Pn Cae ee eR REN sty tnt ST RN fe Fog NN \ 
ad er ys ¥ = ‘ 
CXXXVil 
range, the Atlas of Morocco, can hardly be considered more 
suitable. As regards Hastern Algeria Parnassius mnemosyne 
is excluded as a possible resident, because the food-plant, 
Corydalis, is missing in that district. Moreover, Eastern 
Algeria and Tunis have many affinities with Sardinia and 
Corsica, and these two mountain islands have likewise no 
species of Parnassius. We may, therefore, conclude that 
Parnassius came from the East into Italy when Corsica and 
Sardinia as well as Mauretania were already isolated from 
Kurope. 
Other groups of animals offer corroborative illustrations of 
these phenomena of distribution. Amongst mammals the 
most striking examples of common European species the 
distribution of which stops short at the straits of Gibraltar 
are the wolf, the mole and the Arvicolid mice, such as the 
short-tailed field-mice, water-rat, bank-vole, ete., all of which 
are entirely absent from North Africa. The long-tailed 
field-mouse, the house-mouse and several European shrews 
have crossed over into Mauretania, why not a single vole? 
In the case of birds the Mediterranean can hardly be considered 
an effective barrier to their distribution, yet many European 
species which extend south to the northern shores of the 
Mediterranean are absent from North Africa. We mention 
the Blue Magpie, Snow Finch, Reed Bunting, Hedge Sparrow, 
Long-tailed Tit, Penduline Tit, Bearded Tit and others. If 
we knew the past history of the absentee species, we would 
also have, I think, the explanation of this remarkable kind 
of distribution, for the present-day bionomics of a species 
are built up on its past history, and barriers formerly effective 
are often still respected though they have dwindled down to 
a mere shadow. Take as an illustration of what I mean an 
example from near home. The crested lark is a bird common 
on the Continent and nesting freely on the French and Belgian 
coasts and yet does not come over to England except as an 
occasional straggler. The Channel is no real barrier to a 
lark; the biological conditions in the southern and eastern 
counties of England would be quite suitable; we should give 
the species protection and leave it unmolested, and neverthe- 
less the birds stop short at the other side of the Channel, 
