424 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



their position as long as they can manage to do so ; so that the 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 some- 

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

 great ascendancy over the native vegetable population. This is conspicu- 

 ously so in New Zealand, where an English watercress (Nasturtium 

 amphibium) 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 Phormium 

 tenax, or " New Zealand flax," before it ! The number of British plants 

 has for years been steadily increasing in New Zealand. Similarly does 

 the Canadian Elodea canadensis flourish in England, though we possess 

 the female plant only. It would seem, therefore, 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 the sporadic plants, 

 being generally inconspicuous annuals and wind- or self -fertilising, are 

 independent of insects ; so that they survive in the struggle for existence 

 over their more showy brethren, which cannot propagate fully by seed 

 unless habitually visited. 



In a paper on the " Se]f -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 mentioning a few 

 of the most widely dispersed of our British plants. Thus, e.g., the 

 bitter-cress (Cardamine hirsuta) is found in north-east Asia, tropical 

 Asia, Hong Kong, Kamchatka, Chili, South Australia, Auckland, and 

 Campbell's Islands, Falkland and Fuegia, Tasmania, South Africa, New 

 Zealand, Madeira, &c. Similarly is Cerastium vulgatum dispersed over 

 the same area. Solanum nigrum is also found in California, South 

 Australia, Tasmania, New Zealand, Society Islands, Andaman Isles, North 

 China, Japan, Galapagos Islands, &c. 



Having now considered the present distribution of the British flora, 

 we have to account for it as far as possible ; and here theory must supple- 

 ment facts. In looking back to discover an historical or rather geological 

 origin of our present flora, we soon find that there have been very 

 remarkable changes in the characters of successive floras that peopled 

 our country. Going no further back than the Eocene period — for 

 attempts at deductions as to climatal conditions become more and more 

 uncertain in proportion as the faunas and floras are more remote in time 

 from and unlike their living representatives — we find tolerably certain 

 evidence that the climate of England at that time was tropical, at least so 

 far as palms, Mimosac, Nipaditcs, on the one hand, and turtles, crocodiles, 

 and large water-snakes on the other, justify us in drawing such a 

 conclusion. This period, then, could not have seen the origin of our 

 present temperate and arctic floras. The next epoch, the Miocene, like- 

 wise fails to furnish any members of it. The flora of this period was 

 sub-tropical, but' probably became less and less so as the next — the 

 * Tram. Lin. Sue. 1877. 



