184 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



as they are, have resulted from a gradual process of 

 erosion not unlike that with which the geologist is 

 familiar. 



The botanical problem is complicated by several 

 considerations, so that the parallel between natural 

 selection, which eliminates the ill-adapted forms, and 

 denudation which removes the softer strata, can 

 only be said to hold in a very general w^ay. The 

 formation of adventitious roots by a cutting or slip 

 when placed in moist earth is an instance showing 

 how external agencies which have had to do 

 with moulding the outlines of plants, react on the 

 internal forces of the organism. The laws of 

 s^anmetry and inheritance are equally obscure. All, 

 therefore, we can hope to do at present is to direct 

 attention to some of the more obvious examjDles of 

 adaptation. 



I. Thalloiohytes. The mycelium of a fungus has 

 no definite shape, but consists of a network of fila- 

 ments ramifying through the nidus in which it 

 grows. Not being exposed to the action of wind 

 or water, the vegetative part of a fungus exhibits 

 no adaptation in relation to these agencies ; it is 

 only in the reproductive system we can hope to 

 discover shapes fitted to resist their action. The 

 small size of most fungi, and the sheltered places 

 in which they grow, render it improbable that among 

 them we should find any striking examples of adapt- 

 ation in relation to the wind. Nevertheless the pre- 

 vailing umbrella shape, so typical of the larger fungi, 

 admits of an easy explanation, and illustrates the 

 principles we have been endeavouring to explain. 

 The shape of a mushroom is such as to protect the 

 liymenium from rain, and at the same time admit 

 tlie wind underneath to carry away the spores 

 formed on the under surface of the pileus. Ventila- 

 tors on the toj)s of buildings and chimney-cans are 

 often fashioned on the same pattern. 



The Lichens, which botanists now consider to be 

 the result of symbiosis, include a number of forms 



