ABSENCE OF DATA. 303 



minute organisms of this class wtict demonstrably belong to the 

 West Indian fauna. The only possible explanation of this fact 

 is the assumption that these organisms were borne to us by the 

 higher stratum of the returning trade-wind and deposited in 

 Europe, where the trade-wind gradually sinks. In the same 

 way all those higher organisms are capable of being conveyed 

 through the air which I spoke of in a former chapter as being 

 able to endure long periods of desiccation : the Tardigiada, the 

 Rotatoria, and the eggs of various small Crustacea and Worms. 

 If, in fact, the wind in this way fulfils the function 

 of distributing such organisms, all such passively migratory 

 creatures must exhibit a very wide range ; or else — which is the 

 same thing — a great uniformity must prevail in the fauna of 

 different countries as regards these forms, since the extreme 

 facilities afforded to the migrated individuals for constant cross- 

 ing with those of the parent species which are subsequently 

 introduced, will easily prevent the rapid formation of new species 

 in the new locality. The facts, so far as they are known, agree 

 to a certain extent with these hypotheses ; but it is impossible 

 at present to venture to offer any decided opinion in the matter ; 

 the gaps are still too great in our knowledge of the distribution of 

 these animals, the only creatures which for the moment concern 

 us. We, as zoologists, may perhaps be blamed for this, since it 

 is our duty to collect the observations bearing on the question ; 

 but such a reproach does not touch us very deeply. Each science 

 must determine its own course without regard to any collateral 

 outside interest, and it may even occur that important questions 

 should be for the time set aside from absolute necessity, if, 

 within the province of the special science to which they apper- 

 tain, no key as yet exists to their solution. And this is at 

 present, or has hitherto been, in a conspicuous degree, the case 

 with the point under discussion. So long as the vitality of our 

 museums is kept up by the constant supply, year after year, of 

 thousands of new butterflies and other insects imported from 

 the tropics, so long as they can interest the public by the fact 

 that so many new fishes or birds, bats or snakes, are described 

 in them, so long, naturally, travelling naturalists will pay little 

 heed to the search for those inconspicuous animals which are 



