26 PHILOSOPHY OF ZOOLOGY. 



theory, but stands in opposition, viz. the production of 

 hybrids, or the offspring of the union of the males and 

 females of different species or genera. Thus, in the case 

 of the mule, the produce of the ass and the mare, which 

 partakes of the qualities of both parents, it is obvious, 

 that the pre-existent germ of the female was more than 

 stimulated tolhVby the sexual union, that its structure 

 was likewise changed, and that all the germs of its future 

 offspring were likewise annihilated, since mules are seldom 

 fertile. It fails, likewise, to account for the superior fer- 

 tility of cultivated plants or domesticated animals 



Before dismissing this part of our subject, it is necessary 

 to take notice of those facts illustrative of the origin of 

 organized beings, which have been ascertained by the re- 

 searches of modern geologists. In investigating the struc- 

 ture and composition of the rocks which constitute the 

 crust of the earth, it is observed, that they enclose the 

 remains of animals or vegetables, more or less altered in 

 their texture. Presupposing that those rocks on which all 

 the others rest are the most ancient ; and after dividing 

 them according to their age, as determined by their super- 

 position, it has been ascertained, that the organic remains 

 found in the older rocks differ from those which occur in 

 the more recent strata, and that they are all different from 

 the plants and animals which now exist on the surface 

 of the globe. It likewise appears, that the petrifactions 

 contained in the newer strata, bear a nearer resemblance to 

 the existing races, than those which belong to the rocks of 

 an older date. That the remains of those animals which 

 have always been the companions of man, are only to be 

 found in the most recent of the alluvial deposites. In the 

 older rocks, the impressions of the less perfect plants, such 

 as ferns and reeds, are more numerous than those of the 

 dicotyledonous tribes, and the remains of shells and co- 



