66 SEX AND VARIETY. [introd. 



SECTION XII. 



Parallel between Discontinuity of Sex and Discontinuity 



in Variation. 



The application of the term Discontinuity to Variation must 

 not be misunderstood. It is not intended to affirm that in dis- 

 continuous Variation there can be between the variety and the 

 type no intermediate form, or that none has been known to occur, 

 and it is not even necessary for the establishment of Discontinuity 

 that the intermediate forms should be rare relatively to the 

 perfect form of the variety, though in cases of discontinuous 

 Variation this is generally the case ;-4>ut it is rather meant that 

 the perfect form of the variety may appear at one integral step 

 in Descent, either without the occurrence of intermediate grada- 

 tions, or at least without the intercalation of such graduated forms 

 in the pedigree.^ 



In the case of the tarsus of Blatta we have seen an example 

 of a total and complete Variation affecting single members of a 

 series of repeated parts, not collectively, but one or more at a 

 time 1 . Such an instance of a Meristic Variation occurring in 

 a state which is total as regards members of a series but not 

 total as regards the whole series finds many parallels among 

 Substantive Variations, as, for example, that of the Crab {Cancer 

 pagurus) bearing the right third maxillipede fashioned as a chela, 

 while the left third maxillipede was normal. Variations of this 

 nature in plants are of course well known to all. 



At a previous place (Section vn.) allusion was made to the 

 familiar but very curious analogy between members of a series of 

 Meristic parts and separate organisms. The facts of Variation bring 

 out this analogy in many singular ways, and in speaking of the 

 totality of Variation it is necessary to bear these facts in mind. 

 Not only are there abundant instances of independent division 

 or multiplication of single members of Meristic series, but as 

 has been said, single members of such series may thus inde- 

 pendently and singly undergo qualitative or Substantive Variation, 

 being treated in the physical system of the body as though they 

 were separate units. In Variation, therefore, though it will be 



sumption that the variation may at least sometimes be congenital. Supposing 

 however that this shall be found hei-eafter not to be the case, I do not think that 

 the deductions drawn from the facts will be less valid. The conclusions as to 

 the definiteness of the two types, and the relationships of the several parts of each 

 to the several parts of the other, would still hold good. There are besides in other 

 forms, instances of similar numerical Variation, as for example, in the number of 

 joints in the antenna of Prionidas, where the hypothesis of change on renewal is 

 impossible, from which a similar argument might be drawn ; but on the whole I 

 have preferred to leave the account as it stands, taking the case of Blatta as an 

 example, because it is easily accessible and because, from the fewness of the joints 

 concerned, the issues are singularly clear. 

 1 See Note at the end of Section xr. 



