356 CHANGES IN THE ORGANIC WORLD. 
The changes that have taken place in the forms 
of the animal and vegetable kingdoms, are not less 
striking than those which we have above described in 
the inorganic world. The animals and plants of the 
more ancient strata, are not only such as could not 
now exist in the latitudes which they formerly in- 
habited, but almost all the species, and very many 
of the genera, are no longer to be found in any 
part of the known globe. In the newer deposits, 
on the contrary, we perceive an intermixture of 
existing with extinct species; the proportion of the 
former increasing according to the more recent 
formation of the strata, till, in the deposits of the 
modern era, the remains of existing species alone 
are discovered, and, as we have already remarked, 
in these accumulations of debris, the skeletons of 
man, and traces of the works of art of the early 
tribes of our race, are sometimes found imbedded. 
The extinction of whole genera of animals and 
plants has, no doubt, depended on various causes. 
In the earlier revolutions, the vicissitudes of climate, 
and the mutations of land and water, were, pro- 
of the whole globe, being a perfectly fair conclusion, from what we 
know of continental, insular, and oceanic climates by actual observ- 
ation. Here, then, we have, at least, a cause on ivhich a philosopher 
map consent to reason ; though, whether the changes actually going on 
are such as to warrant the whole extent of the conclusion, or are even 
taking place in the right direction, may be considered as undecided till 
the matter has been more thoroughly examined.” Another possible 
cause of the refrigeration of the earth is to be found in the “ diminution 
of the excentricity of the earth’s orbit round the sun; and which, as a 
general one, affecting the mean temperature of the whole globe, and as 
one of which the effect is both inevitable, and susceptible, to a certain 
degree, of exact estimation, deserves consideration.” — Discourse on the 
Study of Natural Philosophy, p. 146, 147. 
