468 RECENT ADVANCES IN GEOLOGY. 
deep-sea animals than those dwelling on the land; but the 
investigations which I have cited, show that while the sea- 
fauna has undergone slight modifications since the dawn of 
the Cretaceous Epoch, the land-fauna has been subjected to 
the most marked deviations. 
May not, then, these fluctuations of temperature be due 
to causes which operate from the exterior? It is necessary 
to assume that, throughout the lapse of all time, our planet 
has occupied its present relation to the sun, or the solar sys- 
tem? Is not the recession of Sirius, which is now going on, 
an argument against the fixity of the siderial heavens? 
We are assured that ours is not a central sun, but one in 
the great possession of stars which is sweeping towards the 
constellation Hercules ; and that in the region of either there 
are spaces of densely-clustered stars, and other spaces which 
are comparatively barren. Now every star is a sun, emitting 
light and heat, a portion of which is transmitted to us. Our 
planet at this time is moving through one of those starless 
spaces, and therefore is not in a position to receive the full 
influence of such a cause. The distinguished Swiss botanist, 
Heer, to whom we are so largely indebted for our knowl- 
edge of the Miocene flora, has suggested that it is to this’ 
source rather than to telluric causes we are to resort to 
explain the varying distribution of p as mani- 
fested in past geological times. 
Again: Have we the right to assume that, throughout all 
past ages, the poles of our planet have pointed in the same 
direction? We can conceive that, if its axis were to form 
with the plane of the ecliptic, the same angle which it now 
forms with the equatorial plane, there would ensue an entire 
change of climate, and consequently of organic forms. Why 
should the astronomer insist on the immutability of the 
siderial system, when to the geologist is unfolded a record. 
of seas displaced and continents elevated; of great cycles 
of heat and cold; of the disappearance of old, and the ap- 
pearance of new forms of organic life? Change, not con- 
` stancy, is inscribed on every leaf in the volume of Nature. 
