if U& 







CHAPTER V. 



LINEAR SERIES — continued. 



Homceotic Variation in Arthropoda. 



The occurrence of Homoeosis among the appendages of Ar- 

 thropoda is illustrated by a small but compact body of evidence. 

 To this evidence special value may be attached, not because it 

 is likely that in the evolution of the Arthropods variations have 

 really taken place, in magnitude comparable with those now to 

 be described, but rather because these cases give a forcible illus- 

 tration of possibilities that underlie the common and familiar 

 phenomena of Meristic Repetition. Of these possibilities they 

 are indeed " Instances Prerogative," salient and memorable ex- 

 amples, enuntiating conditions of the problem of Variation in 

 a form that cannot be forgotten. Facts of this kind, so common 

 in flowering plants, but in their higher manifestations so rare 

 in animals, hold a place in the study of Variation comparable 

 perhaps with that which the phenomena of the prism held in the 

 study of the nature of Light l . They furnish a test, an elenchus, 

 which any hypothesis professing to deal with the nature of organic 

 Repetition and Meristic Division must needs endure. 



Insecta. 



*75. Cimbex axillaris (a Saw-fly), having the peripheral parts 

 of the left antenna developed as a foot. The right antenna is 

 normal, ending in a club-shaped terminal joint. In the left an- 

 tenna the terminal joint is entirely replaced by a well-formed 

 foot, having a pair of normal claws and the plantula between 

 them (Fig. 16). This foot is rather smaller than a normal foot, 

 but is perfectly formed. The rest of the antenna, so far as the 

 point at which the club should begin is normal in form, but is 

 a little smaller and thinner than the same parts in the right 

 antenna. Kraatz, G., Dent. ent. Ztschr., 1876, xx., p. 377, PI. 



1 See the well-known passage in Nov. Org., n. xxii. 



