OKIGIN 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- 



* Scharff, E. F., " Early Tertiary Land-connection," p. 525. 



