ON THE HYPOTHESIS OF EVOLUTION. 135 



the lamprey and the shark. If the whole number of these parts 

 does not co-exist in the embryonic man, we find in embryos of 

 lower forms, more nearly related to the lamprey, that they do. 

 Later in the life of the mammal but four aortic origins are found, 

 which arrangement, with the heart now divided into two cham- 

 bers, from a beginning as a simple tube, is characteristic of the 

 class of Vertebrates next in order — the bony fishes. The optic 

 lobes of the human brain have also at this time a great predomi- 

 nance in size — a character above stated to be that of the same 

 class. With advancing development the infant mammal follows 

 the scale already pointed out. Three chambers of the heart and 

 three aortic origins follow, presenting the condition permanent 

 in the Batrachia ; and two origins, with enlarged cerebral hemi- 

 spheres of the brain, resemble the reptilian condition. Four 

 heart-chambers, and one aortic-root on each side, with slight 

 development of the cerebellum, follow, all characters defining the 

 crocodiles, and immediately precede the special conditions defin- 

 ing the mammals. These are, tlie single aorta-root from .one 

 side, and the full development of the cerebellum ; later comes 

 that of the cerebrum also in its higher mammalian and human 

 traits. 



Thus we see the order already pointed out to be true, and to 

 be an ascending one. This is the more evident as each type or 

 class passes through the conditions of those below it, as did the 

 mammal ; each scale being shorter as its highest terminus is 

 lower. Thus the crocodile passes through the stage of the lam- 

 prey, the fish, the batrachian, and the reptile proper. 



h. In Time. — "We have thus a scale of relations of existing 

 forms of animals and plants of a remarkable kind, and such as 

 to stimulate greatly our inquiries as to its significance. When 

 we turn to the remains of the past creation, preserved to us in 

 the deposits continued throughout geologic time, we are not 

 disappointed, for great light is at once thrown upon the sub- 

 ject. 



We find, in brief, that the lowest division of the animal king- 

 dom appeared first, and long before any type of a higher charac- 

 ter was created. The Protozoon, Eozoon, is the earliest of 

 animals in geologic time, and represents the lowest type of 

 animal life now existing. We learn also that the highest branch 

 appeared last. No remains of Vertebrates have been found below 

 the lower Devonion period, or not until the Echinoderms and 



