A SPECIES AND A NEW KIND OF TYPE SPECIMEN 477 
There exist the so-called geographical species which show no 
marked morphologic differences. Modern biologists make such 
distinctions in color or in the nature of the hair or muscles; these 
are, however, criteria necessarily lost to the paleontologist, who 
generally has only the hard parts for study. The student of 
vertebrate fossils is further limited, as a rule, in his making of 
species, to a single specimen, while the student of modern biology 
has many cotypes or paratypes. 
Considering the vastness of geological time, it is easy to see 
that two specimens of the same geological formation may have 
been separated by tens of thousands of years, during which impor- 
tant changes not only of habitat but as well of form and habits may 
have occurred. In such a case, nothing of the resulting trivial 
changes in the soft parts may be discerned. 
Relativity of species.—Generally the small unit characters have 
value only when grouped; not always does a specific feature fully 
determine the bounds of a species. But on the other hand, no 
matter how trivial this character may appear to be, if we find it 
occurring constantly in one set of specimens and not in others, it 
is to be regarded as specific in value and typifies the individuals 
of that group. Furthermore, not all the varieties referred to a 
species may have all of the so-called specific characters. Two 
specimens under the same species may have but one or two only of 
the several secondary features supposed to characterize the group 
as a whole. 
Distinguishing characters are of two classes, relative and 
absolute. Sometimes a relative difference may be advanced so 
far that it appears to be, or may actually become, absolute; for 
example, the growth of a bone until it meets a joint and develops 
a facet, or the reduction of a premolar to a point where it becomes 
obsolete. The so-called absolute characters may be thought of as 
unit characters. Relative characters may be illustrated by the 
familiar variation in size, and then we wonder what the limits of 
size are beyond which animals cannot interbreed, for we realize 
how positive a barrier this may become. Im this respect it seems 
safe to say that in mammals a size difference of 30 per cent would 
