ANALOGOUS FORMS. 



225 



mals, 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 spe- 

 cies 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 ex- 

 isted 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 af- 

 ford a favourable example of Wagner's law of mi- 

 gration, as with the difficulties of locomotion, and the 

 improbability of a large subsequent arrival, the se- 

 cluded individuals, under even slightly different influ- 

 ences, 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 



