the Australes, contains species with small ray-pits,
cones dehiscent at maturity and spring-shoots gradually changing, among
the species, from a uninodal to a multinodal form. They are, without
exception, species of the New World.

The third group, the Insignes, contains the serotinous species. The
ray-pits are small and the spring-shoots are, with two exceptions,
multinodal. With two exceptions, P. halepensis and P. pinaster, they are
New World species.

These three groups, being the progressive sequence of a lineal
evolution, are not absolutely circumscribed, but are more or less
connected through a few intermediate species of each group. The
systematic position of these intermediate species is determined by their
obvious affinities. It cannot be expected that the variations, which
take an important part in the evolution of the species, progress with
equal step or in perfect correlation with each other.

As to specific determinations, a little experience in the field
discloses an amount of variation in species that does not always appear
in the descriptions of authors; and species that are under the closest
scrutiny of botanists, foresters or horticulturalists, attest by their
multiple synonymy their wide variation. The possibilities of variation
are indefinite and, with adaptable Pines, the range of variation is
somewhat proportionate to change of climate. In mountainous countries,
where there are warm sheltered valleys with rich soil below cold barren
ledges, the most variable Pines are found. The western species of North
America, for instance, are much more variable than the eastern species,
while in Mexico, a tropical country with snow-capped mountains, the
variation is greatest.

Therefore in the limitation of species undue importance should not be
given to characters responsive to environment, such as the dimensions of
leaf or cone, the number of leaves in the fascicle, etc. Moreover, there
are familiar examples (P. sylvestris, etc.) that show the possibility of
wide differences in the cone of the same species.

In the following classification species only are considered without
attempting to determine varietal or other subspecific forms. But
varieties are often mentioned as one of the factors illustrating the
scope of species. Synonymy serves a like purpose, but synonyms not
conveying useful information are omitted, Roezl's list of Mexican
species, for instance, and variations in the orthography of specifi