INTRODUCTORY ESSAY. 



3. With comparatively few exceptions, plants are confined within well-marked limits, which, 

 though often very wide, are sometimes as much the reverse ; while the instances are rare of sporadic- 

 species, as such are called which are found in small numbers in widely sundered localities. These 

 facts seem incompatible on the one hand with the theory of species spreading from many centres, and 

 on the other with then: varying indefinitely ; for were it otherwise, sporadic distribution would be the 

 rule, insular floras would not necessarily be peculiar, and similar climates would have similar, if not 

 identical species, which is not the case. 



4. A multitude of allied species of plants grow close together without any interchange of spe- 

 cific character ; and there are instances of exceedingly closely allied plants keeping company under 

 many modifications of climate, soil, and elevation, yet never losing their distinctive marks. 



5. The individuals that inhabit the circumference of the area occupied by a species, are not 

 found passing into other species, but ceasing more or less abruptly ; their limits may meet or overlap 

 those of one or more very similar species, when the individuals associate, but do not amalgamate. 



6. One negative argument in favour of distribution from one centre only, is, that taking the 

 broadest view of the dispersion of species, we find that the more extensive families* are more or less 

 widely distributed, very much in proportion to the facilities they present for dispersion. Thus the 

 most minute-spored Cryptogams -f- are the most widely dispersed of all organized nature; plants that 

 resist the influence of climate best, range furthest ; water-plants are more cosmopolite than land- 

 plants, and inhabitants of salt, more than those of fresh water : the more equable and uniform is the 

 climate of a tract of land, the more uniformly and widely will its plants be distributed. 



7. The species of the lowest Orders are not only the most widely diffused, but then - specific 

 characters are not modified by the greatest changes of climate, however much their stature and 

 luxuriance may vary. Fungi offer a remarkable instance of this : their microscopic spores are wafted 

 in myriads through the air ; the life of the individuals is often of very short duration, and many of 

 them being as sensitive as insects to temperature and humidity, they are ephemeral in all senses ; 

 sometimes appearing only once in the same spot, and remaining but a few days, never to reappear 

 within the observer's experience. The specific characters of many reside in the diameter, form, colour, 

 and arrangement of their most minute organs, whose analysis demands a refinement of microscopic 

 skill •; yet the most accomplished and profound botanist in this Natural Order (who has favoured me 

 with the descriptions of the New Zealand Fungi) fads to find the most trifling character by which 

 to separate many New Zealand species from European. 



8. The fact, now universally conceded by all intelligent horticulturists, that no plant has been 

 acclimated in England within the experience of man, is a very suggestive one, though not con- 

 clusive ; for it may be answered, that plants which cannot survive a sudden change, might a slow and 

 progressive one. On the other hand, plants have powers of enduring change when self-propagated 

 that they have not in our gardens ; thus I find a great difference in the hardiness of individual species 

 of several Himalayan plants J, depending upon the altitude at which they were gathered. In these 



* This rule does not extend to the Natural Orders themselves. The Composite, whose facilities for dispersion 

 are proverbial, are amongst the most local ; and the same may be said of Leguminosee and Soiauece, whose seeds 

 retain their vitality in a remarkable degree : a few of their species are remarkably cosmopolite, but the greater num- 

 ber have generally narrow ranges. 



f The fact (first communicated to me by the Rev. M. J. Berkeley) of the spores of Fungi having been found 

 by Professor Ehrenberg mingled with the atmospheric dust that has fallen on ships far out at sea, is one of the 

 most decisive proofs of this. 



% Thus some of the seedling Pines whose parents grew at 12,000 feet appear hardy, whilst those of the same 



c2 



