INTRODUCTORY ESSAY. xiii 



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 are constitutionally best fitted to thrive 

 and to propagate ; that they do not grow where they would most like to, but where they can find 

 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, such 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, Graminecs, 

 and others ; facts which (supposing existing species to have originated in variation) would seem to 

 indicateahat 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 Gramineee of Monocotyledons, and the Chenopodiacece of Dicotyledons. 

 This tendency of the least complex species to be most widely diffused is most marked in Acotyledons, 

 and least so in Dicotyledons,t a fact which is analogous to that already stated (4), that the least 

 complex are also the most variable. 



* Though invariable forms, they may be, and often are, themselves varieties or races of a species that inhabits 

 more fertile spots, as Poa bulbom, 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 

 coantries, and 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 classincatory point of view the relative value of the presence or absence 



