51 



MOKPHOLOGICAL DEVELOPMENT. 



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this truth, in devising appliances by which the greatest 

 strength shall be obtained at the smallest cost of material ; 

 and among organisms, we see that natural selection habit- 

 ually establishes structures conforming to the same principle, 

 wherever lightness and stiffness are to be combined. The 

 cylindrical bones of mammals and birds, and the hollow 

 shafts of feathers, are examples. The lower plants, too, 

 furnish cases where the strength needful for maintaining an 

 upright position, is acquired by this rolling up of a flat 

 thallus or frond. In Fig. 77, 

 we have an Alga which ap- 

 proaches towards a tubular 

 distribution of substance ; and 

 which has a consequent rigid- 

 ity. Sundry common forms 

 of lichen, having the thallus 

 folded into a branched tube, 

 still more decidedly display- 

 ing the connexion between 

 this structural arrangement 

 and this mechanical advantage. And from the particular 

 class of plants we are here dealing with the Acrogens a 

 type is shown in Fig. 78, Riella helicophylla, similarly cha- 

 racterized by a thin frond that is made stiff enough to stand, 

 by an incurving which, though it does not produce a hollow 

 cylinder, produces a kindred form. If, then, as we have 

 seen, natural selection or survival of the fittest, will favour 

 such among these recumbent Acrogens, as are enabled, by 

 variations of their structures, to maintain raised postures ; 

 it will favour the formation of fronds that curve round upon 

 themselves, and curve round upon the fronds growing out 

 of them. What, now, will be the result should such a 

 modification take place in the group of proliferous fronds 

 represented in Fig. 76 ? Clearly, the result will be a 

 structure like that shown in Fig. 79. And if this inrollmg 

 becomes more complete, a form like Junqe^mannia cordifoUn, 



