516 
THE GEOLOGIST. 
The interior higher temperature of the earth, the higher climatal 
temperature of the early geological periods, the lower tempera- 
ture of the glacial periods, the earth's form and density, together 
with the astronomical phenomena of the precession of the equinoxes, 
and the corresponding inequality in the moan's motion, must all be 
accounted for by any complete geological theory. In proceeding, then, 
to theoretical considerations, in the first place we may remark that the 
spheroidal form of the earth could not fail to suggest the idea of fluidity 
— a state to which no conceivable cause but excessive heat could reduce 
it, but which was not necessarily the earth's primitive state, but an 
antecedent state through which it must have passed before it became 
solid. 
Moreover, the observed increase of temperature in descending beneath 
the earth's surface suggested the notion of a central incandescent nucleus 
still remaining in a state of fluidity from its elevated temperature. 
Hence the theory that the exterior crust had assumed its present solidity 
by the radiation of heat into surrounding space and its consequent 
refrigeration. 
This central heat was formerly thought a sufficient cause for the more 
equable climate over the earth's surface at remote geological periods ; but 
it manifestly could not account for a temperature in this glacial period 
so much colder than at present. This fact has led to a more careful 
investigation of these causes, and these are found in the relative dispo- 
sition of land and sea, and the effects to be more especially influenced by 
the ocean-currents of warmer or colder water. 
The influence of the Gulf-stream upon our own climate is well 
known, and with such a powerful example of climatal alteration before 
us, we must be cautious how we appeal to other causes as necessary to 
account for a higher temperature. 
The rival theory of this primitive and central heat is that which 
attributes the internal terrestrial temperature, as well as volcanos, to 
chemical action. 
The question of a continuous progression of our globe from a primi- 
tive towards a final physical state considered with reference to the 
evidence supplied by organic remains on the question of progres- 
sion or non-progression, has, perhaps, been more or less associated in 
the minds of speculative geologists with that of the trammutation or 
permanence of species. 
Mr. Hopkins discusses here the necessary distinction of a species ; 
but without following him in those remarks, we enter at once into the 
main topic. 
The question really involved is that of the transmutation or perma- 
nence of species is, whether species — using the term in its wider and 
more unrestricted sense — have any existence in nature or not. 
That different races of animals have been introduced on the face of 
the globe at successive epochs, is testified by their fossil remains, hence 
the question of transmutation or permanence of species has a high 
geological interest ; and those who wish to study the details of this ques- 
tion, may refer to Sir Charles Lyell's presidential address to the 
