fined to America, forty-two are found in both the northern and
southern continents, twenty-one are confined to South America, while twenty
are found only in North America, the West Indies, or Mexico. This equality
of North American and South American species in the Galapagos is a fact of
great significance in connection with the observation of Sir Joseph Hooker
that the _peculiar_ species are allied to the plants of temperate America
or to those of the high Andes, while the non-peculiar species are mostly
such as inhabit the hotter regions of the tropics near the level of the
sea. He also observes that the seeds of this latter class of Galapagos
plants often have special means of transport, or belong to groups whose
seeds are known to stand long voyages and to possess great vitality. Mr.
Bentham also, in his elaborate account of the Compositae,[65] remarks on
the decided Central American or Mexican affinities of the Galapagos
species, so that we may consider this to be a thoroughly well-established
fact.

The most prevalent families of plants in the Galapagos are the Compositae
(40 sp.), Gramineae (32 sp.), Leguminosae (30 sp.), and Euphorbiaceae (29
sp.). Of the Compositae most of the species, except such as are common
weeds or shore plants, are peculiar, but there are only two peculiar
genera, allied to Mexican forms and not very distinct; while the genus
Lipochaeta, represented here by a single species, is only found elsewhere
in the Sandwich Islands though it has American affinities.

_Origin of the Galapagos Flora._--These facts are explained by the past
history of the American continent, its {289} separation at various epochs
by arms of the sea uniting the two oceans across what is now Central
America (the last separation being of recent date, as shown by the
considerable number of identical species of fishes on both sides of the
isthmus), and the influence of the glacial epoch in driving the temperate
American flora southward along the mountain plateaus.[66] At the time when
the two oceans were united a portion of the Gulf Stream may have been
diverted into the Pacific, giving rise to a current, some part of which
would almost certainly have reached the Galapagos, and this may have helped
to bring about that singular assemblage of West Indian and Mexican plants
now found there. And as we now believe that the duration of the last
glacial epoch in its successive phases was much longer than the time which
has elapse