INTRODUCTION. _ 3 
desirable. The fuller and more accurate the knowledge which the 
geologist has of kindred branches of inquiry, the more interesting 
and fruitful will be his own researches. From its very nature 
geology demands on the part of its votaries wide sympathy with 
Investigation in almost every branch of natural science. Especially 
“necessary is a tolerably large acquaintance with the processes now at 
work in changing the surface of the earth, and of at least those 
forms of plant and animal life whose remains are apt to be preserved 
in geological deposits, or which in their structure and habitat enable 
us to realise what their forerunners were. 
It has often been insisted upon that the present is the key to the 
past; and in a wide sense this assertion is eminently true. Only in 
‘proportion as we understand the present, where everything is open 
on all sides to the fullest investigation, can we expect to decipher 
the past, where so much is obscure, imperfectly preserved, or not 
preserved at all. A study of the existing economy of nature ought 
evidently to be the foundation of the geologist’s training. 
While, however, the present condition of things is thus employed, 
we must obviously be on our guard against the danger of uncon- 
sciously assuming that the phase of nature’s operations which we 
now witness has been the same in all past time; that geological 
changes have taken place in former ages in the manner and on the 
scale which we behold to-day, and that at the present time all the 
ereat geological processes, which have produced changes in the past 
eras of the earth’s history, are still existent and active. Of course 
we may assume this uniformity of action, and use the assumption as 
a working hypothesis. But it ought not to be allowed any firmer 
footing, nor on any account be suffered to blind us to the obvious 
truth that the few centuries wherein man has been observing nature 
form much too brief an interval, by which to measure the intensity 
of geological action in all past time. For aught we can tell the 
present is an era of quietude and slow change, compared with some 
of the eras that have preceded it. Nor can we be sure that, when 
we have explored every geological process now in progress, we have 
exhausted all the causes of change which, even in comparatively 
recent times, have been at work. 
- In dealing with the Geological Record, as the accessible solid 
part of the globe is called, we cannot too vividly realise that at the 
best it forms but an imperfect chronicle. Geological history cannot 
be compiled from a full and continuous series of documents. Owing 
to the very nature of its origin the record is necessarily from the 
first fragmentary, and it has been further mutilated and obscured by 
the revolutions of successive ages. And even where the chronicle of 
events is continuous, it is of very unequal value in different places. 
In one case, for example, it may present us with an unbroken succes- 
sion of deposits many thousands of feet in thickness, from which, 
however, only a few meagre facts as to geological history can be 
gleaned. In another instance it brings before us, within the compass 
3 B 2 
