v, DISTRIBUTION AND EVOLUTION 95 



In pasture land each kind of domestic animal leads to the 

 presence or absence of certain species, while in the vicinity 

 of farms or villages, the presence of geese, pigs, or poultry 

 has a distinctive influence. 



What a new light these researches throw upon the 

 development of the vegetation of each country during past 

 ages ! We see how the indigenous vegetation of oceanic 

 islands, in the total absence of mammalia, must have 

 gradually eliminated some of the chance immigrants by 

 which they were first stocked, and favoured others often of 

 later date, and how, in the competition with each other, those 

 species which were most easily modified into a shrubby or 

 arboreal type would have the advantage. Thus may we 

 explain the composites, lobelias, violets, and plantains of 

 the Sandwich Islands being mostly shrubs or even trees 

 | of considerable size, and so abundant in species as to form 

 a characteristic feature of the vegetation. Numerous 

 Caryophyllaceae, Primulacese, and a Geranium are also shrubs 

 or small trees. In the Azores a Campanula and a Semper- 

 vivum are shrubs. 



Again, the knowledge we have recently gained of the 

 wonderfully rich mammalian fauna of temperate North 

 America in middle and late Tertiary times — camels, 

 ancestral horses and cattle, mastodons, and many others, 

 which disappeared at the on-coming of the glacial epoch — 

 affords us a very important clue to the development 

 of its special vegetation. Every change of animal life 

 that so often occurred in all the continents — the union and 

 separation of the sub - arctic lands at various epochs, the 

 temporary separation of North and South America in late 

 Tertiary times, and that of Africa from Europe and Asia 

 during the Early and Middle Tertiary — must all have 

 profoundly affected the special developments of the vegeta- 

 tion, as well as of the animal life, in the respective areas. 



No less indicative of delicate response to variation of 

 temperature, and therefore of close adaptation to the whole 

 modified environment, is the continuous increase in the 

 number of species with every important change of latitude. 

 Although this increase is but slight for moderate changes, 



