June 15. 1893] 



NATURE 



163 



hides, wool and other objects likely to carry seeds have been 

 largely imported into Europe during the last two or three 

 centuries from I.a Plata, and during the last forty or fifty years 

 from Australia." {Op. cit., p. 340.) Wallace says, "There 

 is good reason to believe that the most effective agent in the 

 exiinction of species is the pressure of other species, whether as 

 enemies or merely as competitors. (" Island Life," p. 63 ) 



It is well known that few Australian plants have found a 

 fooling in Europe notwithstanding the many facilities which 

 commerce offers for their introduction, and the few American 

 weeds which have found their way to Europe do well only in the 

 Mediterranean region. Even in New Zealand but a few 

 Australian plants have become naturalised, as is shown by Mr. 

 T. F. Cheeseman's paper on the naturalised plants of Auckland 

 (read before the Auckland Institute, November 1892). 



In America, the majority of introduced weeds are European, 

 though at first they completely beat the natives, it is noteworthy 

 that now the natives are holding their own, and even beating the 

 strangers, thus showing that competition has gone on long 

 enough for some advantage to be gained by the natives. It is 

 remarkable too that the plants of Eastern America immigrated 

 westward with man, and conquered the western plants at first ; 

 but from a consideration of the facts the great American 

 botanist Prof. Asa Gray was led to prophesy a return wave of 

 western plants, and that is now actually coming. 



The theory that insulated floras are less able to resist the 

 influx of foreign plants is supported by the fact that only in the 

 Neilgherrie Mountains in India have Australian plants been 

 able to compete with others to any extent. It is, I believe, 

 considered that that part of India long existed as an insular 

 region. Therefore we see that the Australian flora, which 

 though isolated, had a large range, is able to get an advantage 

 over the Neilgherrie flora which was for so long developed in a 

 small centrum. 



One cause of the power of spreading of what are commonly 

 called weeds is that they are usually plants with inconspicuous 

 flowers, and as such are generally self fertilised and so can get 

 along without specialised insects to fertilise them. It is manifest 

 that in a new country where the local insect fauna is being 

 destroyed to some extent, the plants which have not to depend 

 on insects for fertilisation will be the more likely to win. And 

 even cross-fertilised plants seem to manage sometimes to find 

 insects to perform that office for them. Moseley points out an 

 instance in the following passage : — " The orange, lemon, and 

 lime, which grow wild all over Tahiti do not appear to de- 

 teriorate at all in quality or quantity of fruit, although in the 

 ferine condition. The fruit almost appears finer for running 

 wild Some native insect must have adapted itself com- 

 pletely to the blossoms of the orange tribe as fertiliser, so 

 abundant is the fruit." ("Notes ofa Naturalist on the Challenger,^' 

 p. 524,) The same is the case in Australia, for although the 

 orange does not seem to grow wild to any extent, lemons ha'-e 

 made themselves at home in the Illawarra district. The flowers 

 of the lemon and the native plant Synoum glanilulosum are 

 much alike in structure, and it may be that the same insect or 

 insects fertilise them. These plants would be on equal terms in 

 this respect, but the lemon from its wide cultivation has gained 

 a power of bearing diverse conditions, which gives it a better 

 footing. I may remark that Synoum is a common plant in 

 Illawarra. 



Among wind -fertilised plants are the grasses. The introduced 

 species so far are not beating the natives. They are equal as far 

 as regards fertilisation, but most introduced species are from 

 cool temperate region^, and so the Australian species being 

 warm temperate, are able to hold their own. The dying out of 

 some Australian grasses is attributable to over stocking and 

 close feeding and not to competition. 



In considering the introduction of weeds in Australia there is 

 a great difficulty viz. that it is hard in some cases to say 

 whether c rtain plants are indigenous or alien. It is considered 

 a safe ul; to take all plants common in the colony in Robert 

 Brown's time as truly indigenous, but as Brown only collected 

 in the neighbourhood of Port Jackson, that course leaves some 

 difficulty still. On this subject Baron von Mueller says in the 

 preface to his "Census of Australian Plants " (ist Edit. 1882) 

 — "The lines of demarcation between truly indigenous and 

 recently immigrated can no longer in all cases be drawn with 

 precision ; but whereas Alchemilla vulgaris, and Veronica ser- 

 pillifolia were found along with several European Carices in 

 untrodden parts of the Australian Alps during the author's 



NO. 1233. VOL. J 81 



earliest explorations, Alchemilla arvensis and Veronica fere' 

 grina were at first only noticed near settlements. The occur- 

 rence of Arabis glabra, Geum umbrosu?n, Agrinionia eupatoria, 

 Eupaloritim caiinabinum, Carpesium cernuiiin, and some others 

 will readily be disputed as indigenous and some questions con- 

 cerning the nativity of various of our plants will probably 

 remain for ever involved in doubts." As will be seen from 

 this, the origin of some plants will and must remain more or 

 less a matter of personal opinion. And on referring to lists of 

 plants of the various colonies it will be found that their authors 

 differ in their placing of these doubtful plants. If we critically 

 examine the Census of New South Wales plants by Mr. C. 

