Time and Change by John Burroughs — Read

Can we fail to see the significance of the order in which life has appeared upon the globe -- the ascending series from the simple to the more and more complex? Can we doubt that each series is the outcome of the one below it -- that there is a logical sequence from the protozoa up through the invertebrates, the vertebrates, to man? Is it not like all that we know of the method of nature? Could we substitute the life of one period for that of another without doing obvious violence to the logic of nature? Is there no fundamental reason for the gradation we behold?

All animal life lowest in organization is earliest in time, and vice versa, the different classes of a sub-kingdom, and the different orders of a class, succeeding one another, as Cope says, in the relative order of their zoological rank. Thus the sponges are later than the protozoa, the corals succeed the sponges, the sea-urchins come after the corals, the shell-fish follow the sea-urchins, the articulates are later than the shell-fish, the vertebrates are later than the articulates. Among the former, the amphibian follows the fish, the reptile follows the amphibian, the mammal follows the reptile, and non-placental mammals are followed by the placental.