Cockayne. — Ecological Studies in Evolution . 15 



some pinnae with pouches and others quite flat, in accordance with, the degree 

 of illumination to which they are exposed. In fact, here the specific dis- 

 tinction does not hold — it is merely epharmonic — and the latest name must 

 be abandoned ; nor can the two " species " be maintained even as " varieties." 



(i.) Discaria toumatou Raoul (Rhamnac.) when growing in positions 

 subject to the attacks of rabbits may form low green cushions made up of 

 leafy spineless shoots. " Normally " it is a stiff branching shrub furnished 

 with abundant spines. 



Many more examples could be cited, but the above show clearly enough 

 how unstable species may be, even when growing under natural conditions. 

 When experimental methods are brought into play the effects from plasticity 

 become still more striking. For example, spine-production may be sup- 

 pressed in Discaria toumatou ; true leaves may be produced in the whip- 

 cord veronicas and species of Carmichaelia (Legum.) ; rolled leaves made 

 flat,* and vice versa ; cushion plants opened out widely. Undoubtedly 

 a series of experiments such, as those of Klebs (1903) would yield results 

 equally surprising. 



It can be seen from the above that this uncertainty as to " normal " 

 form opens up room for great doubt in all discussions regarding the origin 

 of permanent adaptations, for it may quite well be asserted that absolute 

 fixity does not exist. It seems to me all that can be done is to consider 

 as " normal " those forms which predominate and represent the general 

 growth-form of the bulk of the individuals ; but assuredly in no few cases 

 there is no normal form at all. 



3. Response to Ecological Factors. 



Warming has summed up the state of knowledge on this head up to 

 the date of publication of his admirable " Oecology of Plants" (Warming, 

 1909, pp. 16-81), so that only a few local examples are necessary here. 

 First of all, it must be emphatically pointed out that it is virtually 

 impossible in the field, where so many ecological factors are concerned, 

 to say which is the predisposing cause of the internal response of the 

 plant. Generally more than one factor will be concerned. 



(a.) Soil. 



Excess of salt leads to succulence, as in certain salt-meadow species 

 which become less succulent as members of non-halophytic formations. 

 The introduced Silene anglica L. develops more succulent leaves when 

 growing near the sea than inland. Miss Cross examined the anatomy of 

 certain salt-meadow plants and those of the same species grown in ordinary 

 soil in a greenhouse. Her figures show considerable differences in thick- 

 ness of leaves, but other factors besides want of salt doubtless affected 

 the result (1910, pp. 569-71). 



The soil near hot springs containing excess of sulphur, &c, inhibits 

 the erect shrub form of Leptospermum ericoides A. Rich., which then occurs 

 only in the prostrate form. 



Lack of nutritive salts in sand -plains near the mouth of the River Rangi- 

 tikei and elsewhere changes the leaf-form of Selliera radicans Cav. (Gooden). 

 This is in accordance with the much more carefully conducted observations 



* la the case of Olearia cymlifolia Hook. f. the much revolute, boat-shaped leave 

 become flat with moist-air culture, and what was considered an important specifi 

 character, distinguishing the " species " from O. nummviarifolia Hook, f., vanishes. 



