their contemporary the archæopteryx did; for the constant correlative structure with hot-bloodedness is a non-conducting covering for the body. Prof. Huxley, on the other hand, differing from this anatomist, thinks that, judging from the air-passages in their bones, they were warm-blooded, but that, nevertheless, they were reptiles with special modifications for special purposes.
It would, therefore, appear that we are again face to face with a group which the most eminent authorities are far from agreed whether to regard as reptiles or as birds.
We have now passed in review various remarkable forms—living birds and living reptiles, separated by an immeasurable distance from each other, and forms which have so mingled the characters of both as to present great difficulties to their being included among the members of either group. Starting from the groveling crocodile, we have seen that there existed gigantic crocodile-like forms, such as the giant-lizard and the iguanodon, that walked, sometimes at least, oil their hind limbs; others, like the long-necked, long-tailed compsognathus from the Solenhofen slates, that hopped on the ground after the manner of a bird; then "flying dragons," with birdlike brain and bones that cleft the air with their twenty-feet expanse of wing; next, undoubted birds, with toothed bills, the one with reptilian vertebræ, the other with a beaver-like tail; while last of all, omitting the imperfectly known Sheppey fossil, the feathered archæopteryx whose twenty caudal segments bar its entrance to every existing family of birds.
Without by any means asserting—what is not only far from being ascertained fact, but is indeed very improbable; for we are not in a position to state that they appeared on the earth intermediately between the two groups—that these forms are the direct terms in the series of progressions from reptiles to birds, we can, in their intelligent contemplation, without overstraining the imagination or violating our reason, picture still more modified forms wherein the reptilian and the avian types would so harmoniously blend that we should find it impossible to say, "At this point the line between reptiles and birds must be drawn." There can be no reasonable doubt but that the remains, which only through the circumstance of a happy burial have been preserved to us from the second great era of the world's history till now, are no more than a very few examples with many a blank between of the fauna which has lived and died, whose tombs no man knoweth. Moreover, it seems easy enough to believe, after studying these forms, that, could any human eye have followed from that day to this the waxing and waning of the various animal groups, he could have constructed for us a marvelous chain of existences between reptiles and birds, the conformation of whose unknown links we can almost fabricate in our minds, between which no abrupt transitions harshly jarring would occur, no stepping-stones too wide to stride across; and, handing on to us, besides, the traditions of a still earlier time, he could have pictured