130 GEOGRAPHICAL DISTKIBUTION. 



are at this time driven towards the middle of the lake by the sur- 

 face-current of the land-winds, sink during the day, being driven 

 away by the light, into tlie deep water, and thus escape the surface- 

 current of the lake-winds, which would otherwise have carried 

 them again to the shore. Constantly driven farther every night, 

 they remain confined to the pelagic region, as they are not carried 

 back again during the day. Thus a differentiation takes place by 

 natural selection, until at last, after a certain number of genera- 

 tions, there remain only the wonderfully transparent and almost 

 exclusively swimming animals which we know. When this diflfer- 

 entiation has once taken place, the pelagic species is conveyed [in 

 the condition of resting eggs] by the migratory water-birds from 

 one country to another, and from one lake into another, where it 

 reproduces its kind if the conditions of existence of the medium 

 are favourable. In this way we may find the pelagic Entomostraca 

 in lakes which are too small to possess the alternation of winds, 

 the animals having been dififerentiated by the action of the winds 

 in other larger lakes." 



It might, however, be asked with Pavesi, if the general uni- 

 formity of pelagic faunas has been brought about through a method 

 of distribution such as is here indicated, how has it happened that 

 some lakes should be so largely deficient in pelagic forms as com- 

 pared with other, and nearly contiguous, lakes ? The lakes of 

 Northern Italy may be taken in illustration of a condition of this 

 kind. Seeing that identical forms have been scattered to such 

 widely separated quarters of a continent, as Italy, Scandinavia, and 

 the Caucasus, it certainly appears a little surprising that immedi- 

 ately adjoining districts should have been so irregularly stocked 

 with the distributed material. It might, however, be conceived 

 to be a matter of accident, and, indeed, at first sight the condition 

 appears to be more in the nature of a support to the theory stated 

 than as an argument against it. But if accidental conditions of 

 this kind have happened, why has it not also accidentally happened 

 that some of the lakes should have retained a fauna, formed through 

 modification of their own piarticular littoral or deep fauna, distinct 

 from that of any other lake ? Still, the objection here raised is not 

 an insuperable one, and offers much less diflBculty in the way of the 

 partial solution of the problem than does the circumstance of the oc- 

 currence of identical forms in the lakes of Europe and North America. 



