34 ri,.\Nr lifk ok ai.aisama. 



'I'lic <)|)|)(iit unilit's tor tlir dispcision <»!" sjx'cics li\- tlicsc iiioans aro 

 more or less diiiiitiislKMl l»y iii(cr\ cniiiL;' f.\t('ii>i\ r water and desert 

 areas or liiiili iiiountairi I'anecs. 



'riies«' priiiiai'v and secondary t'actoi's. in their e\ cr-varyin*,'' eoinhi- 

 natioiis, are the conditions hy which the \ai"ioiis associations of species 

 and tlieir restriction witliin specitic areas ol" ei-catcM- or h-ss extent are 

 determined. Such areas constitut*' tlie Hoi'al re»;ions: and the system- 

 atic relationsliii) ol' the ditt'erent species, theii- numei-ical pi'opoitions, 

 and their various assembhij^'es impart to each ree-ion its lioral character. 



SuitabhM'iivironment. tliat is, a proper c()ml)inatio!i of conditions of 

 moisture, suflicient room and lig'ht, propei' ex])osure, etc.. (h-termines 

 tlie plai-e in whicli a phmt linds all the icijuirements for its existence 

 met. that is, its Jidhifaf. 



The conditions wiiich outline its hahitat. in comliination with the 

 greater factors of latitude, altitude, rainfall, etc.. d(>termine tlie distri- 

 bution of pUints over wider areas, in which th(> particular plant (species) 

 may find few or maii}^ suitable localities, which areas constitute its 

 range. Within this ranije the plant may be found in tew or many 

 places, isolated or gregarious, Ijut outside of this range it does not 

 occur. 



DISTRIBUTION OF SPECIES AS DEPENDING UPON GEOLOGICAL 



HISTORY. 



The distri})uti()n of plants can not always be explained on the ground 

 of their dependence upon the atmospheric and terrestrial factors. 

 The differences in climatic conditions become too insignificant to 

 explain the continement of many species within extremely narrow 

 limits, and at the same time topogra})hic and other conditions of 

 environment offer no satisfactory account. Premising the theory 

 that the existing ])lants are the descendants of similar tyju's which 

 flourished in past periods of th(^ history of our globe, in most instances 

 an explanation is easily found on geological grounds. Viewed in this 

 light, the occurrence of the hendock (7'sv/f/^/ ca)i(i<le)hsl.s)^ for example, 

 with its northern companion, the sweet birch, on the extreme southern 

 extension of the Allegheny Mountains, in Winston County, Ala., in 

 a completely isolated spot hundreds of miles distant from the range 

 of its distribution, can be accounted for when the}- are regarded as 

 the sole remnants of the northei'ii arjjoreal flora which during the 

 glacial period was jjushed to lower latitudes and which on its recession 

 to cooler zones left these trees l)ehind in the narrow valle\' of the 

 Sipsey River, where at present the former shades the cliff-bound 

 banks. The TorveyA {Tinu ioti ta.vifoliiiiii) and fhe Florida yew {TaxuK 

 floridana) oi the valley of the Apalachicola Kiver in w^estern Florida, 

 the American smoketree or chittamwood {('otlmiK cotinoides) in its 

 isolated localities in north Alabama and southwestern Missouri, and 



