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 (/.<:. 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 different 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 Nägeli in the Sitzungsberichte der kön. 
bayer. Akad. der Wiss. Dec. 15, 1865. 
