Williams] ANIMAL AND PLANT AGGREGATES. 17 
formation to another. This use of the term range is illustrated by 
the phrase "Atry pa reticularis has a long geological range in Paleozoic 
time." The range of fish must be carried below the Devonian and 
Silurian (where it was previously supposed to begin) because of the 
discovery of the wonderful fish remains in the Harding sandstone of 
Canyon, Colo. , associated with a Trenton limestone invertebral e fa una. 
In order to discuss the problems of the time relations of organisms 
it is necessary to use the terms range ami distribution to refer respec- 
tively to geological and geographical space, and to note that the facts 
concerning the range of species and genera are stated in terms signi- 
fying position in and thickness of formations. Range in time, often 
referred to, must be determined by relationship of the faunas or 
species to one another, and this is another method of the discrimina- 
tion of the faunas, a method which is neither geographical nor geolog- 
ical, but, as we shall see, organic, and which is strictly a measure of 
the life history of organisms in evolutional succession one to another. 
The importance of the distinction between range and distribution, 
as applied to fossils, is apparent when it is considered that the evolu- 
tion or modification of the form of organisms may be coincident either 
with change of place during the same epoch of time or with passage 
of time in the same area of space. Fossils can be used as indicators 
of uniformity of geological horizon only within the limits of their 
modification by conditions of geographical distribution. If the form 
of a fossil varies according to the nature of the sediments in which it 
is buried, indicating different conditions of life, the extent of that 
variation and the relation of the change of form to the particular 
nature of the sediments must be observed before the characters of the 
fossils can be accurately applied in discriminating their age. 
It has been ascertained, as will be illustrated beyond, that a fossil 
species may recur at successive zones for a thousand or more feet of 
thickness of strata without showing greater modification of form 
than is expressed in specimens of the same species obtained from the 
same stratum. It can also be shown that the species making up the 
fauna of rocks not over 100 miles distant from each other, which by 
other means are proved to be at the same geological horizon, may 
present greater differences than the successive faunas of a single sec- 
tion extending over a range of many hundreds of feet. These facts 
lead to the discrimination of the idea of variation and to the applica- 
tion of that term to indicate differences expressed by specimens of 
the same species — differences arising coincidently with extension of 
geographical distribution and change in conditions of environment; 
while the term mutation is technically applied to those changes of form 
that are coincident with passage of time, and hence to generational 
succession under conditions of life so nearly /he same that extinction 
of the race does not residt. 
In treating of the relations of organisms to time and of their evolu- 
Bull. 210—0:5 — r-2 
