928 OR TO IN OF SPECIES. 



which is small in the variety, larger in the species, and still larger in the genus. But in 

 all three cases the points of difference are accompanied by a much greater amount of 

 resemblance ; and since in the phenomena of variation vi^e learn that from forms which 

 are similar others are derived which are constantly becoming more different by the 

 continual accumulation of differences, we assume that the higher degree of variation 

 of similar forms which we express by the terms Species and Genus has resulted from 

 the accumulation of new characters in the variation from one ancestral form. 



Sect. 37. — Causes of the progressive development of varieties. The 

 characters of the cultivated varieties of one parent-form show, as Darwin was the 

 first to point out, a constant striking and remarkable relation to the purpose for 

 which the plant was cultivated by man. The varieties of Wheat differ from one 

 another only slightly in the form of the haulm or leaves, which are of but small im- 

 portance to mankind ; but they show a great variety and extent of difference in the 

 form and size of the grains, and the quantity of starch and proteid contained in 

 them, z. e. in the characters of that part of the plant for the sake of which Wheat is 

 cultivated, and in those properties of this part which under various circumstances are 

 especially useful to mankind. The varieties of the Cabbage, on the other hand, scarcely 

 differ at all in their seeds or even in their seed-vessels or flowers, the external pro- 

 perties of which are useless to man, and the internal properties only of value because 

 the seed has to reproduce the variety; the varieties of Cabbage differ exclusively in 

 the development of those parts which are used as vegetables, and to which therefore 

 cultivation is directed. The object of cultivation is therefore, retaining the taste and 

 value as food for man, sometimes to increase the succulence of the tissues, sometimes 

 to attain as large a size as possible, sometimes to alter the time of the year at which 

 the vegetable can be used. These and a number of other properties are furnished 

 by the different varieties. The varieties of Beet differ only slightly in their flowers, 

 more in their leaves, according as they are grown in the garden as ornamental 

 foliage-plants or as agricultural crops ; the varieties in the latter case differ from one 

 another in the size and shape of the roots and the amount of sugar they contain, 

 properties which make the plant valuable on the one hand as food for cattle, on the 

 other hand for the manufacture of sugar. Fruit-trees of the same kind differ but 

 little in general in their roots, leaves, flowers, or stems, but to an extraordinary 

 extent in the size, shape, colour, smell, taste, period of maturity, and keeping-pro- 

 perties of the fruit, according to the special purpose or prevalent mode in which it 

 is employed. In garden-flowers it is generally the flowers and especially the corolla 

 and inflorescence that differ in the varieties of a species, because the greater number 

 are cultivated only for the shape, size, colour, or odour of the flowers. 



This relation of cultivated varieties to the requirements of man is explained if 

 we suppose that only those varieties were cultivated, at first undesignedly afterwards 

 designedly, in which some character useful to man was more strongly manifested 

 than in the others ; those individuals were selected which best answered to a definite 

 requirement ; they alone were further cultivated ; the particular character was again 

 strongly displayed in some of their descendants, and only these individuals were 

 again selected for reproduction ; and the desired character was thus continually in- 

 creased in strength. Other characters of the plant also varied at the same time, but 

 they were disregarded, and the individuals in which they occurred were not preserved 



