86 EVOLUTION OF LIFE. 
an 2ھ"‎ form of the Sporendonema, the common fly 
fungus. It was long ago observed by Carus that the 
portions of a salamander which were above the surface of 
the water produced a Mucor (fungus), while those immersed 
gave rise to an Achlya (alga). 7 While the Algae pass very 
gradually into the Fungi through intermediate forms like 
Vaucheria, Peronospora, Achlya, and Sporendonema, the 
transition from the Fungi to the Lichens is equally easy. 
LICHENS. 
Lichens are dry plants, covering stones and rocks, or 
creeping over trees, walls, and fences. They are found as 
gray, brown, yellow patches; as wrinkled, leathery, horny 
crusts (Fig. 108); and however unattractive, as a general rule, 
in appearance, are of great importance in the economy of 
nature, and therefore of interest to the botanist. The 
Lichens are widely distributed, being found in the icy 
recesses of Mont Blanc, amidst the recently poured-out 
lava of Vesuvius, and crowning the summits of most 
barren rocks. The Lichens, being aerial in habit, and more 
insensible to changes in climate than any other plánts, 
survive and flourish where all other vegetation would 
perish. The decaying parts of their bodies furnish the 
subsoil in which future mosses, ferns, and flowering plants 
can take root. Their importance, therefore, cannot be 
over-estimated. A Lichen (Fig. 109) is made of threads, 
and colorless and green spores. The threads resemble the 
mycelium, or threads of a Fungus; the green spores 
(gonidia) are like the spores of the Algae (the spores of 
Fungi, being colorless, resemble the other spores) Most 
Lichens derive their nourishment from the air. This 
peculiarity is usually regarded as distinguishing them from 
Fungi, which live parasitically on plants and animals. But 
as certain forms of Fungi (according to Berkeley) are 
