vfiLLiAMS.1 ANIMAL AND PLANT AGGREGATES. 19 
question of permanency. The evanescence of the varietal character in 
generation is the reason for calling it varietal; when it becomes fixed 
and is repeated without change its rank in the vital economy deter- 
mines whether it he classed as a specific, generic, ordinal, or class 
character. The changing of the characters of all ranks of taxonomic 
value and the length of the reproduction of the several characters 
without change are chiefly the measures of that taxonomic rank, since 
the classification of the organisms into the taxonomic groups, species, 
genus, order, etc., is regarded as natural only when the groups of 
higher rank are strictly inclusive of those of next lower rank; 
and this could happen only when the higher characters were present 
before the distinctions of lower rank were produced. For instance, 
it would be impossible to conceive of the distinctions between two 
genera arising by evolution before the ordinal characters had been 
evolved — i. e., in a natural classification. Hence, the higher the rank 
of the zoological character of an animal the more ancient the history 
of that character. The application of the principle may be expressed 
by saying that in identification of fossil specimens for purposes of 
correlation it is imperatively necessa^ to know the taxonomic rank 
of the characters by which the identification is made. If a generic 
character be interpreted as evidence of a particular species, the cor- 
relation inferred from the fact may be false, since the range of the 
specific character in most cases must be far shorter than that of a 
generic character of the same group of organisms. 
From the preceding remarks it follows that fossils, either as taxo- 
nomic aggregates based on genetic affinities or as aggregates asso- 
ciated on the basis of living together, can not be considered simply by 
morphological features, but that their chronological relations must be 
distinctly noted. In considering a species, the paleontologist must not 
only consider all the descendants of a common parent and those differ- 
ing from them no more than they differ from one another, but must con- 
sider the descendants which do differ, and the length of time during 
which generation continues in the race with retention of the specific 
characters. The idea of continuity of race is an element in the geo- 
logical study of species. 
In like manner a fauna at any particular instant of time includes all 
the species of animals living together under a particular, though very 
complex, combination of environmental conditions. The paleontolo- 
gist has to extend this idea to include also the length of tim,e through 
which the fauna persists without loss of the characters essential to the 
fauna. 
Thus the paleontologist is not only forced to consider the time rela- 
tions of species and faunas, but it is by means of the relations of fossils to 
one another that periods and epochs of geological time are distin- 
guished. 
A living species may be classified by its taxonomic characters and be 
