928 
ORIGIN 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 v^^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 charact'^rs 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, I.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 
