MINOR FAUNAS. 799 
How are we to account for their origin and wide distri- 
bution ? 
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, ¢.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. 
(4) 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, ¢.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, ¢.g. a waste product, while 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 pzgmzent—that is, of 
‘a character which was not acquired in the course of natural selection ? 
