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 1 . 



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. 



II. 



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, n. 125. 



