158 The American Geologist. seiit.Miiher, 1892 
Also different genera exhibit different rates of mutation. It 
results therefore that the law of mutation must be studied sep- 
arately for each genus, and even then the aceelerative effect of 
changed environment is not known, although it is within the 
reach of investigation. 
Another difficulty in the way of close application of these laws 
in determination of age is that <i priori it is impossible to tell 
whether the differences exhibited by two closely allied forms are 
varietal and associated with changed environment, or mtifational 
and associated with the paleontological evolution of the race. 
The study of these problems must therefore be intimately' asso- 
ciated with minute regard to stratigraphic sequence, just as in 
deciphering a manuscript, the succession of the words is essential 
to a correct interpretation of the writing. 
The waj' in which the paleontologic record supplements the 
stratigraphic evidence is seen in the fact that the paleontology is 
capable of showing gaps or omissions, the length and nature of 
which cannot l)e calculated from the strata themselves. 
Another mode of investigation has been employed in which 
the modifications of a particular part of an organism are made 
the subject of inquir}'. 
The case of the toes of the ancestors of the horse, from the five- 
toed Eohippus to the one-toed modern horse, the camels, from the 
Poebrotherium of the Miocene to the Pliocene and recent Auch- 
enia, as shown in the bones and in the teeth, are examples. 
A typical illustration is the development of the sutures of the 
tetrabranchiate cephalopods. 
It will be remembered that the distinguishing feature of the 
cephalopod shell is its chambers, thus separating it from the 
shell of the gasetropod. The edges of the partitions forming the 
chambers where they meet the external walls of the shell, are 
technically called sutures. In the Xautilian shell, whether ex- 
hibiting a simple elongated cone as in Orthoceras, or a curved 
horn as in Gyroceras or Cj'rtoceras, or a close coiled shell as in 
Nautilus, the sutures are simple. In other groups of chambered 
shells the suture line is wavy, forming lobes and saddles or vari- 
ously crimped, as in Goniatites or Ceratites. 
These suture lines form a regular series which both in time of 
initiation and in the period of dominance express a simple law of 
evolution. 
