04 LINEAR HEPKTITION. [iNTROD. 



addition to this the statement that the varying limb is perfect 

 conveys a number of ideas that cannot be readily formulated ; for 



mple. that tin- joints are to all appearance properly proportioned 

 and -' -i \ ic. -able, shewing no sign of untitness: they have in fact 

 nnii-li tin- siime appearance as they have in those of the Orthoptera 

 in \\hich tin- tarsus is normally four-jointed. But besides these 

 attributes, which though useful enough for ordinary description 

 are -till in their nature funnies, and of no precise application, 

 th- mother which in the case of these varying legs we are 



.-lit it l.-d to make. We have said that these four-jointed tarsi are 



ill a]i]n arance normal, .save for the number of the joints. Now 

 the niea-iireinents which, at my suggestion, Mr Brindley has been 

 kind enough to make, entitle us to go beyond this, and to assert 

 that tin- tour-jointed tarsus has another character by reason of 

 \\hich it i- actually in a sense a "normal" form. A brief considera- 

 tion of tin- will clearly illustrate the meaning of the term "per- 

 feeiion ' ;iji|,lj,-il to Variation. 



W'- -aw above that in a monomorphic form, the frequency with 

 which, in respect of any given character, it departs from its mean 

 condition follows a curve of Frequency of Error. This is, indeed, 

 \\hat i- meant by the statement that the mean condition is a 

 normal. 



Taking the tive-jointed tarsus, measurements shewed that the 

 ratio of the length of any given joint to the length of the whole 

 tarsus varied in this way about a mean value. Measurement of 

 the joint- of the four-jointed form shewed that the ratios which 

 they bear to the total length of their respective tarsi vary in a 

 similar way about their mean values, and that there is thus a 

 "normal" four-jointed conditi< n just as there is a "normal" five- 

 jointed condition. In the .-aiue way, then, that the ratio of the 

 length of e.,e|i of the ti\e joints to that of the whole tarsus is not 

 alua\s identical but exhibits small variations, so the ratios of the 



ral joints of the four-jointed tareua to the length of the whole 

 tarsus al-o \ar\, but in each case the ratio has a mean value 

 which is approached with a frequency conforming to a curve of 

 Ki .' i <r. 



The measurements established also another fact which is of 

 consequence to an appreciation of the nature of totality in 

 Variation. It not only appealed that the departures from the 

 mean \alin- of these ratio- in the tour-jointed variety were dis- 

 tributed about the mean in the same \'vav as those of the five- 

 joint. -d form, but it was also shewn that the absolute varia- 

 lOnfl tioni the mean \abies of these ratios were not on the 

 "' greater in the tour-jointed tarsi than in the five-jointed 

 In other words, the tour-jointed tarsus occurring thus 

 sporadically, as a variety, is not |e>s definitely constituted than 

 ive jointed type, and the proportions ,f it s several joints 

 oatant. It i- scarcelj accessary to point out that 



