154 The American Geologist. September, i892 
This principle involves the fact that each species has a limited 
life period, but it does not involve, in Lyell's first usage of it, 
the fact that one species is necessarily descended from another. 
Before this latter law was accepted as a fact, the natural se([uence 
of species, and of genera and orders was known. Lamarck had 
advanced the idea of the spontaneous origin and progressive de- 
velopment of the organisms of the earth, but it is interesting to 
note the fact that the natural sequence of ditt'erent orders of be- 
ings was generall}' accepted before it was granted that natural 
descent was the explanation of the sequence. Deshayes maintained 
that the species of the Cretaceous were all extinct, and in this fact 
was found the ground for separating the ♦Tertiary order of rocks 
from the Secondar}', the former alone containing shells identical 
with those now living. 
The laws of geographical distribution, with resultant modifica- 
tions of those combinations of species called fauna and flora, and 
the modification of the species themselves in their adjustments to 
changed environment, are sufficient to explain the imperfections 
of the Lyellian principle of determining relative antiquity of for- 
mations by the mere numerical proportion of recent species they 
contain, but as a general principle it is satisfactory. The same 
principle of numerical comparison when applied to genera, fami. 
lies and orders furnished the basis for the classification of the geo- 
logical series into Cenozoic, Mesozoic and Paleozoic. 
The mere tiumerical comparison of faunas is incapable of very 
miiuite application in marking the chronological scale, for the 
reason that the life period or range of species is often ecpial to that 
of a geological perioci, and the life period of a genus may span 
two or three systems. In these particulars I refer specially to 
invertebrates. Land verteljrates express a much greater sensi- 
tiveness to changes of environment, but as a means of determin- 
ing the geological age of strata, they are of such rare occuri'ence 
as to be practically useless for the general geologist. Vertebrates 
when they are present, as well as plants, are of extreme value as 
time indicators. Thus it is evident that we owe to comparative 
paleontology, and not to stratigraphy or lithology, the primarv 
classification of the geological scale, and the means of distinguish- 
ing the chronological position of each formation. 
A second invaluable service of paleontology to geology is found 
in the application of the law of succession of the great groups of 
