32 Transactions. 



as to the likelihood of extremely small* variations being preserved or the 

 contrary. It must be understood that the " struggle " is not only between 

 the individuals of the competing species, but also between these and their 

 environment. This was distinctly stated by Darwin, who refers to the 

 struggle for life against the drought on the edge of a desert (1899, p. 46). 

 In manv instances this struggle with outer circumstances is the more im- 

 portant ; it is also the deciding factor as to what plant-form can gain a 

 rooting in the first instance. 



The formations themselves offer various conditions according as they 

 are " open " or " closed," for in the former there is apparently room for 

 new-comers, whereas in the latter it is almost impossible for a species 

 from without to gain admittance. This fact is of major importance, 

 for, amongst other matters, it has a strong bearing on the much-debated 

 question regarding former land connections with distant islands as op- 

 posed to bird carriage, &c, across wide"\ areas of ocean. The case of 

 New Zealand as a whole is of great interest in this regard, especially 

 as many misstatements;}: have crept into evolutionary writings regarding 

 the spread of the introduced plants and their rapid ' replacement " of 

 the indigenous flora. I will state briefly what I believe to be the true 

 state of affairs. 



There have been recorded for New Zealand up to the present some 555 

 species of introduced plants, but less than 180 can be considered common, 

 whilst others are local, rare, or even not truly established as wild plants. 

 Many at first sight appear better suited to the soil and climate than are the 

 indigenous species, and over much of the land they give the characteristic 

 stamp to the vegetation ; but this is only the case ivhere draining, cultivation, 

 constant burning of forest, scrub, and tussock, and the grazing of a multitude 

 of domestic animals have made absolutely new edaphic conditions which ap- 

 proximate to those of Europe, and where it is no wonder that the European 



* During the discussion following the reading of this paper the expression " ex- 

 tremely small " was criticized as not giving a fair representation of the views of Darwin 

 and his followers. Darwin, however, writes (1889) — p. 45, " Variations, however slight " ; 

 p. 58, " any advantage, however slight " ; p. 59, " extremely slight modifications " ; 

 p. 69, " individual differences, too slight to be appreciated by us." Weismann puts 

 the case more strongly still (1910, p. 25) : " For the question is not merely whether 

 finished adaptations have selection value, but whether the first beginnings of these, 

 and whether the small, I might almost say minimal, increments which have led up from 

 these beginnings to the perfect adaptation have also had selection value." Wallace, 

 on the other hand (1889, pp. 126, 127), claims that though Darwin used the word " slight " 

 and " small amount," these terms are " hardly justified," since the variability of 

 many important species is of considerable amount, and may very often be properly 

 described as large. 



t The case of Krakatoa, important as it otherwise is, seems to me to have 

 but little bearing on this question, since the distance from the mainland is too 

 trifling. 



% Wallace (1889, pp. 28, 29) refers to Tri folium repem exterminating Phormium 

 tenux; excellent pasture destroyed in three years by Hypochoeris radicata, which can 

 even drive out white clover; and Sonchvs oleracens growing all over the country up to 

 an elevation of 6,000ft. Kirk (1896, p. 18) not only attributes the "displacement " 

 ot Phormium to grasses and clovers, but also Mariscus ustulatus, and even Pteridivm 

 escidentum (bracken fern). Further on (p. 19) he states that Aciphylla Colensoi is 

 gradually replaced by self-sown pasturage plants. However, he also calls attention 

 to the effect of grazing and trampling by cattle and horses as aiding the plants 

 in their work, which, of course, is a very different matter from the effect of plants 

 alone. 



