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 and even sometimes to existing families. Thus, in the Eocene we have remains of the opossum family; bats apparently belonging to living genera; rodents allied to the South American cavies and to dormice and squirrels; hoofed animals belonging to the odd-toed and even-toed groups; and ancestral forms of cats, civets, dogs, with a number of more generalised forms of carnivora. Besides these there are whales, lemurs, and many strange ancestral forms of proboscidea.

The great diversity of forms and structures at so remote an epoch would require for their development an amount of time, which, judging by the changes that have occurred in other groups, would carry us back far into the Mesozoic period. In order to understand why we have no record of these changes in any part of the world, we must fall back upon some such supposition as we made in the case of the dicotyledonous plants. Perhaps, indeed, the two cases are really connected, and the upland regions of the primeval world, which saw the development of our higher vegetation, may have also afforded the theatre for the gradual development of the varied mammalian types which surprise us by their sudden appearance in Tertiary times.

Notwithstanding these irregularities and gaps in the record, the accompanying table, summarising our actual knowledge of the geological distribution of the five classes of vertebrata,