44 
IOWA ACADEMY OF SCIENCE Vor.. XXVII, 1920 
what they call a 'new species/ use the term to signify what was 
meant by it twenty or thirty years ago.” ^ It may be agreed that 
concepts must change with the development of knowledge, but 
it would seem that scientific terminology ought to remain as near 
constant as possible. 
After this rather brief historical survey of the species concept 
it will be germane to inquire as to what concrete criteria have been, 
or can be applied. The analysis shows that there are three such 
criteria, viz., the genetic, the physiological, and the morphological. 
The idea that hereditary descent is the essential test of specific 
rank seems to be the oldest and original point of view. As in- 
timated, this was Ray’s conception. This criterion is definite, 
but it fails in allowing for no expansion, no evolution. By virtue 
of continuity a species is always the same species. And this is 
manifestly in contradiction to the modern viewpoint. This cri- 
terion furnishes the basis for the modern principle of intergrad- 
ation. 
The criterion of relationship can have little value, because all 
forms and all groups, including subspecies, species, genera, etc., 
are related in this sense. So that relationship is a common prop- 
erty, and not a differential character. Furthermore, in nature it 
is usually impossible to know the parentage of forms. 
The physiological test of species has had a long and honorable 
past. Many older writers were quite firmly convinced that true 
species could not interbreed. So that, interspecific sterility was 
accepted as a true test. of a proper species. Time has shown, 
however, that it is not. 
There are recorded cases of sterility in hybrids; there are 
recorded cases where sterility results from a cross in one direction, 
and fertility results from a cross in the other direction ; there are 
recorded cases where fertility results from a cross in both direc- 
tions; and there are, apparently, a few cases in which the hybrid 
shows a greater degree of fertility than in normal fertilization. 
Such facts indicate, no doubt, that all species do not possess 
the same degree of difference, physiologically, at least. But they 
also show that there is no constancy in the matter of sterility in 
hybrid offspring, and that such a criterion cannot be used in the 
test of species. 
Aside from the matter of reproduction some functions have 
been regarded as having specific value; for example, in the pro- 
1 ‘"On the Osteology of the Chimpanzees and Orangs.” Trans. Zool. Soc., 1858. 
