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 cr 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 to life by 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, hkewise, 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 dividmg 
them according to their age, as determined by their super- 
position, it has been ascertaimed, 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- 
