1899] ORIGIN OF AUSTRALIAN FLORA 205 



of these economic plants have been introduced from the north, and 

 at the same time the seeds of other plants accustomed from time 

 immemorial to flourish in association with them, as well as the seeds 

 of species which have been allowed, for the sake of old recollections, 

 to obtain a foothold in the new homes of the race. We have seen 

 how advantageous it is for a migrating species to be herbaceous, and 

 a still greater advantage should obtain where migration has been so 

 effectually assisted by human effort. Then again, a point we ought 

 to have information about, for it has material bearing on the case, is 

 whether the indigenous herbaceous vegetation has benefited by the 

 introduced changes. 1 



But the case becomes still stronger when we take Australia into 

 consideration. The fierce droughts experienced by so large a part of 

 that country have brought about the survival of a vegetation to a very 

 large extent xerophilous. Now there is one peculiar feature about all 

 desert countries except the very dryest, a feature necessarily tending 

 to favour the spread of any herbaceous vegetation of which the seeds 

 may chance to be introduced into them, namely, that at least during 

 some part of the year there are always places where water is apt to 

 collect, and where the ground will remain moist during the short time 

 while the life-history of a herb is being enacted. 2 This is simply 

 what one sees in the interior of Western Australia. For a period 

 Ion" enough to ensure the maturation of their seeds, introduced plants 

 enjoy, in normal seasons, conditions precisely similar to those obtain- 

 ing in their native habitats. But no sooner does the sun gain in 

 power, and the ground become dry and warm, than these herbs com- 

 pletely disappear ; they show, in fact, none of that capacity for adapt- 

 ing themselves to their altered surroundings which we should expect 

 members of an " aggressive " flora to possess. This is, however, not 

 the only advantage " Scandinavian " species enjoy when introduced 

 into a country with a dry climate such as Australia. If one or more 

 seasons of drought supervene, what happens ? Considerable though 

 varying power of latency is possessed by the great majority of seeds, 

 and under these circumstances the introduced herb is in precisely the 

 same position as the indigenous, both having to await a favourable 

 season in order that their seeds may germinate. Contrast this now 

 with the fate awaiting seeds of dry southern climates introduced into 

 a country with a climate like ours. A short spell of warmth sets in, 



1 Authoritative information on this suhject has recently come to hand (vide T. Kirk, 

 Presidential Address to the Wellington Philosophical Society, 1895 ; abstracted in Journ. 

 of Botany, 1896, p. 338). From this it is clear that in some cases indigenous species have 

 benefited by changes due to human agency. 



2 The conditions in Australia are specially favourable to the introduction of cold 

 temperate herbs, inasmuch as it is only when the temperature is low, that is, when the con- 

 ditions approximate to those of the summer of Northern Europe, that the ground remains 

 moist for any length of time. Then is the only chance for herbaceous vegetation whether 

 endemic or introduced. 



