I 



PRIMARY REQUIREMENTS FOR PLANT GROWTH. 11 



Locality, Death Valley, California. 



Station, edge of the salt marsh about 400 meters east of Bennett 

 Wells, Death Valley, California. 



Habitat, densely alkaline moist soil, apparently only that containing 

 compounds of boracic acid. 



It is believed that the current signification of most other terms teeh- 

 nieally used in connection with the subject of geographic distribution 

 is sufficiently specific to make their definition unnecessary, but a few 

 other words are explained in the text as occasion requires. 



PROBLEM OF DISTRIBUTION. 



In our present consideration of the distribution of plants we adopt 

 the hypotheses— that existing species have developed from similar pre- 

 existing ones; that, in general, the successive steps in this develop- 

 ment have originated directly from changed environment, and have 

 become fixed by natural selection; that usually a species has held the 

 relation to its parent of a variation connected with it by a series of 

 intermediate forms and spreading from a single geographic center, but 

 having a distinct range or a different habitat; and that these geo- 

 graphic variations have become species by the disappearance of the in- 

 termediate forms. These hypotheses have so nearly attained the rank 

 of demonstrated propositions that it is considered unnecessary to re- 

 peat here the facts and arguments supporting them. The writer is not 

 to be understood, however, as necessarily holding that species and even 

 generic types have not in some cases originated from their parent 

 species without the intervention of a complete series of connecting 

 forms, or that the same variety or even species has never been devel- 

 oped from a common parent in two widely separated regions indepen- 

 dently. 



The existence of an individual plant in a particular locality rests 

 upon two primary requirements— first, that the seed or other reproduc- 

 tive body be transported there; and second, that the local conditions 

 permit its growth. 



There are undoubtedly many separated regions— as, for example, the 

 Mohave Desert and the Sahara Desert— whoseenvironmental conditions 

 for plants are practically identical, but whose floras consist mostly of 

 totally distinct types, because natural transportation of seeds has been 

 impossible. The same may be stated of certain portions of Australia 

 as compared with the tropical and subtropical regions of America. A 

 long list of similar cases could be made up, the two regions in each 

 pair of names being situated in essentially different continental areas. 



An excellent example of the second requirement for the growth of a 

 plant, the proper environment, is afforded at Cajon Pass in the San 

 Bernardino Mountains. The flora of the Mohave Desert, lying to the 

 north ami east, is here abruptly substituted, within a distance of 3 to 5 

 kilometers, for the flora of the San Bernardino Valley. There is abundant 



