in the Carboniferous era a creature twenty-seven centimetres in length.  Among the mammals of Tertiary times this same law of steady increase in size has been operative, as seen in the Felidae, the stag, and the antelope.  Man himself has, no doubt, been under the same law, and is probably a much larger animal than any of his Tertiary ancestors.  In the vegetable world this process, in many cases, at least, has been reversed, and the huge treelike club-mosses and horsetails of Carboniferous times have dwindled in our time to very insignificant herbaceous forms.

Animals of overweening size are handicapped in many ways, so that nature in most cases finally abandons the gigantic and sticks to the medium and the small.

III

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.

It almost seems as if nature hesitated whether to produce the mammal from the reptile or from the amphibian, as the mammal bears marks of both in its anatomy, and which was the parent stem is still a question.

The heart started as a simple tube in the Leptocardii; it divides itself into two cavities in the fishes, into three in the reptiles, and into four in the birds and mammals.  So the ossification of the vertebral column takes place progressively, from the Silurian to the middle Jurassic.

The same ascending series of creation as a whole is repeated in the inception and development of every one of the higher animals to-day.  Each one begins as a single cell, which soon becomes a congeries of cells, which is followed by congeries of congeries of cells, till the highly complex structure of the grown animal with all its intricate physiological activities and specialization of parts, is reached.  It is typical of the course of the creative energy from the first unicellular life up to man, each succeeding stage flowing out of, and necessitated by, the preceding stage.