NS an ERRE ane De 
ee ar eee ee a a = 
oe pipes 
1893.) Cretaceous Plant Population. 337 
which to-day variation is most rapid and plasticity is greatest 
for each species and even for every individual. 
The attempt to show that the high preponderance of metas- 
permic remains in the Cretaceous rocks is a sufficient proof of 
the high preponderance of pines, cycads, ferns and club-mosses 
in the Cretaceous forests is paradoxical enough to excite grave 
suspicions of the method by which such a result is reached, 
but the writer hopes to show that the absurdity of the state- 
ment is solely a superficial one, and that it is really capable of 
a defence both from experiment, observation and legitimate 
deduction. I have already presented in my recent work on 
the plants of the Minnesota valley the skeleton of the argu- 
ment, but desire here to amplify and extend the discussion, 
somewhat, to more fully cover the ground. If the conclusions 
that I have drawn are legitimate, and my knowledge of the 
literature of the subject sufficient to uphold my belief that 
certain new laws are determined, both of plant-distribution 
to-day and paleontological interpretation, it will be clear that 
the argument is an important one, and should be examined for 
what it may prove to be worth. 
In the first place let us examine some of the observed facts 
of plant-distribution to-day. It is apparent that while species 
themselves are generally limited in some more or less definite 
manner and occupy a more or less considerable area of the 
earth’s crust, there are several degrees of aggregation in which 
species play asubordinate part. We note the coniferous forest 
as distinct from the hardwood forest, the prairie knoll from 
the prairie swale, the mountain-side herbage and shrubbery 
from the herbage and shrubbery of the valley. The presence 
of these different zonal groups at different attitudes and lati- 
tudes was first pointed out in the modern way, by von Hum- 
boldt2 Since his time, the various groups of species, condi- 
tioned in some definite manner and, under climatic influences, 
acquiring characteristic habits or habitats, have been studied 
1Metaspermz of the Minnesota Valley, p. 602 (1892). 
- 2Essai sur la Geographie des Plantes ,suivi d'un Tableau Physique des Regions 
De Distributione Geographica Plantarum, secundum cceli Temperium et altitudi- 
nem montium, Prolegomena 
23 
