186 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



would appear to be purchased at the expense of 

 their size. The small resistance they offer to the 

 coarser inorganic agencies seemingly allows the finer 

 forces of the organic world to have free play, and 

 thus it comes that the smallest organisms exhibit 

 the most beautiful designs with Avhich in Nature we 

 are acquainted. Physical forces have certainly a 

 good deal to do in determining the limits of size to 

 which the larger organic forms attain — the height of 

 trees, as we have seen, is limited by the force of the 

 wind ; it is at least probable that the same mechani- 

 cal principles are concerned in determining the size 

 of the smaller organisms. Many of these minute 

 forms of life are pelagic in habit — that is, they are 

 not confined to one i)lace, but drift about in the sea 

 at different depths. Fixed organisms have greater 

 need to be adapted in relation to currents than those 

 Avhich drift. The fixed diatom E.xilaria flahellata is 

 a good example of the fan type. The disc is ex- 

 emplified in Micrasterias, The boat-shaped diatoms 

 Navtcida, Navicella, and Fragilarla, afford as good 

 an illustration as can be found in the vegetable 

 kingdom of an external configuration which reduces 

 mechanical resistance to a minimum. Many of these 

 diatoms have the j)Ower of moving through the 

 water by some unexplained mechanism. The shuttle- 

 like shape is calculated to increase their si:>eed and 

 economise their power. Volvox, another locomotive 

 Alga, is spherical in form and has a rotatory motion. 

 From its shape, a rectilinear movement like that of 

 Navtcida would hardly be XDossible. One of the largest 

 fresh-water ^/(yte is the filamentous Lemania fluviatilis. 

 It grows in swift-running streams, often on the 

 edges of cascades. One cannot help ^vondering at 

 its ability to bear the force of the water without 

 being swept away. Its tough knotted filaments 

 spring from a cartilaginous disc, so firmly applied to 

 the rocks that it requires considerable force to de- 

 tach the plant. Reference has already been made 

 to t!io larger marine Algcc. In studying them, the 



