8^6 ORIGIN OF SPECIES. 



If notwithstanding this many wild plants and some cultivated ones are very constant and 

 produce no varieties which can be distinguished externally, this is mainly the result of the 

 fact that the newly produced varieties are unable to exist in the conditions by which they 

 are surrounded, or at least soon disappear, a point to which I shall recur more in detail. 

 The hereditary transmissibility of acquired characters exhibits itself in a most marked 

 way when it does not affect the whole of the parent-plant, but only a particular 

 branch. A still more remarkable case was observed by Bridgman. He noticed that 

 the spores from the lower inner part of the lamina of the leaves of the varieties Scolo- 

 pendrium indgare laceraium and S. vulgare Crista-galli, which were of the normal form, 

 uniformly produced plants of the normal parent-form, w'hile those produced on the outer 

 abnormal part of the leaf reproduced the special varieties \ 



Sect. 34.— Accumulation of new characters in the reproduction of 

 varieties. The difference between a variety and its parent-form, or between the 

 varieties of a common parent-form, is usually at first small and affects only a few 

 characters. But the descendants of the variety may again vary, the new characters 

 may thus become intensified, and other new characters of a different kind may be 

 added to them. The amount of difference between parent-form and variety and 

 between the various varieties of the same parent-form thus becomes greater : and if 

 the tendency to become hereditary of the characters increases with the increase of 

 their difference, the variety comes at length to differ so greatly from the parent-form 

 that their genetic connection can only be proved historically or by the existence of 

 transitional forms. This is the case with many of our cultivated plants, as e.g. the 

 pear, which varies much even in the wild state, but in cultivation has altered its mode 

 of growth, form of leaf, flower, and especially its fruit, to such an extent that it 

 would be impossible to suppose the finest sorts of pears to be descendants of the 

 wild Pyrus communis, if Decaisne had not proved their genetic connection by the 

 study of the transitional forms (Darwin I.e. vol.1, p. 350). In the same manner it 

 scarcely admits of a doubt that all the cultivated kinds of gooseberry are descended 

 from the wild Ribes Grossularia of Central and Northern Europe ; and Darwin 

 brings forward historical evidence to show that the size of the fruit has been con- 

 tinually increased by cultivation since 1786, so that in 1852 it had attained the 

 weight of 895 grs. Darwin found that a small apple 6| inches in circumference 

 weighed as much (/. ^- p. 356). The different varieties of cabbage are all descended 

 from one parent-species, or, according to Alph. De Candolle, from two or three 

 closely related ones still growing in the neighbourhood of the Mediterranean. In 

 this case hybridisation has also cooperated; the varieties are for the most part 

 hereditary but without any great constancy. The extent of the variation which has 

 taken place under cultivation is shown by the existence on the one hand of shrubby 

 forms with branching woody stems, 10 to 12 or even 16 feet high, on the other 

 hand of the round cabbage with a short stem and a spherical, pointed, or broad head 

 consisting of leaves closely packed one over another ; and again of the savoy with 

 its curled blistered leaves, the kohl-rabi with its stem swollen below, the cauliflower 

 with its crowded monstrous flowers, &c.^ 



* Ann. and INIag. Nat. Hist, third series, vol. VIII, 1861, p. 490 ; Darwin /. c. vol. II, p. 379. 

 2 See Metzger, Landwirthschaftliche Tflanzenkunde, Frankfiirt a. M. 1851, p. 1000 ; and 



Darwin /. c. vol. I, p. 323. 



