Each of the geological periods has its dominating representative type of life. Perhaps it may he asked: "How can we be satisfied that the members of this long series are strictly the successive descendants by evolution from older forms, and in their turn the progenitors of the later? How do we know that they have not been introduced by sudden creations, and removed by sudden extinctions?" Simply for this reason: The new groups make their appearance while yet their predecessors are in full vigor. They come under an imperfect model which very gradually improves. Evolution implies such lapses of time. Creation is a sudden affair.
A striking illustration of this is offered by two of the most imposing types of life, the reptile and the mammal. The former is the characteristic of the Secondary, the latter of the Tertiary period. In the Secondary, when reptile life was at its culmination, there were reptiles flying in the air, swimming on or in the sea, crawling on the land, or climbing the trees. After this type of life had reached its culmination, and extinction began to set in, that process went forward in a gradual and orderly way. The flying lizards were the first to disappear, then those of the sea; they now have scarcely any representative left. The fluviatile and terrestrial ones, though greatly diminished both in numbers and size, still maintain a struggle for life; but the complete dying out of animated forms, though irresistible, requires for its completion countless centuries.
While reptile life was in full vigor, mammal life was introduced. It came under the lowest forms, the imperfect orders appearing first.
What does this coexistence of two different forms of life, through immense lapses of time—the one declining and on its way to disappearance, the other marching forward to increase—what does this overlapping mean? Not sudden creation, but slow development. The environment is slowly becoming unsuitable to the one, and slowly becoming suitable to the other.
If time permitted, I would ask your close attention to rudimentary organs, for they illustrate strikingly the theory of evolution. They are organs existing in an apparently undeveloped and useless condition, such, for instance, as the incisor teeth in the mid-bone of the upper jaw in embryos of common cattle, the rudimentary wings of the penguin and dodo, the mammæ of the male mammalian, the subcutaneous feet of certain snakes. In the embryos of whales teeth are found in the jaw, precisely as we find them at birth in the human infant. In the latter instance, we think we see a wise provision and foresight of Nature, which does not give to man these masticatory organs before the time they are wanted. But what are we to make of the parallel case of the whale? Shut up as these rudimentary teeth are in the interior of the jaw, never to be developed and never to be used, does not that look something like a useless work? And why has Nature,