GEOGEAPHICAL DISTEIBUTION.
The present distribution of the Arctic-Alpino flora is usually explained on an hypothesis
of alternating migrations.' This hyphothesis may be summarised as follows :—
A circumpolar flora initially uniform in character and, during the warmer period
antecedent to one or other of the more general glacial periods, compact in distrihition,
eommenccd to migrate southwards under the influence of the gradual decrease of temperature
that preceded the glacial period proper. During this advance new forms that
appeared did so under conditions that were uniform for the migrating flora. At the same
time the progressive enlargement of the area subject to these conditions mitigated the
severity of the struggle for existence between rival forms and diminished the swamping
effect of the presence alongside of them of the original types. A large proportion of the
forms were thus enabled to survive. Here aud there accident determined the detention of
stragglers, while occasionally a particular meridian o£ migration ended in a cul-de-sao. In
these cases necessity compelled the development of a power of accommodating themselves
to circumstances which proved sufficient in a few forms to ensure their survival. Wide seas
and lofty mountain chains on whose summits a rigorous climate bad already become
established formed natural obstacles to the advance, aud where tliese were encountered the
advance was checked or deflected. Elsewhere the line of furthest advance formed a
continuous zone of distribution. The transition between congeneric forms in a given
section of this zone was a serial one and represented the evolution of the genus along
one meridian of migration; the transition between forms of successive adjacent meridiansections
was in a closed curve. The natural conditions uniform for the invading flora
were progi-essively unfavourable for the displaced one, and few of the forms characteristic
of the latter were able to accommodate themselves to adverse circumstances and at the
same time to hold their own against members of an invading flora for which tliese
conditions were suited.
"When the advance ceased there ensued a period during which the natural conditions
were comparatively stable. The struggle for existence within the invading flora was
therefore intensified, since the number of forms continued to increase while the area they
inhabited ceased to enlarge. Internal pressure tended to make the invading flora extend
to localities less favourable for its existence. But in these localities the conditions were
favourable for the displaced flora, and here it was able not only to survive, but to resist
invasion. Hard pressed from within, successfully opposed from without, many forms
gave way in the struggle, and as here one, there another, kind of variation proved more
adapted to local conditions, some local colouring became imparted to different districts of
the zone in which the Arctic-Alpine flora now lived.
This period of stability came also to an end. With the gradual increase of temperature
that marked the disappearance of the ice, the invading flora began to migrate
northwards. Like the advance, the retreat was a meridional one moditied by local
conditions. Intervening seas checked or deflected it as they did the advancc. Mountain
ranges, however, played a different part. In place of turning aside the retreating flora
they largely attracted it, offering their flanks as routes of safe retreat and their summits
as asylums where retreat was altogether cut off. But even with this the area available
for the Arctic-Alpine flora was steadily curtailed, the struggle for existence was intensilied,
the tendency to vary, especially in districts where retreat was delayed, was greatly
stimulated, the variations deviated more and more from the original type, while for
reasons that are not yet demonstrable, now more, now fewer, forms survived.
' Tiiis tkeory was first advanced by Professor £ . Forbes (Brit Assoc. Report, IS-IS, and Mem. Gaol. Survey i, 33(5-432
1846), but TTtts greatly extended and elaborated by tlie late Mr. Darwin [Origin of Specias, cliap. xi [edit. 0, cliap. xiij), and
«peeially applied by Sir J . D. Hooker {Trans. Linn Soc. sxiii, 253).
HYPOTHESIS OP MIGR.ÍTION-S. 4 1
The mountains to the north of the zone of furthest advance were not alone in
attracting the retreating flora. To a less but still to an appreciable extent certain forms
found an asylum and perhaps a base for further advance on the slopes or summits of
mountains to the south.
During the retreat the local colouring peculiar to forms of a particular area became
imparted to all the districts along the meridian of retreat. In this way the circumpolar
flora of the second period of stability, while as compact in distribution as the initial one
had been was less uniform in character. On the other hand, mountain systems that had
during the advance formed impassable barriers between adjacent meridians of migration,
attracted to their opposite flanks during the retreat the forms they had hitherto kept asunder,
and pei-mitted through their passes an interchange of influence and of individuals. Individuals
ao-reeing in morphological characters did not all regain the original seat of the flora, but
were content or were compelled to remain in one or more of the Alpine localities en route.
Their characters these detained individuals at first transmitted unchanged. But when
the period of retreat had ceased, there came a time when natural conditions were once
more stable. And now even in the moat suitable localities slight modifications ensued,
while in localities where retreat had been earliest and most completely cut off, this period
of stability allowed the retreating flora to develope its latent powers of accommodati<jn &nd
permitted the survival of forms that must otherwise have altogether disappeared. But the
somewhat unsuitable conditions of these localities greatly stimulated the tendency to vary,
the more suitable varieties surviving and supplanting the original types. This went on'
till a balance was established and a period (of which the present is probably an example)
of approximate equilibrium resulted. This equilibrium was earliest attained in the new
circumpolar region, where the natural conditions were those that had characterised the
true home of the flora, whatever its geographical position and whether in advance or in
retreat. Its greatly circumscribed area caused an enormous reduction in the number of
the forms it contained, but owing to continuity of conditions there were necessarily among
the survivors fewer variations from the original types, and these variations were slight.
I t is thus that in general terms the Circumpolar province is poorer in number of forms,
that these forms are generally more archaic in structure and the endemic percentage of
its separable forms is lower than in other provinces, while in provinces nearer to the
circumpolar, as compared with those nioi*e remote, the same things hold good.
The behaviour of the Equatorial Flora during these alternatii^g migrations has only
an indirect bearing on the distribution of the particular genus under review. It need
therefore be but briefly alluded to. It was of necessity in all points the opposite of the
other—while the Circumpolar flora advanced and was characterised by a zonal distribution,
the Equatorial retreated and had a meridional character imparted to its distributionwhile,
on the other hand, the Circumpolar flora retreated, the Equatorial advanced aud its
mode of distribution became necessarily more zonal. It has been pointed out on a former
page that a few forms oí Fcdicularis are present in the Intermediate Flora.» But Sir J. D
Hooker has clearly shown- that there are in reality only two Floras. These two are "the
Circumpolar and the Equatorial; and this Intermediate Flora, which it is convenient
to recognise practice since it simplifies the treatment of the other two, has been composed
mainly of forms left behind by the Equatorial when the Circumpolar was advancing, by
the Circumpolar itself when it was retreating. According to the hypothesis of migrations
the present period is one of approximate equilibrium subsequent to a Circumpolar retreat •
' P.,22,
' Godman and Salvi : Biologia Centrali-Am my; Commentary on Mr. Semüeys Introduc
, CAI.CUTTA, VOL. ILL,