34 THE ORIGIN OF SPECIES n 



either in form or in structure. There is always a 

 certain amount of deviation, not only from the 

 precise characters of a single parent, but when, as 

 in most animals and many plants, the sexes are 

 lodged in distinct individuals, from an exact mean 

 between the two parents. And indeed, on 

 general principles, this slight deviation seems as 

 intelligible as the general similarity, if we reflect 

 how complex the co-operating " bundles of forces " 

 are, and how improbable it is that, in any case, 

 their true resultant shall coincide with any mean 

 between the more obvious characters of the two 

 parents. Whatever be its cause, however, the 

 co-existence of this tendency to minor variation 

 with the tendency to general similarity, is of vast 

 importance in its bearing on the question of the 

 origin of species. 



As a general rule, the extent to which an 

 offspring differs from its parent is slight enough ; 

 but, occasionally, the amount of difference is much 

 more strongly marked, and then the divergent 

 offspring receives the name of a Variety. Multi- 

 tudes, of what there is every reason to believe are 

 such varieties, are known, but the origin of very 

 few has been accurately recorded, and of these we 

 will select two as more especially illustrative of 

 the main features of variation. The first of them 

 is that of the " Ancon " or " Otter " sheep, of 

 which a careful account is given by Colonel 

 David Humphreys, JF.R.S., in a letter to Sir 



