LIFE AT GREAT DEPTHS. 395 
ontologists who began to assert that periods were perfectly artificial 
notions — that it did not follow, because one set of deposits was 
forming in one part of the world, others exactly corresponding to 
it elsewhere, so. far as the organic remains are concerned, were 
contemporaneous — and that life had progressed on the globe con- 
tinuously and without a break from the dawn of it to the present 
time. 
The persistence of some species through great vertical ranges 
of strata, and the relation between the world-wide distribution of 
forms and this persistence, were noticed by D’Archiac, De Ver- 
neuil, Forbes and others. The identity of some species in the re- 
mote natural-history provinces of the existing state of things was 
established in spite of the dogmatic opposition of authorities ; 
and then geologists accepted the theories that there were several 
natural-history provinces during every artificial period, that some 
species lived longer and wandered more than others, and tha 
some have lasted even from the paleozoic age to the present. 
Persistence of type was the title of a lecture delivered by Pro- 
fessor Huxley * many years ago; and this persistence has been 
admitted by every palzontologist who has had the opportunity of 
examining large series of fossils from every formation from all 
parts of the world. 
Geological ages are characterized by a number of organisms 
which are not found in others, and by the grouping of numerous 
species which are allied to those of preceding and succeeding 
times, but which are not identical. Certain portions of the 
world’s surface were tenanted by particular groups of forms dur- 
ing every geological age; and there was a similarity of arrange- 
ment in this grouping under the same external physical conditions. 
To use Huxley’s term, the ‘‘ homotaxis” of certain natural-history 
provinces during the successive geological ages has been very 
exact. The species differed; but there was a philosophy in the 
consecutive arrangement of high-land and low-land faunas and 
floras, and those of shallow seas, deep seas, oceans and reef-areas. 
he oceanic ł conditions, for instance, can be traced by organic 
remains from the Laurentian to the present time, and the deep-sea 
corals now under consideration are representative of those of older 
deep seas. 
* Royal Institution. See also President’s ee Geol. Soc., 1870. 
P. M. Duncan, Quar. Jour. Geol. Soc., No. 
