You do not allude to one very striking point enough, or at all--viz., the classes having been formerly less differentiated than they now are; and this specialisation of classes must, we may conclude, fit them for different general habits of life as well as the specialisation of particular organs.
Page 162 (192/3.

On page 163 Lyell refers to the absence of Cetacea in Secondary rocks, and expresses the opinion that their absence "is a negative fact of great significance, which seems more than any other to render it highly improbable that we shall ever find air-breathers of the highest class in any of the Primary strata, or in any of the older members of the Secondary series.") I rather demur to your argument from Cetacea: as they are such greatly modified mammals, they ought to have come in rather later in the series.

You will think me rather impudent, but the discussion at the end of Chapter IX.

on man (192/4.

Loc.