160 The Americmy Geologist. September, isoa 
2d. It is the order in which the several types become domi- 
nnnt. 
3d. It is tlie order of ehxhoration in tlie ontogenetic growth 
of the indiAidual. 
4th. Tt is the normal order of physical relation borne by the 
several types to each other; each type is a physical elaboratioa 
of the next preceding t3'pe. 
The convolutions of the suture are crimpings of the edge of a 
more or less flat disc — the septum — and these convolutions are 
the simplest mode of adjustment of the edge of such a disc, 
whose circumference increases more rapidly tnan its radius. 
Assuming this sutural feature to be the only difference in- 
volved in the comparison of the several types, it would be cor- 
rect to state that it would be physically Impossible for the Am- 
monites, septum and suture to be formed without passing through 
the stages represented by the Nautilus, (loniatites and Ceratites. 
In other words the exhaustive anal^'sis of this one element of 
structure of cephalopod shells shows us that the actual history 
of the organisms has been exactly that which a serial classifica- 
tion on the basis of differences of this part would suggest, but 
that no other classification or order of succession could take 
place l)y natural descent. It is unneccessary to speak of the 
value of such series for purely stratigraphical purposes. 
It is but one of a great many such mutations to be discovered 
in the stud}' of comparative paleontology'. The general law in- 
volved is this: in a series of genetically related forms in which 
the .later representatives present a character which is but the 
physical elaboration of that found at a much earlier stage, there 
is implied the presence in the intermediate formations of species 
in which the character is in an intermediate stage of develop- 
ment, and of a continuous series connecting the extreme forms. 
I have thus far spoken of the general scope and application of 
comparative paleontology. I might cite many cases in which 
particular problems have been settled by the use of these methods, 
and refer to the works of Neumayr, Waagen, Kayser, Barrois, 
Gosselet, Freeh, Tschernej'schew, and others in Europe, to the 
investigations of Hall, Whitfield, Whiteaves, White, Mai-sh, 
Cope, Walcott, Herrick, Hill, Prosser, Keyes, Clarke, and of 
Meek, who was one of the keenest of our earlier paleontologists. 
And there are many extremely valuable special investigations 
