410 Dr. A. G. Biitlcv on the Butterjlies 



be free from error, and, furthermore, I am quite aware that 

 one sliould consider species and genera rather than groups. 

 But 1/et I venture with due respect to direct attention to the 

 subject in the hope that zoologists who are dealing with plank- 

 ion will not confine their attention merely to the adult groups 

 of the same, hut will, in addition, arrange for the examination 

 of the larvce therein in view of the distribution of the different 

 groups of littoral animals. 



Considering eacli side of the question, remembering in 

 particular the faunas of oceanic banks, so far as we know 

 them, I venture to suggest that there are no banks to which 

 an abundant variety of Crustacean larvae cannot pass, that 

 the maximum regular passage for Echinoderm (not Crinoid) 

 and Enteropneust larvaj is about twenty days, while for 

 Sipunculids, Aimelids, MoUusca, and Crinoidea it is pro- 

 gressively less, the series passing on to Muller^s larva, 

 found in the Turbellaria, and ending with regular planulse 

 not as a rule exceeding more than four or five days of oceanic 

 life, and probably in many forms averaging much less. 



Take the Chagos Archipelago as a case in point. Crusta- 

 cean larva3 should reacli it freely from the Sej'chelles or 

 Africa and Australia, and the Crustacean faunas of the tliree 

 localities should be approximately the same for all forms of 

 the group possessing larval development. Many of the 

 Echinoderms and Enteropneusta should be common to the 

 Seychelles and Western Australian, but the Chagos forms 

 might reasonably be expected to show some small variations 

 from their possiljly parent stocks on either side. These 

 differences should be progressively more important in Sipun- 

 culids, Chsetopods, and Echiuroids, while the corals and 

 Turbellaria should have no more resemblance to those of the 

 Australian than to those of the African shore, and should 

 for the most part, indeed, have begun to take on forms which 

 are distinct varieties or subspecies of those found in the 

 Seychelles. 



Zoological Laboratory, 

 Cambridge. 



LV. — The Butterflies of the Group Callidryades and their 

 Seasonal Phases. By ARTHUR G. BuTLER, Ph.D., F.L.S., 

 F.Z.S., &c. 



Between the years 1S97 and 1899 I revised a number of 

 genera of Pieridine Butterflies in the pages of the ' Annals,' 

 and indicated their seasonal phases; but I unaccountably 



