PREFACE vil 
species may be considered to be separated by arbitrary lines ; arbi- 
trary lines as regards a minor number of recalcitrant specimens ; 
but natural lines nevertheless, in their broader interpretation, be- 
cause doubtless representing real currents.of descent, currents in 
an onward flow which continues as a mass to move forward, in spite 
of the presence of cross-currents and lateral blending. 
And yet, concerning some of the new species of Aster here 
described, some authorities may question the title to specific rank. 
It is indeed seldom that there would be entire agreement between 
any two monographers of a long series of critical forms. But the 
result which is of first importance is, that these forms, being clearly 
distinct entities in nature, should be put on record and described. 
I once myself suspected that certain of these species were but 
forms assumed by A. divaricatus or A. macrophyllus when under 
special conditions, and that they could not be relied on to show the 
same characters in successive offshoots of the same colony. My 
field studies since 1886 have been largely devoted to the deter- 
mination of this subject, seeking for the limits of variation in un- 
changed environment, and for the beginnings of new permanent 
species. Many of the species here first published have been 
watched in their natural environment for periods varying from 
seven to ten years ; and without change of characters. I have not 
spared expense or time or travel, while keeping selected plant- 
colonies under repeated observation, in widely separated localities, 
in New England, about Lake Erie, the Hudson Valley and the 
Potomac. I have called on each form to show its permanence 
while in unaltered natural conditions before according it the rank 
of an independent species. I have often held back some remark- 
able form from specific recognition (as I do still the obolarian form, 
p. 113) with the thought that it must surely be but a passing phase 
of the speeies, induced by the particular environment ; only as often 
to meet the same form again in a new and distant locality, present- 
ing not only the same characteristics possibly due to environment, 
but also presenting identical additional characters apparently not 
connected with its environment at all. Insuch casesI have thought 
it better to describe such a type as a distinct species. How cautious 
I have endeavored to be may be seen from the number, some 250, 
of plant-types under observation which are listed here merely as 
