INTRODUCTORY ESSAY. XV 
these are mundane, that is, found in all or most temperate or tropical climates, as Ranunculus, Cle- 
matis, Senecio, and many Grasses and Ferns; and we cannot yet tell whether the difficulties are greater 
with them than with the more local or endemic genera, as Coprosma, Celmisia, Alseuosmia, and Dra- 
cophyllum. Of the mundane genera again, some are chiefly composed of species which are local (as 
is the case with the three first mentioned), while of others the species themselves are widely distri- 
buted, as those of Potamogeton, Lemna, and many Ferns. 
The fact of a plant having a wide range implies its being exposed to climatic differences that 
often induce change, and the consequent propagation of forms or races that cannot be recognized as 
members of one species, without full series of specimens from many localities. If we allow a suffi- 
cient time, it is quite reasonable to suppose that geological or other natural causes (producing a 
change of climate) may isolate by sea or desert, or by the intrusion of stronger plants that monopolize 
the soil, the outlying abnormal states of a species that was once uniformly spread over an area. 
To connect those dissevered members is often a work of great difficulty, for individuals of such races 
freguently retain their character even when they have been under cultivation for many years. 
Hybridization has been supposed by many to be an important element in confusing and masking 
species“. Nature, however, seems effectually to have guarded against its extensive operation and its 
effects in a natural state, and as a general rule the genera most easily hybridized in gardens, are not 
those in which the species present the greatest difficulties. With regard to the facility with which 
hybrids are produced, the prevalent ideas on the subject are extremely erroneous. Gärtner, the 
most recent and careful experimenter, who appears to have pursued his inguiries in a truly philoso- 
phical spirit, says that 10,000 experiments upon 700 species produced only 250 true hybridst. It 
would have been most interesting had he added how many of these produced seeds, and how many of 
the latter were fertile, and for how many generations they were propagated. The most satisfactory 
proof we can adduce, of hybridization being powerless as an agent in producing species (however 
much it may combine them), are the facts that no hybrid has ever afforded a character foreign to that 
of its parents, and that hybrids are generally constitutionally weak, and almost invariably barren. 
* Unisexualf trees must offer many facilities for the natural production of hybrids, which, nevertheless, 
have never been proved to occur, nor are such trees more variable than hermaphrodite ones. 
nothing in common but their facility for transportation. From the above-mentioned hypothesis it would hence result 
that whilst the differences of one degree (specific) are small and inconstant, those of a higher degree (generic) are 
great and trenchant. To a certain extent, however, these facts are not incompatible, for we can imagine a flora 
wholly composed of a few genera as well marked (generically) as Coprosma and Alseuosmia, whose species may yet 
be as undefinable ; or again, species may be well marked, yet variable in characters which would in no one's opinion 
be of specific value. 
* Hybridization as an agent in confusing species is ‘a very favourite argument with those who are fond of 
founding species on inconstant characters; when shown a specimen combining two such spurious species, they at 
once pronounce it a hybrid—a very simple way of getting rid of a difficulty. In Ferns, the most variable of all 
plants, hybrids were once generally admitted to exist, but the observations of Suminski have led to the discovery 
of their sexual organs, whose arrangement and structure seem to preclude the possibility of such a phenomenon. 
T See his observations on muling, Hort. Soc. Journ. vol. v. and vi. 1850-1851. 
i Unisexual plants are very interesting in many points of view, and in none more than in the varying de- 
velopment of the sexes according to circumstances. Observations on this subject are very much wanted : it has 
been stated to depend on local circumstances whether the seeds of a bisexual plant shall come up male or female ; 
and the fact of both kinds of flowers, or even of hermaphrodite flowers, often occurring on a plant that usually per- 
fects one sex only (as in the monwcious Hop-plant described by Mr. Masters in Gard. Chron. 1847), shows that 
we may even speculate on the possibility of dicecious plants having sprung originally from a single parent, whose off- 
