288 THE AMERICAN NATURALIST. {[VoL. XXXIII. 
groups of similar individuals, in which case all the circumfer- 
ential possibilities of form and structure would have been real- 
ized, with a result comparable to a uniformly solid sphere. 
Arguments and phylogenies are often based on this assumption, 
that all organic types have been connected by intermediate 
forms; but there is no valid reason to suppose that more than 
very few of such theoretical possibilities have been projected 
into the actual; the oceans of our vital globe are not only deep, 
but are in overwhelming proportion to the space otherwise 
occupied. Segregation has everywhere accompanied differen- 
tiation in structure, and the forms which might have main- 
tained connection between the various branches of life are not 
in most cases to be thought of as extinct; they have never 
come to expression, never existed. 
Having been accustomed to consider organic types as form- 
ing connected series, those who have approached systematic 
biology from the paleontologic standpoint have been especially 
prone to overlook the distinctions which arise from the fact 
that all such series are, in fact, lineal, and that their successive 
members have always been separated by chasms of nonexist- 
ence, so to speak, from all except phylogenetically related forms. 
Moreover, the points of actual contact with these last are few 
and momentary if the extent of biological history be taken into 
account ; a grave fallacy is accordingly involved in the theory of 
classification which attempts to render nature at large compre- 
hensible through methods logically applicable only to phyloge- 
netic series. From the practical standpoint it is as though 
topographic expedients were limited to the numbering of houses. 
As such designations are of no utility unless streets are also 
known, so is the lineal method of classification of use only in 
dealing with ascertained series. Notwithstanding the theories 
of interlacing phylogeny which some authors have propounded, 
it may be asserted with confidence that the analogy of streets 
and numbers has but a limited application in nature. There 
are no cross-thoroughfares ; all the lines radiate in general from 
a center, and while they may sometimes become closely parallel, 
we have as yet no adequate evidence that under natural condi- 
tions they ever reunite after more than the briefest divergence. 
