318 TAXONOMY. 



627. No two individuals are exactly alike ; and offspring of 

 the same stock may differ (or in their progeny may come to differ) 

 strikingly in some particulars. So two or more forms which 

 would have been regarded as wholly distinct are sometimes 

 proved to be of one species by evidence of their common origin, 

 or more commonly are inferred to be so from the observation of 



f a series of intermediate forms which bridge over the differences. 

 Only observation can inform us how much difference is compat- 

 ible with a common origin. The general result of observation 

 is that plants and animals breed true from generation to genera- 

 tion within certain somewhat indeterminate limits of variation ; 

 that those individuals which resemble each other within such 

 limits interbreed freely,, while those with wider differences do 

 not. Hence, on the one hand, the naturalist recognizes Varieties 

 or differences within the species, and on the other Genera and 

 other superior associations, indicative of remoter relationship of 

 the species themselves. 



628. Varieties are forms of species marked by characters of 

 less fixity or importance than are the species themselves. They 

 may be of all grades of difference from the slightest to the most 

 notable : they abound in free nature, but assume particular 

 importance under domestication and cultivation ; under which 

 variations are most prone to originate, and desirable ones are 

 preserved, led on to further development, and relatively fixed. 



629. If two seeds from the same pod are sown in different 

 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 differences 

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

 produce and increase them artificially is one of the objects of 

 cultivation. Striking as they often are (especially in annuals 

 and biennials) , they are of small scientific consequence. When 

 spontaneous they are transient, the plant either outlasting the 

 modifying cause or else succumbing to its continued and graver 

 operation. But, in the more marked varieties which alone de- 

 serve the name, the cause 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 con- 

 sequently through all that proceeds from it by propagation from 

 buds, as by offsets, layers, cuttings, grafts, &c. 



630. Some varieties of cultivation originate in comparatively 

 slight deviations from the type, and are led on to greater differ- 

 ences by strict selection of the most marked individuals to 

 breed from. Most appear as it were full-fledged, except as to 



