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 . 
