EUROPEAN OCCUPATION 529 



very numerous seeds ; and still another, that a large proportion of them 

 come to maturity very rapidly, and that their seeds germinate quickly. 

 These characters are all retrogressive from one point of view — ^that is to 

 say — ^the plants exhibiting them have tended to become less instead of 

 more specialised in their development; but by this degradation of their 

 reproductive organs they have really become better adapted for the 

 peculiar conditions which are imposed upon them in their struggle with 

 the gardener and agriculturist. 



Commenting on this paper D. Petrie wrote as follows: 



The spread of weeds is mainly due to useful plants — their competitors 

 — being regularly checked and eaten down, while the weeds are mostly 

 allowed to grow without check of any important nature. Almost all weeds 

 found in our northern pastures owe their spread to this; e.g., several 

 buttercups, numerous docks, pennyroyal, Holcus mollis and H. lanatus, 

 and many weedy grasses, various spurges, mallows, mulleins, and so 

 forth. In many cases their spread is facilitated by the ready germination 

 of their seeds, by the long time that the seeds retain their vitality in the 

 soil, and by the readiness with which their earliest roots strike deep down 

 into the soil, which allows the plants to establish themselves in hot, dry 

 weather. Black Medick {Medicago lupulina), meadow plantain {Plantago 

 lanceolata), the docks and spurges, all start and thrive in hot, dry weather, 

 when more superficial-rooting seedlings die off. The introduced speed- 

 wells and poor man's weather-glass {Anagallis arvensis) are much in the 

 same case. The dechne of plants that have taken possession of a district 

 for some years is no doubt due to temporary exhaustion of some element 

 of plant-food needful for their vigorous growth. This principle lies at the 

 base of the theory of rotation of crops. In Central Otago, when I first 

 knew it, Carduus lanceolatus was the prevailing weed on open downs and 

 dry hill-slopes. Some years after C. pauciflorus ^ completely replaced it, 

 and this will, no doubt, be now giving way to something else. The doctrine 

 that the Scandinavian plants possess extraordinary vigour, which is the cause 

 of their aggressive character, seems to me very doubtful. In each single 

 species particular advantages can generally be assigned that will readily 

 explain their rapid spread. In the peninsula north of Auckland there are 

 very large areas of land on which European weeds have but slightly 

 established themselves, though the ground is frequently cleared of all 

 native vegetation by fires. In these areas native plants mostly grow up 

 with great readiness, especially species of Leptospermum and Pomaderris, 

 Haloragis tetragyna and H. minuta, besides various cyperaceous plants. The 

 pre-eminence in aggressive characters of North European plants is 

 decided enough, but many non-European plants are now widely spread 

 here and are indeed very aggressive. I may instance Modiola multifida, 

 a North American malvaceous plant, an Australian Plantago, two species 

 oiErigeron, and Kyllinga. The rat-tail grass, too, is no doubt introduced, 

 and has been most aggressive, while the South African Cyperus {minimus}) 

 is nearly as ubiquitous as sorrel. 



Here, as in the South, a few native plants are spreading : Aristotelia 

 racemosa. Fuchsia excorticata, Pomaderris phylicifolia, ErechtiUs, etc. ; but 



^ ? C. pycnocephalus. 



T. N. z. 34 



