ORGANIC AGENCIES. 121 



in what sense this is true. Species, then, come in grad- 

 ually on the borders of their range, reach their highest 

 development in number and vigor about the middle, and 

 pass out gradually in number and vigor on the other 

 border, other species taking their place, and the two 

 ranges overlapping on their borders. Thus, in Fig. 67, 



b' 



a a' is the north and south range of species A, and b b' of 

 species B the height of the curve the number and vigor 

 of the individuals, and b a' the overlap of ranges. 



3. But in specific character there is no such gradual pas- 

 sage of one species into another no evidence of trans- 

 mutation of one species into another, nor of derivation of 

 one species from another. From this point of view spe- 

 cies seem to come in at once in full perfection, remain 

 substantially unchanged throughout their ranges, and 

 pass out at once on the other border, other species taking 

 their place as if by substitution, not transmutation. It is 

 as if each species originated, no matter how, somewhere 

 in the region where we find them, and then spread in all 

 directions as far as physical conditions and struggle with 

 other species would allow. 



We can best make this plain by illustrations : The 

 sweet-gum or liquidambar-tree extends from the borders 

 of Florida to the banks of the Ohio. It is most abundant 

 and vigorous, indeed, in the middle regions, and dying 

 out on the borders, where it is replaced by other species : 

 but is everywhere the same species, unmistakable by its 

 five-starred leaf, winged bark, spinous burr, and fragrant 

 gum. Again, the Red- wood (Sequoia) ranges from south- 

 ern California to the borders of Oregon. It may be most 



