TAVERNER — TEST OF THE SUBSPECIES 
125 
It is beside the question that in some particular cases such a proceeding 
might conceivably be an advantage for it is recognized that extremes 
of obvious subspecies sometimes differ more in apparent characters 
than do other distinct species. It is thus apparent I think, from the 
standpoint of mere expediency, that the amount of divergence as a 
test of the lower systematic units is open to serious que&tion. 
The only logical ground for applying a quantitative rather than an 
intergradational test to the subspecies is that of the instability of species. 
If species are hquid quantities flowing imperceptibly into each other 
the amount of difference by which they are characterized is the only 
practical means for their recognition. If, however, the species is a 
definite entity it must be cut off sharply from all other similar entities 
and degree of divergence becomes unimportant and isolation (discon- 
tinuity) its final test. Herein lies a conflict of ideas. 
Those who concentrate their attention on the paleontological evidence 
are prone to regard the species as a mere concept, an ever varying 
quantity in constant state of development, adopted for convenience 
in referring to arbitrary points along a continuous line of progress. 
The modern zoologist however finds species the termination of lines 
of descent, and each sharply marked off from the other. As both of 
these reasonings are demonstrable it is apparient that in the word 
“species,’^ as generally accepted, we have lumped two separate concepts. 
Certainly contemporaneous and consecutive species bear fundamentally 
different relations to each other and between themselves, and eventually 
will probably have to be differentiated by systematists. It is only the 
fragmentary nature of our geological evidence that has heretofore 
concealed the essential difference between species merging into each 
other along a line of descent and species the outcome of independent 
lines of descent. Intergradation is a concomitant of the first but 
incompatible with the latter. 
It can be urged that evolution is an always present activity, that the 
processes of the past are continuing in the present and any system 
founded upon the stability of the species is doomed to eventual con- 
fusion. This may be correct philosophically, but in practice need hardly 
be considered in dealing with modern material. Within historical times 
we have absolutely no evidence of serious evolutionary change. A 
system that would have sufficed for three thousand years in the past 
will probably do for an equal time in the future. By the time evolution- 
ary change introduces serious disturbance in the present scheme of 
things it is probable that our whole classification system will have been 
