488 ^ 
CONCLUSION. 
Chap. XIV. 
be able to gauge with some security the duration of 
these intervals by a comparison of the preceding and 
succeeding organic forms. We must be cautious in 
attempting to correlate as strictly contemporaneous 
two formations, which include few identical species, 
by the general succession of their forms of life. As 
species are produced and exterminated by slowly act¬ 
ing and still existing causes, and not by miraculous 
acts of creation and by catastrophes; and as the most 
important of all causes of organic change is one which 
is almost independent of altered and perhaps suddenly 
altered physical conditions, namely, the mutual relation 
of organism to organism,—the improvement of one being 
entailing the improvement or the extermination of 
others; it follows, that the amount of organic change in 
the fossils of consecutive formations probably serves as a 
fair measure of the lapse of actual time. A number of 
species, however, keeping in a body might remain for a 
long period unchanged, whilst within this same period, 
several of these species, by migrating into new countries 
and coming into competition with foreign associates, 
might become modified; so that we must not overrate 
the accuracy of organic change as a measure of time. 
During early periods of the earth’s history, when the 
forms of life were probably fewer and simpler, the rate 
of change was probably slower; and at the first dawn 
of life, when very few forms of the simplest structure 
existed, the rate of change may have been slow in an 
extreme degree. The whole history of the world, as at 
present known, although of a length quite incompre¬ 
hensible by us, will hereafter be recognised as a mere 
fragment of time, compared with the ages which have 
elapsed since the first creature, the progenitor of innu¬ 
merable extinct and living descendants, was created. 
In the distant future I see open fields for far more 
