IN THE WAKE OF THE “ CHALLENGER.' 
29 
which all traces were obliterated. The application of this 
hypothesis might reconcile the discrepancy between biological 
theory and geological fact. 
1. There is evidence that the present species of animals and 
plants have arisen by gradual modification of pre-existing 
species. 
2. There is also evidence that, geologically speaking, this 
process has extended over a longer period than that of the 
fossiliferous rocks. 
3. Nevertheless, beneath the latter there lies a great thick- 
ness of formations almost entirely azoic. 
Such researches consequently lend support to the views of 
those geologists who find the explanation of the past history of 
the earth in the present operations of Nature — views held half 
a century ago by Sir H. de la Beche, and developed by Sir C. 
Lyell, the able supporter of the Uniformitarianism enunciated 
by Hutton. 
Want of space and pressure of time permit of no more than 
a passing allusion to the interesting controversy which has been 
going on for several months between Dr. Carpenter and Mr. 
Croll, of the Scottish Geological Survey, on the subject of 
“ The Crucial Test of the Challenger .” * Mr. Croll’s position 
seems to be that all the great movements of ocean-water, deep 
as well as superficial, depend upon the action of winds upon its 
surface. Though admitting that the Polar water finds its way 
along the floor of the great ocean basin into the Equatorial 
area, he affirms that this is merely the reflux of the current 
which has been driven into the Polar basin by the agency of 
winds. Dr. Carpenter admits that the current movements of 
surface- water are, for the most part, produced by winds, but 
all these belong to a horizontal circulation which tends to 
complete itself ’, a surface indraught being produced when a 
surface outflow is kept up, but that the deep movements are 
the result of a vertical circulation , maintained by the continu- 
ance of a disturbed equilibrium between Polar and Equatorial 
columns, occasioned by the surface action of Polar cold and 
Equatorial heat. It seems, too, that Dr. Carpenter’s views as 
regards a vertical circulation are in accordance with the opinions 
of Emil Lenz, a physicist who accompanied Kotzebue in his 
second voyage of circumnavigation in 1823-26. As, however, 
the controversy does not yet appear to be at an end, and as the 
subject properly concerns a physicist, any expression of opinion 
on the part of the writer would be of but little value. 
* See u Nature,” vol. ix. p. 423, and other papers in the same periodical, 
as well as a series extending over several numbers of the (i Philosophical 
Magazine.” 
