THE TAXONOMIC UNIT 
45 
duction of certain secretions, such as gums, alkaloids, etc. Most 
especially, in the study of large groups of minute parasitic organ- 
isms, like the bacteria and other fungi, the effects of their meta- 
bolic activities upon living hosts or upon culture media not only 
are characteristic, but are quite easily discerned. The minute size 
of many of these organisms makes the application of the morpho- 
logical test somewhat difficult. And while expediency may justify 
the use of physiological characters in such cases, this should not 
blind us in recognizing the inadequacy of this principle in general. 
We may now consider the morphological criterion, viz., that 
similarity of structure brings individuals within the limits of the 
specific group, regardless of ancestry — known or unknown. 
It goes without saying, almost, that we can have no other cri- 
terion for extinct species, whose only remains are structural. 
In the examinatiofi of structure we are able to measure and 
compare. All of the data are present. It remains but to fix the 
limits and bounds. Such a criterion of species harmonizes with 
the conception of a variable and mutable species. 
But when the species varies or mutates beyond the confines of 
the defined species it becomes something else, under our eyes, 
just as we assume others have done in the prehistoric past. For, 
in the words of L. H. Bailey, ‘‘This notion that a species, to be 
a species, must have originated in nature’s garden, and not in 
man’s, has been left over to us from the last generation.” ^ 
The taxonomists of the present generation in science have not 
entirely graduated from the Linnaean conception of species, par- 
ticularly as it includes the idea of fixity ; although they are prone 
to look with disdain upon his meager binomial vocabulary. They 
mistake continuity for fixity and immutability. In their laudable 
efforts to harmonize classification with the probable phylogenetic 
history they forget that all groups above the individual are, in a 
measure, artificial and arbitrary, and of necessity must be, since 
we have no authentic record of their phylogenesis. 
Our conclusion is, then, that the only true and scientific cri- 
terion of species is the one based upon morphology. 
To what extent may the differentiations of living organisms be 
useful to science? What degree of difference should be recog- 
nized taxonomically ? We may readily understand that where 
such living forms are under experimental observation for the 
purpose of determining genetic relationships, considerable care 
in cataloguing minute variations may be necessary; but where 
2 Survival of the Unlike, page 110. 
