CHAPTER XII. 



DISTRIBUTION. 



104. THERE is a distribution of organisms in Space, and 

 there is a distribution of organisms in Time. Looking first 

 at their distribution in Space, we observe in it two different 

 classes of facts. On the one hand, the plants and animals of 

 each species have their habitats limited by external condi- 

 tions : they are necessarily restricted to spaces in which their 

 vital actions can be performed. On the other hand, the 

 existence of certain conditions does not determine the pres- 

 ence of organisms that are fit for them. There are many 

 spaces perfectly adapted for life of a high order in which 

 only life of a much lower order is found. 



While, in the inevitable restriction of organisms to environ- 

 ments with which their natures correspond we find a negative 

 cause of distribution, there remains to be found that positive 

 cause whence results the presence of organisms in some places 

 appropriate to them and their absence from other places 

 equally appropriate or more appropriate. Let us consider the 

 phenomena as thus classed. 



105. Facts which illustrate the limiting influence of sur- 

 rounding conditions are abundant, and familiar to all readers. 

 It will be needful, however, here to cite a few typical ones 

 of each order. 



The confinement of different kinds of plants and different 



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