244 EXPLORATION OF THE CANONS OF THE COLORADO. 
ability to define species satisfactorily is a very good gauge of our ignorance 
of the whole truth. 
But naturalists practically work, for the most part, upon the surface 
of the subjects presented to their examination, not necessarily concerning 
themselves at all times with what lies hidden underneath. There are occa 
sions to speculate and theorize, and there are other times when a naturalist 
may legitimately ignore underlying principles, and properly content himself 
with statement of certain observed facts. Working on this plane, as I am 
in the present instance, it is my business to render an exact account of what 
I find the case of Thomomys to be, without reference to abstract questions 
involved ; and to sum the statement in such nomenclatural terms as I may 
judge most suitable to express the relationships believed to subsist between 
the several differentiations which the genus has undergone. In describing 
the several forms of Thomomys, I waive the question of species ; no harvest 
would be garnered if the laborers waited till they learned how the grain 
grew. In studying my specimens, I find that one of them differs from all 
the rest to such a degree that its characters are totally irreconcilable with 
those of any others. My description of it is merely an amplification of this 
statement. I give it a name, and call it a species, conventionally, simply in 
recognition of this fact, and for the usual purpose of readily indexing the 
items of information the specimen affords. I find, furthermore, that all the 
other specimens collectively present a varying sum of characters, according 
to difference in the emphasis of each one of these characters common to all ; 
and that their interrelation or intergradation is so intimate and complete that 
no lines of precise distinction can be drawn ; but that, nevertheless, an average 
difference in three directions may be readily perceived and described intel 
ligibly. It is an undoubted fact, furthermore, that these three differentiations 
are related in some way to certain geographical areas, for the simple reason 
that all the specimens of one style are produced in certain portions of the 
country, and all of the rest in certain other regions ; and that the Thdinomys 
existing on the confines of the several areas share each others' peculiarities. 
It is, once again, within the experience of those who have studied such 
subjects in their general bearings, that the aspects of the case presented by 
Thomomys tally exactly with those determined in a great many other cases. 
