ORIGIN OF GALAPAGOS FAUNA 
317 
to propound the theory that North and South America might 
have been joined by means of a land bridge between Mexico 
and Chile across the Galapagos islands at a time when the 
greater part of South America was still submerged.* But 
that land connection is evidently one which must have ceased 
to exist in its entirety, at least in early Tertiary times (see 
Fig. 16). 
I have likewise alluded above to a supposed affinity of some 
of the Galapagos birds to species found in the Sandwich 
islands, pointing out that in other groups this relationship 
undoubtedly exists, although only to a slight extent. I have 
mentioned examples among the mollusks. Among plants the 
composite Lipochaeta is a good example. The genus is con¬ 
fined to the Sandwich islands, except one species which in¬ 
habits the Galapagos archipelago. Instances of affinity of 
the Sandwich islands with Mexico and Central America also 
occur, though as a rule the relationship is a remote one. 
Still, it has been tentatively suggested by several writers that 
some time or other in the past Mexico or California were 
joined by a land bridge to these islands. It might be con¬ 
sidered somewhat beyond the scope of this work to discuss a 
faunistic relationship of this nature, but it really forms part 
of the great problem as to the origin of the older Asiatic stock 
on the American continent. 
What I described above (p. 97) as the latest or most recent 
immigration of Asiatic types into North America took place, I 
think, in Pliocene and perhaps early Pleistocene times. 
A large proportion of the more northerly animals and plants 
now living in North America readily reveal their Asiatic origin 
as slightly modified descendants of a stock once common to 
both Asia and North America. These we must regard as 
the offspring of the latest Asiatic invasion. The American 
species which thus indicate their Asiatic lineage were des¬ 
cribed as Asiatic invaders principally in the early chapters. 
In the fourth chapter the time and the place of their invasion 
across Bering Strait were more fully discussed. Throughout 
the subsequent chapters I drew attention to the existence 
of severally geologically more ancient invasions from Asia, ex- 
Sckarff, R. F., “Early Tertiary Land-connection,” p. 525. 
