D. 
INTRODUCTORY ESSAY. xi 
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 their 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- 
cifie 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, twhen 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 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 their 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) fails 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 plantst, 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 Leguminose and Solanee, 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. 
T 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 
