. The discoveries of the fossil species of horse,[1] _Eohippus, Hipparion_, and so forth, clearly establish a developmental series, and the ancient forms are claimed as the ancestor of the modern horse; but these (Professor Owen tells us) differed more from one another than the ass and the zebra (for instance) differ from the horse. Still, of course it may be that there are still undiscovered intermediate forms; and in any case there need be no desire to detract from the value of the series, as really pointing towards a gradual perfection of the horse from a ruder ancestor up to the latest type. But having reached the type, and though that type exhibits such (considerable) variations as occur between the Shetland pony, the Arab, and the dray-horse, we have still no difficulty in recognizing the essential identity; nor is there any evidence or any probability that the horse will ever change into anything essentially different. All the fossil bats, again, were true bats: and so with the rhinoceroses and the elephants. Granting the fullest use that may be made of the imperfection of the geological record, it is difficult to account for this, and still more for the absence of intermediate forms (particularly suitable for preservation) of the _Cetaceae_. The Zeuglodons from Eocene down to Pliocene, the Dolphins in the Pliocene, and the _Ziphoids Catodontidae_, and _Balaenidae_ in the Pliocene, are all fully developed forms, with no intermediate species. [Footnote 1: The series is thus (Nicholson, p. 702):--1. _Eohippus_--Lower Eocene of America; fore-feet have four toes and a rudimentary thumb or pollex. 2. _Orohippus_ (about the size of a fox)--Eocene. 3. _Anchitherium_--Eocene and Lower Miocene; three toes, but 2 and 4 are diminutive. 4. _Hipparion_--Upper Miocene and Pliocene; still three toes, but 3 more like the modern horse and 2 and 4 still further diminished. 5. _Pliohippus_--later Pliocene, very like Equus. 6. _Equus_--Post-Pliocene.]