12 THE AMERICAN NATURALIST. [VOL. XXXIII. 
species from the deciduous zone, where the soils are compara- 
tively heavy, are able to push their way southward into the 
marl of the tension zone, and we may also note the effect of 
the chemical composition of the soil on the growth of these 
species, as indicated by their decreased size wherever they 
occur on areas of sand or gravel. In similar manner we can 
understand the reason for the northward extension of Pinus 
Virginiana at Perth Amboy, on account of the sandy or gravelly 
surface soil, and its abrupt limitation in that locality, at the 
terminal moraine, the soil of which is more compact and heavy. 
Quercus Phellos and Q. nigra are, without doubt, limited in 
their distribution by the same causes, and we are justified in 
assuming that if the soil conditions which favor them, or, what 
is the equivalent, the geological formation on which they grow, 
had a further northward extension, the species mentioned would 
be found upon it irrespective of climatic conditions.? 
One thing, however, which should not be ignored in regard 
to characteristic species of any zone is that they do not always 
exist where they are found by reason of the environment being 
the most favorable one for them, but because the environment 
may be unfavorable for other species. For example, Pinus 
rigida, as previously noted, exists in parts of the sand barrens 
of Burlington, Ocean and Atlantic Counties, almost to the ex- 
clusion of all other trees; but it is stunted and conspicuously 
less vigorous in appearance as a whole than where it. occurs as 
scattering groves or individuals further north in richer soil. 
The natural inference is that it would exist to better advantage 
in a different soil from that in which it is most abundant, but 
that other more aggressive species are able to occupy and hold 
such soils against it, and that in the sand barrens it merely 
exists by reason of freedom from competition. This inference 
is further strengthened if we consider its wide geographic 
range, which extends from New Brunswick to Georgia and 
1 a following references may be found of interest in this connection : 
On 
- Britton, N. L. the Existence of a Peculiar Flora on the Kittatinny 
Saue of Northwestern New Jersey, Bull. Torrey Bot. Club, vol. xi (1884), 
p- 126. 
2 Hollick, ER Plant Distribution. as a Factor in the Interpretation of 
Geological Me Trans. N. Y. Acad. Sci., vol. xii (1893), p- = 
