224 "^^^ DOCTRINE OF DESCENT. 



Another example of so-called vicarious or " analo- 

 gous " 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 excellent observations. In accordance with other 

 botanical and zooligical facts, he has established that 

 the shell fauna of Spain and North Africa forms a whole, 

 so that the Algerian snails appear a mere appendage to 

 those of Southern Europe, notwithstanding the separa- 

 tion 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 up- 

 heaval 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 unmean- 

 ing 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 ani- 



