SOME PROBLEMS OF TAXONOMY 
291 
affecting all existing life, for the possibility of variation within 
some limit of degree and time is a fundamental provision of our 
theories of phylogeny. These facts, however, are axiomatic, viz,, 
that an organism’s power to vary and to adapt itself to change 
of environment decreases with every progression, in whatever 
direction, from the primitive condition of its ancestors, and that 
there is an ultimate point beyond which the process of its evolu- 
tion cannot go. That such a point exists, a point at which the 
species itself enters a state of senility wherein any considerable 
demand for further adaptation can result only in extinction, is 
abundantly proved by the dinosaurs, pterodactyls, and other ex- 
tinct forms of the past. Among existing animals such forms as 
the birds of paradise and the head fishes appear to be senile to an 
appreciable degree, and less conspicuous examples are found in 
the Lepidoptera already mentioned. Such forms may easily be 
recognized as natural entities. 
For the .purposes of a practical system of classification it is 
necessary to reach some basic idea of our unit and the way that it 
is to be applied, and this idea must necessarily be applicable to 
the entire subkingdom, if not to all living organisms. Many of 
the so-called species of the present are in a state of senility as 
pointed out, some are primitive but constant, while others are 
either primitive or generalized, and very actively variable. It 
is obvious both from past experience and from a logical consider- 
ation of these facts, that the application of a single rigid concept 
to the limitation of all of these groups is a difficult task. Here 
the neglected element of time enters. Combined with senility, it 
furnishes us with an ideal of the species as a natural unit which 
we may define as follows : A group of individuals may exist as a 
natural entity at any period of time in the phylogeny of the line 
to tvhich it belongs ivhen the specialization of this group has pro- 
ceeded to such a point as to preclude (a) the production of a 
marked degree of future modification and (h) discernible inter- 
gradation with other similar units, and in this state may be re- 
garded as an ideal species. 
After the elimination of such forms there remains the im- 
mense number of living organisms which are not, at present, di- 
vided into rigidly delimitable species. Perhaps after the passage 
of many thousands of years these will have resolved themselves 
into such species as were defined in the last paragraph, but here 
again time is the important factor. These variable groups must 
