126 
JOURNAL OF MAMMALOGY 
scrapped for something better or else altered beyond recognition. In 
the meantime I think we are safe in basing our working system on the 
convention that existing relationships are practically stable. 
Though all standards of taxonomic measurement are not mutually 
transferable between paleontology and modern zoology it does not 
follow that paleontological evidence should be neglected by modern 
systematists. As its evidence increases and its lessons become plainer, 
paleontology must be, even more in the future than it has been in the 
past, the rule and guide of our classification. We should however 
bear in mind that concepts that apply to the one may require modifi-' 
cation before they can be transferred to the other. 
However otherwise it may have been in the past or may be in the 
future, at the present moment or on any one given geological horizon, 
the species is a definite entity and its essential character is its genetic 
isolation. Absence of intergradation with other forms is the only test 
of the species as it exists at present. There is a barrier that isolates 
modern specific groups one from another, individualizes contemporary 
species and prevents wholesale mongrelization. Just what this barrier 
may be we cannot say with confidence, nor is it altogether necessary 
to the present argument to do so, but the agent that seems most capable 
of producing present results is the degree of fertility between such 
groups. When fertility between divergent forms breaks down, when 
differentiation progresses to the reproductive processes sufficiently to 
form a handicap to crossbreeding, genetic isolation ensues that forever 
separates the varying branches of a common stock and a new species 
is born. Whatever the mechanics may be that tend to hold a species- 
true and prevents promiscuity, subspecies are incipient species, and I 
do not see what they can be but variations tending towards, but not. 
reaching, specific status until connection with other forms (inter- 
gradation) is broken down and isolation established. , Thus inter- 
gradation is not only an indication of a condition but it is the condition 
itself and the refusal to recognize it as the essential quality of the sub- 
species seems to be a denial of fact. It should be admitted, and cam 
be without discrediting the fundamental argument, that intergrades- 
will not always be discovered. Material from critical localities may 
not be available or connecting distributions may be obliterated through 
geographical or ecological changes. Subspecific variations may appear’ 
in disconnected communities and give rise to discontinuous distribution 
where even intergradation through individual variation may conceiv- 
ably be obscured, without in any way denying the relative status of 
