LECTUEE XXXII 



INFLUENCE OF ISOLATION ON THE FORMATION 

 OF SPECIES 



Introduction — Isolated regions are rich in endemic species— 13 isolation a condi- 

 tion in the origin of species ? — Moriz Wagner, Romanes — ' Amiktic ' local forms, the 

 butterflies of Sardinia, of the Alps, and of the Arctic zone — Periods of constancy and 

 periods of variation in species — Amixia furthered by germinal selection — The thrushes 

 of the Galapagos Islands — The intervention of sexual selection — Humming-birds — 

 Central American thrushes — Weaver-birds of South Africa — Papilionidoe of the Malay 

 Archipelago — Natural selection and isolation — Snails of the Sandwich Islands — 

 Influences of variational periods — Comparison with the edible snail and with the snail 

 fauna of Ireland and England — Changed conditions do not always give rise to variation 

 — Summary. 



In an earlier lecture I endeavoured to show, hy means oi" Dar- 

 winian arguments and examples, how important for every species, in 

 relation to its transmutation, is the companionship of the other 

 species which live with it in the same area. We saw that the 

 ' conditions of life ' operated as a determining factor in the com- 

 position of an animal and plant association quite as momentously as 

 any climatic conditions whatsoever, and, indeed, Darwin rated the 

 influences of vital association even more highly, and attributed to 

 them an even greater power of evoking adaptation than he granted 

 to the physical conditions of life. 



We are, therefore, prepared to recognize that even the trans- 

 ference of a species to a difierent fauna or flora may cause it to vary, 

 and this occurs when the species gradually extends the area of its 

 distribution, so that it penetrates into regions which contain a 

 materially difierent association of forms of life. But these migra- 

 tions are not necessarily only gradual, that is, due to the slow 

 extension of the original area of distribution in the course of genera- 

 tions as the species increases in numbers ; they may also occur sud- 

 denly, when isolated individuals or small companies of a species 

 transcend in some unusual manner the natural boundaries of the old area, 

 and reach some distant new region in which they are able to thrive. 



Species-colonies of this kind may be due to the agency of 

 man, who has spread many of his domesticated animals and plants 

 widely over the earth, but who has also intentionally or unin- 

 tentionally forced many wild animals and plants from their original 



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