Cockayne. — Ecological Studies in Evolution . 35 



in the umbel and the involucral bracts. A. diversifolia has been found so 

 far on only one mountain on which A. carnosula is not known to occur; 

 but the species are so much alike that they could only be recognized when 

 in bloom and examined closely. 



To trace the evolution of the shingle-slip plants it seems clear that one 

 must go back to the origin of the shingle-slips themselves from their small 

 beginnings before the eastern peaks of the Southern Alps were disintegrated 

 into rounded summits. If for any reason the climate were wetter,* there 

 would be a similar condition of affairs to what governs the shingle-slips 

 of Westland to-day where true shingle-slip species are absent. On the 

 embryonic debris slopes many plants could settle down, and to the be- 

 liever in natural selection nothing could appear more probable than for 

 these to have been gradually changed in accordance with the slowly chang- 

 ing environment, species after species going to the wall, until only the 

 few highly differentiated should remain. Even these are absent over wide 

 areas of the most extensive and unstable of these alpine deserts. 



An exactly similar argument to the above would apply to water forma- 

 tions, especially as there are cases where true water-plants — e.g., Pota- 

 mogeton Cheesemanii Bennett — flourish in situations where they are quite 

 uncovered for considerable periods. Even for unstable dunes, where there 

 is certainly no struggle between plant and plant, and where no non-sand - 

 binding form could possibly become established, a similar argument would 

 apply, since all degrees of sand-movement exist in a dune-area. But in 

 all the above cases we do know that ecological factors can evoke structures such 

 as are essential, and we do not know for a fact that selection can intensify a 

 character beyond a certain limit. In the tussock - grass Poa caespitosa the 

 power to respond to sand-movement is already present, although its 

 adaptations fit it for other conditions ; thus it has occupied the recent 

 drifting sands of Central Otago. Cases such as these, of stony debris, 

 water, and dune, should be decided not on preconceived opinions or 

 theories, but on the most reasonable conclusions from the observed facts. 



Kock-vegetation, although open, affords plenty of scope for the struggle 

 for existence both between the individuals and with the environment, 

 since, leaving the lithophytes out of the question, the space for rock-crevice 

 plants is very limited. 



On the recent roches moutonnees alongside the Franz Josef Glacier the 

 occupation of rock is now in progress. The pioneer plant is a dark-coloured 

 species of moss, which when it happens to grow in a crevice forms a soil, 

 an essential for the successful germination of seeds in such a station. The 

 first-comers are all plants of some neighbouring association, mostly xero- 

 phytes, some herbs, and other shrubs, or even trees, whose long roots can 

 penetrate into the chinks. Exceptions to this are the filmy fern Hymeno- 

 phyllum, multifidum. Swz., the epiphytic or rock-dwelling orchid Earina 

 autumnalis Hook, f., and Lycopodium varium R. Br. ; but it must be re- 

 membered the atmosphere is nearly always saturated with water-vapour. 

 The above first-comers react one upon another, the most vigorous finally 

 conquering ; but this vigour depends rather upon age than on greater 



* Speight, in a carefully considered paper (1911), brings forward a good deal of very 

 suggestive evidence as to the probability of a wetter climate on the east than the pre- 

 sent one following the steppe climate. The most important fact adduced is the former 

 presence of extensive forests where steppe alone now exists, since such forest could only 

 te established during a period with many rainy days, and no other explanation seems 

 to fit the ease. 



2*— Trans. 



