74 Variation 
it is in fullest harmony with the great principle laid down by 
Darwin. In order to be acted upon by that complex of environ- 
mental forces, which Darwin has called natural selection, the changes 
must obviously first be there. The manner in which they are pro- 
duced is of secondary importance and has hardly any bearing on the 
theory of descent with modification’. 
A critical survey of all the facts of variability of plants in nature 
as well as under cultivation has led me to the conviction, that 
Darwin was right in stating that those rare beneficial variations, 
which from time to time happen to arise,—the now so-called muta- 
tions—are the real source of progress in the whole realm of the 
organic world. , 
IL. 
External and internal causes of variability. 
All phenomena of animal and plant life are governed by two sets 
of causes; one of these is external, the other internal. As a rule 
the internal causes determine the nature of a phenomenon—what an 
organism can do and what it cannot do. The external causes, on the 
other hand, decide when a certain variation will occur, and to what 
extent its features may be developed. 
As a very clear and wholly typical instance I cite the cocks-combs 
(Celosia). This race is distinguished from allied forms by its faculty of 
producing the well-known broad and much twisted combs. Every 
single individual possesses this power, but all individuals do not exhibit 
it in its most complete form. In some cases this faculty may not be 
exhibited at the top of the main stem, although developed in lateral 
branches: in others it begins too late for full development. Much 
depends upon nourishment and cultivation, but almost always the 
horticulturist has to single out the best individuals and to reject 
those which do not come up to the standard. 
The internal causes are of a historical nature. The external 
ones may be defined as nourishment and environment. In some 
cases nutrition is the main factor, as, for instance, in fluctuating 
variability, but in natural selection environment usually plays the 
larger part. 
The internal or historical causes are constant during the life-time” 
of a species, using the term species in its most limited sense, as 
designating the so-called elementary species or the units out of 
which the ordinary species are built up. These historical causes are 
simply the specific characters, since in the origin of a species one or 
more of these must have been changed, thus producing the characters 
of the new type. These changes must, of course, also be due partly 
to internal and partly to external causes. 
1 Life and Letters, 11. 125. 
