THE PRINCIPLES OF CLASSIFICATION. 329 



the most variant form in the particular line or lines, until it be- 

 comes fixed by heredity and as different from the primal stock 

 as the conditions of the case allow. In nature, the analogous 

 selection, through innumerable generations, of the exceedingly 

 small percentage of individuals (as ova or seeds) which ordi- 

 narily are to survive and propagate, is made by competition for 

 food or room, the attacks of animals, the vicissitudes of climate, 

 and in fine by all the manifold conditions to which they are 

 exposed. In the Struggle for Life to which they are thus inevi- 

 tably exposed, only the individuals best adapted to the circum- 

 stances can survive to maturity and propagate their like. This 

 Survival of the Fittest, metaphorically expressed by the phrase 

 natural selection, is in fact the destruction of all weaker com- 

 petitors, or of all which, however they might be favored by other 

 conditions, are not the most favored under the actual circum- 

 stances. But seedlings varying, some in one direction, some in 

 another, are thereb}' adapted to different conditions, some to one 

 kind of soil or exposure, some to another, thus lessening the com- 

 petition between the two most divergent forms, and favoring their 

 preservation and farther separation, while the intermediate forms 

 perish. Thus an ancestral type would become diversified into 

 races and species. Earlier variation under terrestrial changes 

 and vicissitudes, prolonged and various in geological times since 

 the appearance of the main types of vegetation, and the attendant 

 extinctions, are held to account for genera, tribes, orders, &c., 

 and to explain their actual affinities. Affinity under this view 

 is consanguinity ; and classification, so far as it is natural, ex- 

 presses real relationship. Classes, Orders, Tribes, &c., are the 

 earlier or main and successful branches of the genealogical tree, 

 genera are later branches, species the latest definitely developed 

 ramifications, varieties the developing buds. 1 



6G1. Except as to those changes in size, luxuriance, or depau- 

 peration and the like, in which plants, especially seedlings, 

 respond promptly to external influences, as to heat or cold, 



1 For the inception of this theory of descent in the form which has within 

 the last twenty years profoundly affected natural history, and developed a 

 copious literature, see a short paper On the Variation of Organic Beings in 

 a State of Nature ; On the Natural Means of Selection ; and On the Com- 

 parison of Domestic Races and True Species, by Charles Darwin, also On 

 the Tendency of Varieties to depart indefinitely from the Original Type, by 

 Alfred Russell Wallace, both read to the Linnean Society, July 1, 1858, and 

 published in its Journal of the Proceedings, iii. (Zoology) 45-62. For the 

 development of the doct: ine, soe Darwin's " Origin of Species by Means of 

 Natural Selection," "The Variation of Animals and Plants under Domes- 

 tication," and various other works; Wallace's "Geographical Distribution 

 of Animals," &c. For some expositions, see Gray's " Darwiniana." 



