59 
Practically every worker has had difficulty with some of these varia- 
tions. Some are good species at one locality, i.e., with no connecting forms, 
at others not. It looks as though in some areas, as at White's localities 
and the Minnewanka region, the various forms interbred fully, producing 
imperceptible gradations between the different types; whereas if one of 
these forms, as opimus or boonensis, migrated into another area, where 
the types with which it could interbreed were lacking, it would breed 
true. The result would be a true species in the latter area, provided 
the regions where interbreeding had occurred were not found. Should 
the finding of intermediate forms in some distant region invalidate a 
species?; it, of course, does so among living forms today. If, however, 
two species do not interbreed at present but did in the late Pleistocene, 
distant a few thousand years, they would still be considered true species. 
The answer to the question then depends upon the synchronism of the con- 
taining strata. If species A, from locality A, and species B, from locality B, 
interbred in locality C at the same time they were living at A and B they 
would be considered as forming a single species; if, however, they ceased to 
interbreed or died out at C, and other interbreeding areas, while continuing 
to live separately at A and B, they would then be distinct species. Exact 
synchronism, which this demands, is practically impossible of attainment, 
correlation of strata being usually based upon similarity in the succession 
of faunas and floras, that is, upon homotaxis rather than upon synchronism. 
The forms we are discussing come from the Pennsylvanian, a period 
very many times longer than from the close of the Pleistocene to the 
present, and which in the widely separated areas under discussion, it is 
impossible, at least at present, to correlate more closely $han in a broad 
way as Lower, Middle, or Upper Pennsylvanian; it is hence impossible to 
say within the limits of tens of thousands of years whether the inter- 
breeding areas existed at the same time as the separate forms in other 
regions. With the increase in number of paleeontologic facts the connecting 
links between more and more species are being discovered, many of which 
are known to have interbred at the time of their branching into different 
species, and we look forward to the time when practically all species, past 
and present, will be arranged in such a unity. Since it is thus practically 
impossible to apply the zoological definition of species to fossils we should 
denominate as true species all forms which large collections from a single 
locality show to be distinct, even though in some distant area in rocks of 
approximately the same age they are found to grade into another species. 
This will likewise result in more exact correlation and especially in the de- 
velopment of palseogeography, for in this new science facts of migrations 
are very desirable. In the areas where intergrading forms occur it should 
first be determined if the intergradations occur within a single stratum, 
for otherwise an evolving line found in a succession of strata will be con- 
sidered as intergrading; this error we are inclined to believe has at times 
occurred. When an area is determined to have been an interbreeding one 
for certain limiting forms these forms should be described separately as 
form A, form B, etc., and their relationship noted; if, however, these 
forms have already been described as distinct species these names should 
be used. It is here that this definition of species would differ from the 
zoological one, but it seems unavoidable by reason of the uncertain syn- 
chronism of this area and that of the described species. 
