160 SYSTEMATIC BOTANY. 



which " sport " out into numberless varieties when raised from seed 

 under highly artificial conditions. The occurrence of such variations is 

 less common and, when it occurs, generally less marked in wild plants, 

 as might naturally be expected, from the likelihood of wild plants 

 maintaining their footing best in a position where the conditions are most 

 natural to them ; but we do find remarkable cases of variation in many 

 wild species, as of colour in the common Milkwort and the Columbine 

 (Aquileijia) ; but most of those kinds which exhibit the tendency now 

 and then in a wild state, become extremely variable under culture. Some 

 of the variations are dependent simply upon modifications of the cell- 

 contents of certain tissues, as in the commonest of all variations, those of 

 colour, and in the not uncommon appearance of white patches and streaks 

 (" variegation ") on the leaves. Other variations are teratological, and 

 result from the over-stimulation of the vegetative system, causing the 

 reproductive organs to degenerate (of which the ordinary " doubling " of 

 flowers by the degradation of their stamens into petals is an example) 

 or, vice versa, the application of stimuli at particular epochs, producing 

 remarkable development of flower or fruit. All these A r ariations, more 

 especially those involving serious teratological changes, tend to disappear. 

 Common variations, of slight importance, mostly die out at once in the 

 descendants through seed, especially if the conditions are varied ; serious 

 departures from the typical structure (teratological variations) lead to 

 barrenness and incapability of continuing either the variety or the species 

 by seed. 



It is important to note here a fact which will be more minutely 

 examined in another place, namely, that although the peculiar characters 

 of varieties are commonly lost in seeds, the peculiar form is capable of 

 indefinite propagation by vegetative multiplication through cuttings &c., 

 the special idiosyncracy being possessed in common throughout all the 

 leaf -buds, both while forming part of the parent and after they have been 

 detached from it to form new plants, grafts, &c. 



A certain number of species which vary more or less in a wild state 

 exhibit a remarkable peculiarity under systematic cultivation. By strictly 

 maintaining a certain set of conditions, varieties originating accidentally 

 or through^intentional treatment are made to manifest their additional 

 peculiarities so strongly, that they transmit the tendency to present 

 similar peculiarities to their seeds ; and such transmission goes on for an 

 indefinite number of generations, provided the requisite external conditions 

 are kept up. In this way arise what are called Races, series of individuals 

 connected by common characters and by inheritance, like species ; but, 

 unlike them, liable to lose, in one or a few generations, under change of 

 conditions, part or all of the essential characters by which they are distin- 

 guished. We have examples of such races in most of our esculent vege- 

 tables, especially in the many varieties of form, more or less permanent, 

 derived from the wild Cabbage (Brassica okracea}. 



These, together with Hybrids, or the produce of cross-fertilization 

 between individuals of distinct species, will be referred to again among the 

 phenomena of the Physiology of Reproduction. The determination of the 

 limits of species is greatly obstructed in many cases by the frequent 

 occurrence of varieties, and more particularly of races to which hybrids 

 add another complication, probably of less importance than many modern 

 authors suppose. It appears probable that the number of real species is 



