14 Plant-Breeding 



Fortuitous variation. It will probably never be pos- 

 sible to refer every variation to a distinct cause, for it is 

 probable that some of them have no antecedent. If we 

 conceive of the forms of life as having been created with 

 characters exactly uniform from generation to generation, 

 then we should be led to look for a distinct occasion or 

 cause for every departure from the type ; but we know, as 

 has already been pointed out, that heredity by its very 

 nature is not so exact as to carry over every attribute, and 

 no other, of the parent to the offspring. Plasticity is a 

 part of the essential constitution of all organic beings. 

 There is perhaps no inherent tendency in organisms 

 towards any ultimate or predetermined completion of 

 forms, as the older naturalists supposed, but simply a 

 laxity or indefiniteness of constitution which is expressed in 

 numberless minor differences in individuals. 



That is, some variation may be simply fortuitous, an 

 inevitable result of the inherent plasticity of organisms, 

 and it may have no immediate inciting cause. 



Action of natural selection on variation. If we were to 

 assume that every minor difference is the result of some 

 immediate cause, then we should expect every individual 

 plant or animal to fill some niche, to satisfy some need, to 

 produce the definite effect for which the cause stands. 

 But it is apparent to one who contemplates the operations 

 of nature that very many certainly more than half of 

 the organisms which are born are not useful to the per- 

 petuity of the species and very soon perish. From these 

 fortuitous variations nature selects, to be sure, many 

 individuals to be the parents of other generations because 

 they chance to be fitted to live, but this does not affect 



