154 OUTLINES OF EVOLUTIONARY BIOLOGY 



obvious reason. In extreme cases the organisms exhibiting such 

 mutation are often spoken of as sports or monstrosities ; the 

 latter term, however, is also frequently applied in the case of 

 purely artificial modifications, which must not be included under 

 this heading. Such artificial modifications are not, at any rate 

 as a rule, inherited by future generations, while true mutations 

 usually, if not invariably, are. 



Mutations differ from fluctuating variations not only in that they 

 usually deviate more widely from the type of the species, but 

 also in the much greater rarity of their occurrence and in the 

 fact that they do not fluctuate about the old mean or average 

 condition ; nevertheless it may be questioned whether we can 

 logically draw a hard and fast distinction between the two. 



Mutations may be either meristic or substantive. Human 

 beings with six fingers or toes, in place of the normal five, 

 are not infrequently met with, and this meristic mutation (hexa- 

 dactylism) is well known to be transmitted by heredity. 



The classical instance of the Ancon or otter sheep, on the 

 other hand, affords one of the best known examples of substantive 

 mutation. To quote the words of Huxley, 1 " It appears that one 

 Seth Wright, the proprietor of a farm on the banks of the Charles 

 Kiver, in Massachusetts, possessed a flock of fifteen ewes and a ram 

 of the ordinary kind. In the year 1791, one of the ewes pre- 

 santed her owner with a male lamb, differing, for no assignable 

 reason, from its parents by a proportionally long body and 

 short bandy legs, whence it was unable to emulate its relatives 

 in those sportive leaps over the neighbours' fences, in which they 

 were in the habit of indulging, much to the good farmer's vexa- 

 tion." The inheritance of this peculiarity was so strong that 

 this single individual actually became the starting point of a new 

 breed. 



Mutations, more or less pronounced in character, are also not 

 infrequently met with in the vegetable kingdom. Foxgloves, for 

 example, are sometimes found in which some or all of the petals 

 are converted into stamens, and it has been proved by experi- 

 ment that this peculiarity is handed down from parent to 

 offspring. 



Professor Hugo de Vries has, as is well known, made a special 

 study of mutation amongst plants. In the year 1886 this dis- 

 guished botanist found a large number of specimens of the evening 



4 "Darwiniana," p. 35. 



