EVOLUTION 



1069 



reduction. Thus in many of the Cactuses and Euphorbias that live 

 in arid regions, where the rains come at rare intervals and water 

 has to be stored within the plant, there is a practical suppression of 

 leaves, which are represented by spines, and the green stem takes on 

 the ordinary functions of the leaf (absorption of air and light, and 

 transpiration of water- vapour) , but, despite its enlargement, with 

 a greatly reduced surjace, and reduced functioning accordingly. 

 Thus there are quaint shapes — spherical, oval, cylindrical, and so 

 forth — with a minimum or approximation towards a minimum of 

 surface in proportion to volume. Thus undue loss of water in the 

 desert is evaded. 



There are other advantages in surface increase besides those 

 associated with nutrition. Thus one of the striking features of many 



Fig. 187. 



Adapta'tion Illustrated by the Floating Barnacle [Lepas fascicularis). From a 

 specimen. ST, stalk; SH, shell; C, cirri or thoracic limbs. The floating 

 barnacle, to the right, attaches itself to seaweed (W) and the like, which 

 it tends to drag down as it increases in weight. But this is obviated by 

 a secreted buoy (B). In the ordinary ship-barnacle {Lepas anatina) shown 

 to the left, attachment is effected to floating timber and the like, which 

 cannot be submerged by the relatively trivial weight. The shell of the 

 Floating Barnacle is very lightly built. 



of the smaller pelagic organisms — ^both plants and animals — is 

 a notable increase of surface. This facilitates flotation, and makes 

 the creatures almost unsinkable. We refer, for instance, to the long 

 outgrowths, sometimes feathery, that extend from the chitinous 

 cuticle of minute Crustaceans, like out-riggers or catamaran spars. 

 It is interesting to compare the squat heavy-bodied Angler-fish 

 {Lopkhis piscatorius) , not infrequently cast up on the shore, with 

 the free-swimming pelagic larva which bears extraordinary — one 

 might say exuberant — ^frills and tassels of delicate skin. It is 

 highly probable that these greatly increase the flotation-power of 

 the larvae; and they gradually disappear as the animal changes 

 its habitat, and begins to feed on the floor of shallow seas. (Fig. 124). 

 When pioneer animals made the several times repeated transition 

 from the sea to dry land, whether via freshwater routes or directly, 



