IHE 
RMERICAN NATURALIST 
Vou. XXXIII. April, 1899. No. 388. 
FOUR CATEGORIES OF SPECIES. 
O. F. COOK. 
THERE are at least four different sorts of species, so called, 
involved in the study of as many theoretically related, but also 
theoretically and practically distinct, taxonomic problems. The 
result of indiscriminately confusing these problems and their 
specific criteria has been the occasion of many destructive 
criticisms of systematic biology. Such objections”are fre- 
quently valid, but not as uniformly pertinent, since they rest, 
in reality, on faulty analysis rather than upon any inextricable 
difficulties or logical inconsistencies connected with the several 
tasks of biological taxonomy. 
To vary somewhat the familiar comparison of organic nature 
to a tree and its branches, evolutionary theories permit us to 
think of life rather as originating at a point which, by the accre- 
tion of countless successive individuals, has become the center 
of a sphere upon whose surface existing vitality appears as 
islands in the sea of nonexistence. The section of such a 
sphere would differ from that of the earth in that the oceans 
would have enormous depth, for the solid core would separate 
from near its center into numerous and repeated divisions. 
It is perhaps conceivable that life might have reached variety 
and complexity without accompanying segregation into limited 
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