INTRODUCTORY ESSAY. Xlll 
The universality of this feature (of groups having defined areas) affords to my mind all but 
conclusive evidence in favour of the hypothesis of similar forms having had but one parent, or pair 
of parents. And further, this circumscription of species and other groups in area, harmonizes well 
with that principle of divergence of form, which is opposed to the view that the same variety or 
species may have originated at different spots. It also follows that, as a general rule, the same species 
will not give rise to a series of similar varieties (and hence species) at different epochs ; whence the 
geological evidence of contemporaneity derived from identity of fossil forms may be relied upon. 
The most obvious cause of this limitation in area no doubt exists in the well-known fact that 
plants do not necessarily inhabit those areas in which they arc constitutionally best fitted to thrive 
and to propagate; that they do not grow where they would most like to, but where they can in id 
space and fewest enemies. We have seen (13) that most plants are at warfare with one or more 
competitors for the area they occupy, and that both the number of individuals of any one species and 
the area it covers are contingent on the conditions which determine these remaining so nicely balanced 
that each shall be able at least to hold its own, and not succumb to the enervating or etiolating or 
smothering influences of its neighbours. The effects of this warfare are to extinguish some species. 
to spare only the hardier races of others, and especially to limit the remainder both as to area and 
characters. Exceptions occur in plants suited to very limited or abnormal conditions, sucli as desert 
plants, the chief obstacles to whose multiplication are such inorganic and principally atmospheric 
causes as other plants cannot overcome at all; such plants have no competitors, are generally widely 
distributed, and not very variable."* 
15. The three great classes of plants, Acotyledons, Monocotyledons, and Dicotyledons (Gymno- 
spermous and Angiospermous), are distributed with tolerable equality over the surface of the globe, 
inasmuch as we cannot indicate any of the six continents (Europe, Asia, Africa, North and South 
America, and Australia) as being peculiarly rich in one to the exclusion of another. Further, the dis- 
tribution of some of the larger Orders is remarkably equable, as Composites, Leguminosa, Graminea, 
and others ; facts which (supposing existing species to have originated in variation) would seem to 
indicate that the means of distribution have overcome, or been independent of the existing apparent 
impediments, and that the power of variation is equally distributed amongst these classes, and con- 
tinuously exerted under very different conditions. I do not mean that all the classes are equally 
Variable, but that each displays as much variety in one continent as in another. 
16. Those Classes and Orders which are the least complex in organization are the most widely 
distributed, that is to say, they contain a larger proportion of widely diffused species. Thus the 
species of Acotyledons are more widely dispersed than those of Monocotyledons, and these again 
more so than those of Dicotyledons ; so also the species of Thallophytes are among the most widely 
dispersed of Acotyledons, the Graminea of Monocotyledons, and the Chenopodiacea of Dicotyledons. 
This tendency of the least complex species to be most widely diffused is most marked in Acotyledons, 
and least so in Dicotyledons, f a fact which is analogous to that already stated (4), that the least 
complex are also the most variable. 
* Though invariable farms, they may be, and often are, themselves varieties or races of a species that inhabits 
more fertile spots, as Foa bulbosa, which is a very well-marked and constant form of P. pratensis, occurring in dry 
sandy soil, from England to North- western India, its "meadow" relative being a very variable species in the same 
countries, aud always struggling for existence amongst other Grasses, etc. 
f Very much, no doubt, because of the difficulty in classifying Dicotyledons by complexity of organization ; in 
other words, of our inability to estimate in a classificatoiy point of view the relative value of the presence or absence 
