224 THE DOCTRINE OF DESCENT. 



Another example of so-called vicarious or "analogous" 

 species, affording an easier basis for induction, is provided 

 by the comparison of the snails of Southern Europe, 

 and especially of Spain, with those of North Africa, on 

 which we are indebted to Bourguignat for some excel- 

 lent observations. In accordance with other botanical 

 and zoological facts, he has established that the shell 

 fauna of Spain and North Africa forms a whole, so that 

 the Algierian snails appear a mere appendage to those 

 of Southern Europe, nothwithstanoing the separation 

 by the Straits of Gibraltar. Now it is proved that, in 

 geologically recent times, this region of North Africa 

 was in fact a peninsula of Spain, and that its union 

 with Africa was effected on the north by the rupture 

 of the Straits of Gibraltar,^ and on the south by an 

 upheaval to which the Sahara owes its existence. The 

 shores of the former Sea of Sahara are still marked by 

 the shells of the same snails that live on the shores of 

 the Mediterranean. But all North African species are 

 not identical with those of Spain ; of many African 

 sorts, only "analogous" species are found on our side. 

 Now if certain Spanish species do not themselves occur 

 in Africa, but are yet replaced by very similar forms, 

 our standpoint at once connects with the otherwise 

 unmeaning word " analogous " species the idea of the 

 common derivation of the forms replacing one another, 

 and of the local variations superinduced by isolation 

 and altered conditions. 



A severe test is applied to those who believe that 

 species were separately created, by the air-breathing 

 land snails (pulmo-gasteropoda), when it is seen that in 

 isolated islands and island groups these earth-bound 



