EVOLUTION THE RESULT OF CHEMICAL FORCES. 107 



found in various parts of the world and north of the equator, 

 fossil specimens of a far greater variety of marsupial species. 

 Among them are gigantic skeletons corresponding to the hippo- 

 potamus and rhinoceros, and to the large hoofed-animals and 

 insectivora, to the giant sloth and armadillo, to lions and the 

 ape-footed animals. Thus it is seen that there were at one time, 

 spread all over the continents, classes of marsupial animals filling 

 all the spheres and presenting all the similitudes of the quadru- 

 peds of the present day. Why they were not sufficient for all 

 the requirements of nature and did not persist to the end, are 

 questions which natural selection does not answer. 



But for some reason, whatever it may have been, the marsu- 

 pial type was not found to be entirely satisfactory. Apparently 

 it was not the best that nature could do, and so it was allowed 

 quietly to pass away, and another order of animals, with im- 

 proved methods of reproduction and a better quality of brain, 

 was. brought forward to take its place. The distinguishing 

 quality of the placental mammalia is that the fetus has direct 

 connection, by means of the membrane called the placenta or 

 after-birth, with the circulatory systems of the parent, and is 

 brought forth in nearly a full state of development. It was a 

 great advance over the clumsy arid imperfect processes of the 

 marsupials, and it was at once accompanied by more graceful 

 and symmetrical forms, by larger and more convoluted brains, 

 and a much higher standard generally of animal life. But not 

 the least trace of any intermediate races, or connecting links be- 

 tween the two orders, have ever been found. And it is impossi- 

 ble to conceive that there could be any. Either there must have 

 been full uterine connection and sustenance of the fetus until 

 mature, or there could have been none at all. A fractional or 

 partial development of a placenta would have answered no pur- 

 pose ; for that would not have sustained the fetal life. Any 

 gradual lengthening of the period of gestation, even if we could 

 conceive of any means by which it could be brought about, 

 would seem to be out of the question ; for what possible advan- 

 tage could it be to a kangaroo for instance to carry its unborn 

 young six weeks instead of five ? 



