12 DARWIMAFA. 



and animal in a primitive stock, which reproduces its 

 like from generation to generation, and so continues 

 the species. 1 Taking the idea of species from this 

 perennial succession of essentially similar individuals, 

 the chain is logically traceable back to a local origin in 

 a single stock, a single pair, or a single individual, 

 from which all the individuals composing the species 

 have proceeded by natural generation. Although the 

 similarity of progeny to parent is fundamental in the 

 conception of species, yet the likeness is by no means 

 absolute ; all species vary more or less, and some vary 

 remarkably — partly from the influence of altered cir- 

 cumstances, and partly (and more really) from un- 

 known constitutional causes which altered conditions 

 favor rather than originate. But these variations are 

 supposed to be mere oscillations from a normal state, 

 and in Nature to be limited if not transitory ; so that 

 the primordial differences between species and species 

 at their beginning have not been effaced, nor largely 

 obscured, by blending through variation. Conse- 

 quently, whenever two reputed species are found to 

 blend in Nature through a series of intermediate forms, 

 community of origin is inferred, and all the forms, 

 however diverse, are held to belong to one species. 

 Moreover, since bisexuality is the rule in Nature 

 (which is practically carried out, in the long-run, far 

 more generally than has been suspected), and the 

 heritable qualities of two distinct individuals are min- 

 gled in the offspring, it is supposed that the general 



1 " Species tot sunt, quot dixersas formas ab initio produxit Infini- 

 tum Ens ; quae formge, secundum generationis inditas leges, produxere 

 plures, at sibi semper similes." — Linn. Phil. Bot., 99, 15?. 



