g22 ORIGIN OF SPECIES. 



ordinary undivided entire leaves, or has reverted to the primitive form. In the second 

 case new characters not previously displayed arise on particular shoots of a stock. 

 Thus, for instance, single shoots of the Myrtle are sometimes found with leaves in 

 alternating whorls of threes, instead of in pairs ; but these shoots again produce from 

 the axils of their leaves the ordinary branches with decussate leaves. Knight (see 

 Darwin, /. c. vol. I. p. 375) observed a Cherry (the May Duke) with one branch bear- 

 ing fruit of a longer shape which always ripened later. The common 'Moss-Rose' 

 is considered by Darwin (/. c. p. 379) to have probably arisen by * bud-variation' 

 from R. centifolia ; the white and striped Moss-Roses made their appearance in 1788 

 from a bud of the common red Moss-Rose ; Rivers states that the seeds of the simple 

 red Moss-Rose almost always again produce Moss-Roses \ 



Those changes which are produced in a plant by the nature of its food and 

 other external conditions must not be confounded with variation. Specimens of the 

 same plant often differ conspicuously in the size and number of their leaves, shoots, 

 flowers, and fruits, according as their supply of food has been abundant or deficient ; 

 deep shade frequently occasions the most striking changes in the habit of plants that 

 usually grow in sunshine ; but these changes are not hereditary ; the descendants of 

 such individuals revert, under normal conditions of light and nutrition, to the original 

 characters of the species. 



Those characters, on the contrary, which may become hereditary or form the 

 groundwork of varieties, arise independently of the direct influence of soil, locality, 

 climate, or other external influences ; they appear seemingly without any cause. We 

 must therefore assume either that external impulses which are altogether imper- 

 ceptible first cause an imperceptible deviation in the process of development, which 

 is always extremely complicated, and that this variation gradually increases until it 

 becomes perceptible, or that the processes in the interior of the plant itself react 

 upon one another in such a manner as to cause sooner or later an external 

 change. 



The fact that wild plants, when cultivated, usually begin to produce hereditary 

 varieties, shows that the change in the external conditions of life disturbs to a 

 certain extent the ordinary process of development ; but it does not show that par- 

 ticular external influences produce particular hereditary varieties corresponding to 

 them ; for under the same conditions of cultivation the most diff"erent varieties arise 

 simultaneously or successively from the same parent-form. The same is the case 

 also in nature with wild plants ; in the same locality under precisely the same vital 

 conditions a number of varieties are often found by the side of their parent-form, and 

 the same variety is often found in the most diverse localities ^ It is for this very 

 reason — because varieties are to so great an extent independent of external influences 

 — that they are hereditary. A change produced in a plant by moisture, shade, or any 

 similar cause, is not hereditary, because its descendants, when placed under other 

 vital conditions, acquire again other non-permanent characters. That hereditary 

 characters, or those which may become so, are not produced by external influences. 



^ [See also M. J, Masters, On a pink sport of the Gloire de Dijon Rose, Journ. Roy. Hort. Soc. 

 new series, vol. IV. p. 153. — Braun, Abhand. d. Berl. Akad. 1859, p. 219. 



^ Further details on this important subject are given by Nageli in the Sitzungsberichte der kon. 

 ^aver. Akad. der Wiss. Dec. 15, 1865. 



