174 THE EVOLUTION THEORY 



produces periodically, and these are disseminated in the air by the 

 bursting of the sporangia and are carried away by the wind in 

 the form of fine dust; the alga multiplies simply by continual 

 division into two, but it also, like the whole lichen, can survive 

 desiccation, and, after falling to pieces, is likewise carried through the 

 air as microscopic dust. 



The partnership of the two plants rests on a basis of mutual 

 benefit ; the fungus, like all fungi, is without chlorophyll, and cannot 

 therefore decompose carbonic acid gas or elaborate its own organic 

 food-stuffs ; it receives these from the alga. The alga has in the net- 

 work of the fungus a safe shelter and basis of attachment, for the 

 fungus is able to bore into the bark of trees and even into stones ; 

 besides which it absorbs water and salts, and supplies these to the 

 partner alga. We here see the mutual advantage derived from 

 the partnership, which is really an extremely intimate one. Fungus 

 spores, sown by themselves, spring up and develop some branchings 

 of fungoid hyphse, a so-called mycelium, but without the requisite 

 partner alga these remain weak and soon die away. The alga, on 

 the other hand, can, in some cases, though not in all, survive without 

 the fungus if the necessary conditions of its life be supplied to it, but 

 it grows differently and more luxuriantly in association with the 

 fungus. 



The same species of alga may be found associated with different 

 species of fungi, and then each partnership forms a distinct species of 

 lichen of definite and characteristic appearance ; Stahl even succeeded 

 in making new species of lichen artificially by bringing the spores of 

 a lichen-forming fungus into contact with alga-cells, with which they 

 had never been associated in free nature. 



The most remarkable feature of this remarkable association 

 seems to me to be the formation of common reproductive bodies — an 

 adaptation in face of which all doubt as to the theory of selection 

 must disappear. Periodically there are developed in the substance of 

 the lichen small corpuscles, the so-called soredia, each of which con- 

 sists of one or more alga-cells surrounded and kept together by 

 threads of the fungus. When they are developed in large numbers 

 they form a floury dust over the maternal lichen, which ' breaks up ' 

 and leaves them, like the spores of the fungus, to be carried away by 

 the wind. If these alight on favourable soil nothing more is needed 

 than the external conditions of development, light, warmth, and water, 

 to enable the lichen to spring up anew. The great advantage to the 

 preservation of 'species' is obvious, for, when multiplication by 

 the ordinary method occurs among lichens, the spores of the fungus, 



