136 EEV. G. HENSLOW — ORIGIN AND DISTRIBUTION 



of the Pacific Ocean. Tlius, tlie Society Islands contain 3 ; the 

 Sandwich, 5 ; and Fiji, 16 species. 



If now we attempt to find an explanation to the fact of so many 

 plants thoroughly establishing tliemselves in foreign countries, 

 there are two features which strike us as worthy of observance. 

 One peculiarity is that plants do not always flourish best where 

 Nature has, so to say, made their home, but in consequence of the 

 struggle for existence they hold their position as long as other 

 plants will let them grow, so that tlie flora of any locality under 

 normal and existing circumstances has, so to say, long ago arrived 

 at a condition of equilibrium of mutual adjustment. If, however, 

 plants be suddenly transported to other countries, they sometimes 

 at once assume astonishing vigour, and for a long time at least 

 gain great ascendency over the native vegetable population. This 

 was conspicuously so in New Zealand, where the English water- 

 cress grows to twelve feet in length, and three-quarters of an inch 

 in thickness ; while a single plant of Polygonum aviculare will cover 

 several square feet, and the little Dutch clover is driving the huge 

 Fhormium tenax or "New Zealand flax" before it! Similarly 

 does the Canadian Anacharis Alsinastrum flourish in England, 

 though we possess the female plant only. It would seem, there- 

 fore, that the change of climate has somehow introduced new 

 and invigorating elements into their constitution, which the native 

 flora cannot acquire, having been so long adapted to it. This 

 appears to be one cause of introduced plants so readily establishing 

 themselves. Another is that these sporadic plants, being gene- 

 rally inconspicuous annuals and self -fertilising , are independent 

 of insects ; so that they sui'vive in the struggle for existence over 

 their more showy brethren, which cannot propagate fully by seed 

 unless habitually visited. 



In a previous paper on the " Fertilisation of Plants,"* I have 

 shown how this was the case as deduced from statistics, and so will 

 not repeat the evidence now ; but would just illustrate it by men- 

 tioning a few of the most widely dispersed of our British plants. 

 Canlamine hirsuta is found in north-east Asia, tropical Asia, 

 Hong-Kong, Kamskatka, Chili, South Australia, Auckland and 

 Campl)ell's Islands, Falkland and Fuegia, Tasmania, South Africa, 

 New Zealand, Madeira, etc. Similarly is Cerastium vulgatum dis- 

 persed over the same area. Solanum nigrum is also found in Cali- 

 fornia, South Australia, Tasmania, New Zealand, Society Islands, 

 Andaman Isles, North China, Japan, Galapagos Islands, etc. 



I will conclude this portion of the subject with a summary of 

 the general phenomena of botanical distribution, condensed from 

 Hooker's 'Flora of Tasmania.' 



1. Species have a definite or circumscribed area, or " specific 

 centres; " varieties being more restricted than species, and species 

 than genera. Cause of this: — "Plants grow where other plants 

 wiU let them." 



•'Trans. Watford Xat. Uist. Soc.,' Vol. I, p. 201. 



