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.
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