; The iSco2>e of I\deontoIo(/y.~^WiU{onis. 157 
Although it may be rightly o})jected that evidence is in most 
cases extremely imperfect, and in attempts to fill out paleoutolog- 
Ical series imagination has been freely used in filling the gaps, 
there can be no question as to the immense value to geology of 
the knowledge already acquired in this highly theoretical part of 
paleontology. 
It is the Jijfereiices observed on comparing fossils coming from 
different horizons and different regions that are of value in these 
■determinations, and not the ii.umerical 2^i'"po>'f"J" of . identical 
forms, as in the William Smith method of identifying strata. 
It is the direct interpretation of observed variation and muta- 
tion of organic forms into terms of the amount of geological time 
and the extent of change in environment. 
This comparative paleontology, to be accurately used, must 
deal with the finer details of form and structure, because the evi- 
dence of genetic affinity must be perfectly clear before the series 
can be depended upon as expressive of the true order of evolu- 
tion. 
A remark should be made here upon the limitations to the use 
of fossils as indicative of geological age. 
Granting the general proposition that the ditt'erences exhibited 
by different species of the same genus are variations and muta- 
tions in the descendants of a common stock, still it is not possi- 
ble to decide a priori what the rate of the modification may have 
been. Certain modifications are undergone in a season during 
the ontogenetic development of the individual from the germ cell 
to the adult. In the same way examination of a large number of 
cases in different groups of the Animal Kingdom, shows that in 
many cases there is in the early stage of the life history of a new 
genus, rapid expansion in specific modification, and later on each 
specific line appears to express o\\\y very gradual "mutation" iu 
respect of certain characters in which at its early stage it was 
definitely variable. In other words, upon studying the life his- 
tories of species there appears good evidence of an initial stage 
in which the species present characters in a plastic state; later 
these characters became fixed in each genetic line, and the species 
appear, on this account, to be more distinct in their characters. 
Hence the amount of difference exhibited between two species 
will not arbitrarily indicate their distance apart in the genetic 
series. 
