ZOOLOGY. 331 



produced for each species. A more weighty difference of opinion 

 arises from a series of facts, exceptional to those above announced of 

 the endemic or local distribution of plants, namely, the instances, 

 which are numerous, of what is called a sporadic or universal distribu- 

 tion of a species, as, for instance, of the sea sedge (Scirpu* maritimus^) 

 which occurs in similar situations nearly all over the world ; as is the 

 case almost to an equal extent with Samolus Valerandi, and in a less 

 degree with many others. Such very wide distribution of certain 

 plants has led to the opinion that species were created in different 

 places, at different centres, either simultaneously or at different 

 epochs, and that thus different families or stocks of the same species 

 co-exist and share in populating the area inhabited by the species. 

 The instances, however, on which this hypothesis rests, and the almost 

 unlimited powers of diffusion that exist, together with our ignorance 

 of the steps of the migration of plants and the absence of chrono- 

 logical data of the distribution of plants, appear to me to render this 

 view unsatisfactory, and like the preceding to involve supererogatory 

 creative acts, not according with the simplicity so characteristic of 

 the laws under which the natural forces are made to act. Admitting, 

 then, this hypothesis of creation at specific centres, let us see what 

 are the consequences to be deduced from it. We may suppose the 

 species to have been pretty equally apportioned to equal latitudes 

 over the globe, and located in the different regions according to their 

 physiological characters. As they spread they would become mingled 

 together like the circles from rain-drops in a pond, and complex con- 

 ditions at any particular point after the lapse of a long space of time 

 would be explicable by the influence of external agencies upon the 

 individual species, and their influence upon one another, as exerted 

 by those of more vigorous growth bearing down and repressing the 

 growth and dissemination of the more tender. How far the facts we 

 meet with agree with such a view is the test by which the hypothesis 

 must be tried, and the researches of botanical geographers appear to 

 give a favorable verdict in the present state of the inquiry. It is 

 necessary, however to be very comprehensive in our examination 

 of the phenomena, since the interfering causes are so numerous that 

 false conclusions may readily be drawn from the investigation of 

 small areas, or from too little weight being attributed to what often 

 seem to be very trivial external agencies. It would naturally be 

 supposed that the dissemination of species would go on most freely 

 and copiously over large continents, and that the present condition of 

 nature would exhibit more instances of peculiarity in the floras of 

 given regions in proportion as those regions were cut off by some 

 natural barrier, such as mountain ranges, oceans, &c., from other 

 lands; moreover, that islands would present a more characteristic 

 vegetation than tracts of equal extent on a continent, and would 

 approximate in their characters most closely to the main lands nearest 

 to which they were situated. Such is actually the case. The conti- 

 nents lying in the northern hemisphere are almost continuous in their 

 most northern regions, and we find the greatest agreement in their 

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