798 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



1. To explain the uniformity, Darwin referred to the birds which 

 carry organisms from watershed to watershed, to the carrying power of 

 the wind, and to changes of land level which bring different river beds 

 into communication. But this is not enough. 



2. It seems very likely that some of the fresh-water forms have 

 migrated from the sea and seashore through brackish water to rivers 

 and lakes. As the possibility of making the transition depends on the 

 constitution of the animal, it is intelligible that similar forms should 

 succeed in different areas. 



3. There seems much force in what Credner and Sollas emphasise, 

 that many lakes are dwindling relict-seas of ancient origin. Granted a 

 fairly uniform Pelagic fauna, e.g. before Cretaceous times, we can 

 understand that the conversion of land-locked seas into lakes would 

 imply a decimating elimination, and, as the conditions of elimination 

 would be much the same everywhere, the result would be uniformity 

 in the survivors. 



Minor faunas. (a) Of Brackish Water. We are warranted in 

 speaking of a brackish-water fauna, because of its uniformity in widely 

 separated regions. It does not seem to be a mere physiological 

 assemblage, varying in each locality, but rather a transition fauna of 

 ancient date, a relic of a littoral fauna once more uniform. The fact 

 is that the power to live in brackish water is not very common ; it 

 runs in families. 



(b) Cave fauna. -In America, thanks very largely to the labours of 

 Packard, about 100 cave animals are known ; in Europe the number 

 is about 300, the increase being largely due to the occurrence of about 

 100 species of two genera of beetles in European caves. In the famous 

 Mammoth Cave of Kentucky, which has over 100 miles of passages, 

 with streams, pools, and dry ground, there are over 40 different species 

 of animals. The temperature is very equable, varying little more than 

 a degree throughout the year ; it is, of course, dark ; and there are no 

 plants other than a few Fungi. Thus the conditions present some 

 analogy with those of the deep sea. The fauna is of much interest to 

 evolutionists, for we wonder how far the peculiarities of the cave- 

 animals, e.g. absence of coloration and frequent blindness, are due to 

 the cumulative effect of the environment and of disuse, or how far they 

 represent the survival of germinal variations, and the result of the 

 cessation of natural selection along certain lines. Have the seeing 

 animals found their way out, leaving only the blind sports, which crop 

 up even in daylight ? or is the loss of eyes the result of disuse and 

 absence of stimulus? Or again, if it be granted that pigment is an 

 organic constitutional necessity, e.g. a waste product, w r hile coloration 

 is explicable as an adaptation wrought out in the course of natural 

 elimination, then the question arises, whether the cessation of natural 

 selection a condition awkwardly called " panmixia " which might 

 account for the disappearance of the coloration when there is no 

 premium set upon it, can also account for the loss of pigment, that is of 

 a character which was not acquired in the course of natural selection ? 

 (see Beddard's "Animal Coloration''). Our only answer at present is 

 that there is need for experiment. 



(c) Parasitic fauna. It seems legitimate to rank together those 



