EVOLUTION 951 



exhausted promise. With delay of reproduction far greater growth 

 is possible; and fuller flowering, as every orchard shows, and with 

 more fruit accordingly. Yet after the strain of flowering is over, 

 and before the fruit starts fully growing, there is a period of quiet 

 and mature vegetation, of which so many evergreens give fullest 

 developed examples. Is not the simple regular form of their leaves, 

 so often ovoid or oval, a manifestly convergent evolution, reduced 

 from often widely different and more elaborate herbaceous forms? 

 For instance, in ivies, the strongly arborescent Scots and Irish 

 varieties thus tend to quite simple leaves, though the commoner 

 forms are still "ivy-shaped", i.e. with lobes recalling those of the 

 elaborately developed Aralias, so decorative as evergreen foliage 

 plants; while in the larger order of Umbelliferae, to which these 

 essentially belong, everyone is familiar with the elaborate beauty 

 of their compound leaves. The like for many other evergreens, whose 

 leaves are thus simplified by their slow and steady growth, in 

 contrast to the exuberance of development common in their decidu- 

 ous allies, presumably ancestral. That their survival is not so simply 

 a matter of thickness and epidermic resistance as we were wont to 

 assume, can be simply verified by observing that in a late spring 

 frost, when young deciduous leaves are nipped, the young leaves 

 of the common evergreen Euonymus, though still as thin and un- 

 protected as deciduous leaves, conspicuously survive notwith- 

 standing: so here the internal and constitutional quality of the 

 evergreen's life must supplement the usual view, without, of course, 

 disputing survival value to its thickness and epidermic defence in 

 extremer cold, with snow, etc., as with mountain rhododendrons. 



Again, though thorny plants have long been mainly explained in 

 terms of defence against browsing animals, there is evidence as 

 from the common gorse or whin [Ulex eiiropcBiis) that thorny 

 development is on one side a matter of constitutional habit, with 

 active growth of shoot followed by more or less speedy ebb, and on 

 the other of climatic adaptation ; so increasing with heat and dryness, 

 and abated in more favourable conditions. The frequent splendour 

 of cactus flowering, despite leaf-reduction, is again associated with 

 diminished vegetative intensity of growth; for though some attain 

 large and even arborescent dimensions, they need long time for this. 

 In summary, then, the plant is not too simply to be viewed as 

 passive to environment, nor yet as exhibiting indefinite variations 

 of adaptive value. Without denying some truth to each of these, we 

 must also keep the plant in view throughout its phases of life- 

 activity, from earliest to oldest (see Curve of Life), and recognise 

 that its manifold variability bears definite relation to its con- 

 stitutional yet variable balance and direction of self-maintaining 

 and species-continuing energies, and these in the particular phase 

 (or phases) of life which it may especially accentuate in its life 



