xliv 



produced in the same brood, to the well-segregated race living in 

 company with another race referable to the same stock. As such 

 most authors, perhaps rightly, consider these latter as good and true 

 species ; and thus the formation of species out of mere variations is 

 illustrated by the facts of geographical distribution. 



But it is not this branch of the subject with which we are so 

 much concerned, when we wisli to compare the productions of the 

 different Andean valleys and their vertical ranges, as that relating to 

 the nature of barriers to distribution. It has been received as a 

 principle in Zoological and Botanical Geography, that grand physical 

 barriers, such as mountain ranges, form an impassable limit to the 

 faunas and floras of the plains on each side of them. It is repeated, 

 in almost every Manual of Physical Geography, after Humboldt, 

 who I believe was the originator of the statement, that the species 

 are all different on the two sides of the Andes of South America. 

 Such a fact, if well established, would be interesting in many ways. 

 First, it would throw light on the Geology of the country, as proving 

 that the Andes must have existed as a ridge, sufficiently lofty to 

 prevent the creatures of the plains crossing it, before the origin of 

 the species which now people the plains on each side. Now, it is 

 possible that this broad and important generalization may have been 

 made on a too slender foundation of facts. Of course, in those parts 

 of the Pacific coast-region (two-thirds of the whole line within the 

 tropics), where the conditions of soil, climate and vegetation are 

 totally different on the two sides of the Andes, no community of 

 species is possible. A lofty mountain barrier would be here un- 

 necessary, for a few steps of level road, in many parts of the world, 

 would suffice to bring the traveller from the domain of one fauna 

 to that of another — for instance, from an arid plain to a luxuriant 

 forest along some river-valley. This would be a difference of 

 " station," and not of area of distribution, — a distinction long ago 

 recognized in Botany. The question is, then, limited to this : In 

 those parts of the Pacific coast-region, such as Guayaquil, where a 

 humid forest-country exists on both sides of the Cordillera, are the 

 species of the two sides entirely distinct ? This would test the 

 efficacy of mountain -barriers better than almost any 'other case. 

 For the species, at least of insects, which inhabit humid forests 

 near the equator, are probably unable to exist at a higher altitude 

 than 4000 or 5000 feet, and no pass over the Cordillera exists of 

 half this depression, throughout the whole line of the Andes from 



