326 MOSTLY MAMMALS 
The animal of the highest zoological position occurring 
among the true cave-fauna is the aforesaid olm, which is 
the sole representative of the genus Proteus, and is allied 
to the ordinary salamanders and newts. The olm is a 
somewhat eel-like creature, measuring about eleven inches 
in length, and with a uniformly flesh-coloured skin, save 
that the branching external gills are brilliant scarlet. The 
limbs are very short and weak, the front pair being provided 
with three and the hinder with two toes, and the eyes are 
completely hidden, Now it is a most remarkable fact that 
the only other salamander referred to the same family 
(Proteidae) as the olm is a peculiar North American species 
with well-developed eyes, four toes to each foot, and a dark 
brown skin, which constitutes the genus Necturus. From 
this it may be inferred that the ancestral type of the two 
genera formerly inhabited the northern hemisphere, and 
that while its transatlantic descendant has preserved the 
primitive number of toes and adhered to an ordinary mode 
of life, the European species has become more specialised 
in regard to its limbs, and has taken to a completely 
subterranean existence. According to Sir Humphry Davy 
the olm only makes its appearance in the Adelsberg grotto 
when the waters rise to an unusual height, remaining at 
other periods in the streams flowing beneath its floor. 
The only other vertebrate animals belonging to the true 
cave-fauna are fish of several species. By far the most 
celebrated among these is the well-known _blind-fish 
(Amblyopsis spelaea), which has been taken in both the 
Mammoth and the Wyandotte Caves, as well as in the 
intervening subterranean waters. This fish is the typical 
representative of a small family allied to the cyprinodonts, 
which are themselves relatives of the carps. It is quite 
destitute of external eyes, and its body is completely 
