GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 475 



ana), then the tamarack pine {P. contorta), then the P 

 fiexilis, etc. 



2. The ranges of contiguous species shade insensibly 

 into one another by overlapping interpenetration and 

 co-existence on the margins. 



Each species is most abundant and vigorous about 

 the middle of its range, and becomes less and less numer- 

 ous and vigorous on the margins until it ceases and 

 gives place to some other species. Thus, if a a' (Fig. 



^rTTTTTniMfflTTni^^ 



a' b - 



Fig. 333. 



333) be the range of species A, and b V of species .5, 

 then the rising and declining curves represent the rela- 

 tive abundance in different parts of their ranges, and 

 a' b their overlap or area of coexistence on the margins. 



3. But species do not usually grade into other species, 

 which take their place, in specific characters. There is 

 not usually any evidence of transmutation of one spe- 

 cies into another. One species seems to be replaced by, 

 not transmuted ifito, another species. The change is 

 usually by substitution, not by transmutation. I say usu- 

 ally because sometimes such gradation in specific char- 

 acters is found. 



Illustrations. — We take for illustration only two ex- 

 amples: (1) The sequoia, or big tree, and redwood exist 

 to-day only in California; one (the redwood) confined 

 to the coast range, and the other (the big tree) to the 

 Sierra. Now, in commencing south, wherever found 

 these trees are perfect in all their specific characters of 

 bark, wood, leaf, and fruit. They remain substantially 

 unchanged throughout their range, and stop at their 



