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, nothwithstanding 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 
anmeaning 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 
