ANALOGOUS FORMS. 



225 



animals, migrating with so much difficulty, have attained 

 an extraordinary diversity. In the Madeira Islands, 

 134 species of pulmo-gasteropoda were reckoned about 

 ten years ago, of which only 21 were to be found in 

 the Africo-European fauna. These and the 113 other 

 species are mostly confined to narrow districts and single 

 valleys. Are we to suppose that the 113 species for 

 Madeira, and the 21 species for Madeira and Africa 

 with Europe, were each separately created ? Must we 

 not much rather infer that a connection at one time 

 existed between Europe and the present island group 

 of Madeira, and that these 21 species remained 

 what they were before the separation ; while from 

 unknown species still appearing in analogous forms 

 upon the continent emanated the remarkable profusion 

 of new species ? They, and their comrades on other 

 isolated islands, were spared a conflict many sided, 

 and they doubtless afford a favourable example of 

 Wagner's law of migration, as with the difficulties of 

 locomotion, and the improbability of a large subsequent 

 arrival, the secluded individuals, under even slightly 

 different influences, had had a prospect of diverging 

 from the parent species. 



The unscientific opinion, that under like, or nearly 

 like, external conditions, like or similar organisms 

 were created in great numbers, receives a severe blow 

 by the perception that the direct reverse has frequently 

 occurred. Why has America no horses in the present 

 era, although it is proved that the horses introduced, 

 thrive capitally ? It is not necessary for us to explain 

 why the fossil horses which existed in America, as well 

 as in the Eastern hemisphere, became extinct without 



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