EXTERNAI. CONFIGURATION OF PLANTS, 185 



clearly adapted for growing in exposed situations. 

 Lecanora and Parinelia afford examples of the dis- 

 coid type. The filamentous thallus of Alectoria, the 

 shrubby Cornicularice and Roccellce growing on the 

 trunks of trees and on rocks exposed to the violence 

 of the wind, exhibit a marked contrast to the foli- 

 aceous Sfictce and PeUidece which occur among moss 

 in more sheltered positions. The shaggy appearance 

 of the '^ Norseman's Plume " — a lichen which clothes 

 the trunks of trees in the Highlands — is doubtless 

 assumed for the purpose of lessening the resistance 

 which the ]3lant offers to the storm. Microscopic 

 organisms would seem from their minuteness to be 

 able to set at defiance the rules to which the larger 

 plants and animals must conform. At any rate, it 

 is among the minuter forms of life that we meet 

 with the greatest variety. The fantastic shapes of 

 the Rotlfei-a, the exquisite sculj)turings of the Poly- 

 cistincv, the mathematical figures and delicate mark- 

 ings of the Diatoinacece, are without parallel among 

 the larger organic forms, and api^ear to result from 

 the operation of forces far other than those we are 

 now considering. It would indeed seem that in the 

 elaboration of these tiny organisms Nature had 

 called into requisition her finest tools, and employed 

 her most skilful artists. Looking to their extraordin- 

 ary beauty and variety, we may well be tempted to 

 despair of explaining them by mechanical laws. A 

 suggestion of Professor E. Ray Lancaster's is, how- 

 ever, worthy of consideration. This eminent observer 

 is inclined to regard minute size as affording a strong 

 presumption of degeneration. There are not wanting 

 reasons for thinking that the Rotifera are degraded 

 crustaceans, and the mites degenerate spiders. 

 Degeneration in the vegetable kingdom has not yet 

 received much attention. It is quite possible that 

 the minute size of the microscopic Ahjce may partly 

 be the result of their shapes. ApiDarently to a great 

 extent they are exemiDt from the laws which regulate 

 the shapes of larger organisms, but this exemption 



