No. 388.] FOUR CATEGORIES OF SPECIES. 295 
a subject of extreme interest, and it is perhaps even more im- 
portant to record and make accessible all knowledge gained 
regarding the behavior of organized nature under the moulding 
hand of men. Many facts will have places in the treatment of 
all four problems, since these touch at many points; but from 
the standpoint of the execution of the work there is no logical 
necessity that any one task be rendered more difficult by the 
existence of the others. In dealing with each of the categories 
here enumerated, appropriate criteria of so-called species should 
be sought and persistently followed. If we are to continue to 
talk of geological or phylogenetic species, we must understand 
that they can, even in theory, be little more than arbitrary 
sections of the extinct lines of succession leading up to the 
living islands of the present, of which the number, form, and 
relative position can be satisfactorily determined only by direct- 
ing attention to the question of segregation by space, time, or 
mutual sterility. To certify on simple inspection whether an 
individual specimen of a previously unknown plant represents 
a new species, a subspecies, or a hybrid, is, and must remain, 
impossible. Notwithstanding much eminent opinion to the 
contrary, we may insist that the facts of nature and not the 
concepts of the human mind are the primary objects of biologic 
study. In this instance, at least, we may rest assured that no 
refinement of concepts will enable us to know in advance facts 
which must be ascertained by careful and often by extended 
observation. The believers in the doctrine of “amount of 
difference ” have, it is true, an apparent advantage in that by 
the simple application of their individual measuring rods they 
may be ready to assert, without the embarrassment of delay, 
that a new individual represents a new species, since it appears 
“sufficiently different ” from others to meet the demands of their 
“ conception of species.” If these mental phenomena could be 
communicated with uniformity to other naturalists, the method 
would have a practical advantage which it does not now enjoy, 
since the conception is merely individual, and uniformity of 
opinion can never be expected. Segregation or its absence is, 
however, a fact of nature which may be established by careful 
observation, like other phenomena. If some systematists deny 
