DISTRIBUTION OF TRIBES. 481 
they now occupy, thus establishing more or less distinct regions ? 
—these centres of individual races to be carefully distinguished 
from the supposed centres of creation or centres of vegetation, 
from whieh whole diversified floras are supposed to have radiated, 
and to the fallacy of which I specially alluded in my Address of 
1869. 
B. COMPARATIVE ANTIQUITY OF RACES IN CoMPOSIT. 
If we are justified in observing that races, like individuals, 
have successive periods of progressive growth, of full vigour, 
and of gradual decay, we may, in the one case as in the other, 
conclude generally that those which we find to be in the latter 
stage are the oldest—a conclusion, however, which, in the one 
case as in the other, must of course be very much modified 
by the consideration of the numerous constitutional or external 
circumstances which bring on premature decay and extinction. 
A preliminary inquiry, however, is necessary into what con- 
stitutes, what are the evidences of, progress, vigour, or decay 
in a genus, species, or other race of plants, all of which may 
be entirely independent of the evident vigour or decrepitude of 
the individuals the race is composed of. 
The result of the best-founded opinions on this subject which 
to my knowledge have been propounded is that a race of plants, 
be it tribe or genus or species, in its period of full vigour, is widely 
dispersed, accommodates itself to a great variety of climatological, 
physical or other external influences, is numerous and varied in 
subordinate races as well as individuals, these subordinate races, 
especially those immediately subordinate, not being separated by 
wide structural gaps, and not having acquired any very marked 
local characters, but for the most part passing, as it were, into 
each other, their respective distinctive characters not having yet 
acquired any marked degree of correlation. On the other hand, 
a race in a state of decay is represented by subordinate races 
very distinct in structural characters, of restricted areas, and 
requiring for their preservation special climatological or other 
physical conditions, and consequently comparatively few in in- 
dividuals. Of the former, Vernonia, Eupatorium, Aster, Senecio, 
&c.; of the latter, the first five or six subtribes of Helianthoidee 
appear to be good examples among Composite. 
Old decaying and apparently expiring races may, however, 
in some of their branches, owing perhaps to a slight change in 
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