CHAPTER XV. 



LINEAR SERIES continued. 



SYMMETRIES: SEGMENTS ix APPENDAGES. 



.MKIMSTIC Repetition along the axes of appendages is very like 

 that along the axis of the body. Just as particular numbers of 

 -e^meiits or repetitions along the axis of Major Symmetry charac- 

 terize particular forms, so particular numbers of joints characterize 

 particular appendages. Such numbers frequently differentiate 

 species, genera, or other classificatory divisions from each other. 

 In the evolution of these forms therefore there must have been 

 change in these numbers. 



Those who are inclined to the view that Variation is always 

 <-"iit iniious do not perhaps fully realize the difficulty that besets 

 the application of this belief to the observed facts of normal 

 .structure. For in those many groups whose genera or species 

 may be distinguished from each other by reason (amongst other 

 things) of difference in the number of joints in some particular 

 appendage or appendages, will any one really maintain that in 

 all these the process by which each new number has been intro- 

 duced was a gradual one? To take a case: even were evidence 

 as to tlie manner of such Variation wanting, would it be expected 

 that the Longicorn Prionidae, most of which have the unusual 

 number of \'l ant- unary joints, did, as they separated from the 

 other Longicorns which have 11 joints, gradually first acquire a 

 ne\v joint as a rudiment which in successive generations in- 

 creased ? Or, conversely, did the other Longicorns separate from 

 a 12-jointeil form by the gradual "suppression" of a division or 

 of a joint ' If any one will try to apply such a view to hundreds 

 of like examples in Arthropods, of difference in number of joints 

 in appendages of near allies forms that by the postulate of 

 Common Descent we must believe to have sprung from a common 

 ancestor he will rind that by this supposition of Continuity in 

 Variation he is led into endless absurdity. Surely it must be 

 clear that in many such cases to suppose that the limb came 

 through a phase in which one of its divisions was half-made or 



