228 DR. BACHMAN S ZOOLOGY. [CHAP. XVI. 



every evening, for several weeks, near Philadelphia, on two tall 

 oaks, in the autumn, when acorns and chestnuts were abundant, 

 and when they had spare time for play. They were amusing 

 themselves by passing from one tree to another, throwing them 

 selves off from the top of one of the oaks, and descending at a 

 considerable angle to near the base of the other ; then inclining 

 the head upward just before reaching the ground, so as to turn and 

 alight on the trunk, which they immediately climbed up to repeat 

 the same mano3uvre. In this way there was almost a continuous 

 flight of them crossing each other in the air between the two trees. 



I had heard much of the swamp-rabbit, which they hunt near 

 the coast in South Carolina and Georgia, and was glad to see a 

 stuffed specimen. It is an aquatic hare (Lepus palustris), diving 

 most nimbly, and outswimming a Newfoundland dog. 



Dr. Bachman pointed out to me ten genera of birds, and ten 

 of quadrupeds, all peculiar to North America, but each repre 

 sented on the opposite side of the Rocky Mountains by distinct 

 species. The theory of specific centers, or the doctrine that the 

 original stock of each species of bird and quadruped originated in 

 one spot only, may explain in a satisfactory manner one part of 

 this phenomenon ; for we may assume that a lofty chain of 

 mountains opposed a powerful barrier to migration, and that the 

 mountains were more ancient than the introduction of these par 

 ticular quadrupeds and birds into the planet. But the limitation 

 of peculiar generic types to certain geographical areas, now ob 

 served in so many parts of the globe, points to some other and 

 higher law governing the creation of species itself, which in the 

 present state of science is inscrutable to us, and may, perhaps, 

 remain a mystery forever. The adaptation of peculiar forms, 

 instincts, qualities, and organizations to the present geography 

 and climate of a region, may be a part only of the conditions 

 which govern in every case the relations of the animate beings 

 to their habitations. The past condition and changes of the 

 globe and its inhabitants, throughout the whole period when the 

 different beings were entering, each in succession, upon the scene, 

 and all the future conditions and changes to the end of vast 

 periods, during which they may be destined to exist, ought to be 



