296 THE INl'LUENCE OF INANIMATE SURROUNDINGS. 



Mammalia and Reptiles from the Cape de Verde Islands, wMch 

 would be incomprehensible if they had formerly been in actual 

 connection with the European continent ; and as a direct proof 

 he adduces the fact that almost all the birds are of Eui-opean 

 species, and that all the European species of insects which are 

 found on these islands are strong iiyers, while, on the other 

 hand, 45 per cent, of the indigenous species of insects cannot 

 fly at all, being in fact wingless. He has still further con- 

 siderably strengthened his views by an investigation of the 

 peculiarities exhibited by the land mollusca of these islands. 

 These, as I have already said on the strength of Dohrn's re- 

 searches, bear a typical European character, but not one species 

 is identical with a European form. The most important means 

 of transport for land Mollusca are, beyond a doubt, marine cur- 

 rents ; the possibihty of eggs being conveyed by adhering to the 

 feet of birds does not here come under consideration. The 

 direction of the currents in the Atlantic is, moreover, such that 

 the conveyance of European land-snails to these islands might 

 easily be possible. But it is evident that constant winds would 

 be able to transport a much greater number of individual 

 flying creatures within a given time than that of the land mol- 

 lusca conveyed by currents. Hence these last would exhibit a 

 considerably less variety of species than the former ; for we know 

 that the greater facility for free crossing with the parent species 

 renders the formation of new species more difficult, while it is 

 facilitated when a variable species that has been introduced 

 into a new home is by any means prevented from constant in- 

 breeding with the parent form. And this is directly applicable 

 to these islands ; there is no serious hindrance to the transport 

 to them of flying creatures from Europe in great numbers, and 

 accordingly we see that the good flyers among the insects of the 

 Canary Islands are almost all identical with European species ; 

 and it is in perfect agreement with this that the land mollusca 

 which are diflicult of transport have become differentiated into 

 a number of new forms, since the greater difficulty of immigra- 

 tion has prevented the crossing of these varieties with new 

 individuals of the parent stock. 



This, of coiu-se, presupposes, or, rather, it follows from the 



