830 



ORIGIN OF SPECIES. 



characters of a plant, so it is difficult and to a certain extent impossible to define, i.e. to 

 determine by comparison, what amount of differentiation is necessary in order to classify 

 two different but similar forms as species rather than varieties. In the same manner it 

 is left to a great degree to personal judgment to decide w^hether tw'o different but similar 

 groups of forms should be classed as varieties of two species or as species of two distinct 

 genera. The only object actually presented to the eye is the individual (and even this 

 not always as a whole) ; the ideas Variety, Species, Genus are abstract ideas, and indi- 

 cate a progressive scale of the differences between individuals which is small in the 

 variety, larger in the species, and still larger in the genus. But in all these cases points 

 of difference are of less importance than the amount of resemblance ; and since in the 

 phenomena of variation we learn that forms w^hich are similar but are constantly becom- 

 ing more different are derived from the same ancestor by the continual accumulation of 

 differences, so we assume that the higher degree of variation of similar forms which we 

 express by the terms Species and Genus have resulted from the accumulation of new 

 characters in the variation from one ancestral form. 



Sect. 35. — 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 w^heat differ from one 

 another only slightly in the form of the haulm or leaves, w^hich 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 proteine contained in 

 them, 2. 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 w^hich 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 w^hich 

 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 growm in the garden as ornamental 

 foHage-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 differs 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 