 Moore, of Queensland plants by Mr. F. M. Bailey, of Victorian 

 by Baron Von Mueller, and of New South Wales by Dr. Woolls, 

 we shall find abundant evidence of diversity of views in this 

 respect. But very many weeds present no difficulty at all, 

 although the record of their plentiful occurrence in very early 

 days may well surprise us. The Rev. J. E. Tenison- Woods 

 ('■ Proc. Linn. Soc. of N.S. Wales," vol. iv., p. 133) remaiks 

 that Leichhardt found Verbena bonariensis so plentiful in the 

 neighbourhood of Darling Downs, then only five or six years 

 settled, that be named the place Vervain Plains. 



The injury done by introduced weeds Hill be almost entirely 

 by competition, but it is possible that in time, the Australian 

 plants may begin to hold their own and even to some extent 

 drive out the others. This will be more especially the case with 

 the group of plants which are found on the barren and sandy 

 tracts wherever the Hawkesbury Sandstone formation occurs. 

 In such land few aliens get a footing. On the sandstone about 

 Sydney as a rule, and in the Blue Mountains where the same 

 soil occurs, the foreign weeds have no chance. But wherever 

 the soil is fairly good, or where it has been broken up, there 

 they triumph and exclude the indigenes. 



To some, extent however, the weeds will work their own 

 destruction. They increase so rapidly that competition is most 

 severe, not between them and the native?, but between 

 individuals of the same alien species, or between distinct alien 

 species. Sisymbritim officinale was once a pest near Mudgee, 

 the fallow and unoccupied land being covered with a thick mass 

 of it ; but after the lapse of a few years it became quite rare, 

 and Erigeron canadense took its place. I think that in some 



; cases the fact of a heavy crop of weeds occurring in a locality 



: one or more years is a reason for expecting its scarcity in the 



I following years. The soil becomes exhausted of the particular 

 constituents demanded by the planls,and they fail in consequence. 

 I had often read doleful prophecies of the damage that might be 

 expected when the Cape weed {Cryptostemtna calendulaceum) 

 became common. When I first saw it appear in Illawarra, I 



i was therefore prepared to see much land infested by it in a 



' short time. It spread to a great extent in certain spots for a 

 couple of years and then almost disappeared. In my garden 

 half-a-dozen vigorous plants came up, and as I left them for 

 the purpose of observation, they flowered and seeded plentifully. 

 I fully expected a large crop the following year, but to my sur- 

 prise not a single plant was to be found, nor has there been on 

 Mr. T. Kirk, in a paper on the naturalised plants of Port 

 Nicholson, N.Z., says : — "At length a turning point is reached, 



I the invaders lose a portion of their vigour, and become less 

 encroaching, while the indigenous plants find the struggle less 



\ severe and gradually recover a portion of their lost ground, the 

 resultbeing the gradual amalgamation of those kinds best adapted 

 to hold their own in the struggle for existence with the intro- 

 duced forms.and the restriction of those le>s favourably adapted to 

 habitats which afford them special advantages." (Trans. N. Z. 

 Inst., vol. X., p. 363 ) And Mr. T. F. Cheeseman, from whnse 



j paper on the " Naturalised plants of the .Auckland District" I 

 have quoted the above, coincides with this opinion to some 



j extent and says, " Speaking generally 1 am inclined to believe 

 that the struggle between the naturalised and the native floras 

 will result in a limitation of the range of the native species rather 

 than in their actual extermination. We must be prepared 



i to see many plants once common become comparatively rare, 



i and possibly a limited number — I should not estimate it at more 

 than a score or two — may altogether disappear, to be only known 

 to us in the future by the dried specimens in our museums."' If 

 this is likely to be the case in a territory so limited as New 



i Zealand how much more is it probable in Australia with the 

 vast extent of area, diversified surface and various climates from 

 tropical to cold temperate. 



j 1 Paper read before the Auckland Institute, November 1E92. 



