SPECIES AND VAKIETIES. 355 



690. Specific identity is not of course inferred from every strongly 

 marked resemblance ; for the resemblance may be only that of genus, 

 and individuals so related are inferred not to have had a common 

 origin. Nor is it denied on account of every difference ; for individu- 

 als of the same stock may differ considerably ; in fact, no two plants 

 are exactly alike, any more than two men are. Such differences 

 when they become distinctly marked give rise to 



691. Varieties. If two seeds from the same pod are sown in dif- 

 ferent soils, and submitted to different conditions as respects heat, 

 light, and moisture, the plants that spring from them will show 

 marks of this different treatment in their appearance. Such differ- 

 ences are continually arising in the natural course of things, and to 

 produce and increase them artificially is one of the objects of culti- 

 vation. Such vai-iations in nature are transient ; the plant often 

 outlasting the cause or outgrowing its influence, or else perishing 

 from the continued and graver operation of the modifying influ- 

 ences. But in the more marked varieties which alone deserve 

 the name, the cause of the deviation is occult and constitutional ; the 

 deviation occurs we know not why, and continues throughout the 

 existence and growth of the herb, shrub, or tree, and consequently 

 through all that proceeds from it by propagation from buds, as by ofi^ 

 sets, layers, cuttings, grafts, &c. In this way choice varieties of Ap- 

 ples, Pears, Potatoes, and the like, are multiplied and perpetuated. 



692. Since the progeny inherits or tends to inherit all the char- 

 acters and properties of the parent, constitutional varieties must have 

 a tendency to be reproduced by seed, — ■ a tendency which might 

 often prevail, within certain limits, over that general influence which 

 would remmd the variety to the normal state, were it not for the 

 commingling which so commonly occurs in nature, through the cas- 

 ual fertilization of the ovules of one mdividual by the pollen of other 

 individuals of the same species. By assiduously pursuing the oppo- 



origiti of each species from a single individual or a single pair, — a question 

 which science does not furnish grounds for deciding. It is evidently more 

 simple to assume the single origin, where there is no presumption to the con- 

 trai-y, as there may be in the case of tricEcious or of organically associated plants 

 or anim.als ; but the contrary supposition does not affect our idea of species, if 

 we suppose the originals to have been as much alike as individuals proceeding 

 from the same parent are, and to have had a common birthplace. The invest!- 1 

 gation of the geographical distribution of plants more and more favors the idea! 

 of tlie dissemination of each species from a centre of its own. { 



