§4 PAPILIO. 



natural ; and if it were broken up by attaching un- 

 due importance to very subordinate characters, no 

 partial change would suffice; for any principle that 

 might be thought to justify the establishment of 

 one genus, would render it necessary, if consistently 

 acted upon, to create not fewer than thirty or forty. 

 One of the most obvious differences among the spe- 

 cies is the presence or absence of a tail ; but an 

 arrangement founded on this circumstance, sepa- 

 rates, by a wide interval, kinds which are in other 

 respects most closely allied. Nay, the tail itself is 

 often more or less developed in the same species, 

 being sometimes distinct in the one sex while it is 

 inconspicuous or wanting in the other; its value as 

 a diagnosis of genera is thus in a great measure de- 

 stroyed. 



Considerable differences likewise prevail in the 

 appearance of the caterpillars, but these are too im- 

 perfectly known to be made the groundwork of an 

 arrangement, even if they were likely to be avail- 

 able for such a purpose by indicating natural groups 

 or affording additional means of characterising them. 

 " Some of them," says Dr. Boisduval, " such as 

 those of Mackaon, Alexanor, Asterias, are cylindri- 

 cal and smooth; others (Crassus, Philenor), are 

 protected with rather long fleshy prominences ; in 

 a very great number (Pammon, Memnon, Ghalchas, 

 &e.) the two first segments are attenuated, and 

 capable of being retracted under the third and 

 fourth, which are dilated and often ornamented 

 with ocular spots analogous to those presented by 



