92 NATURAL SCIENCE [February 



number of details, the number of different stages, which its study 

 reveals to us.^ 



Probably more ancient in its origin, is what I have called the 

 second direction in which the movable veins express the evolution 

 of the primary wing. This is the reduction of the number of the 

 branches of the radius of primaries by absorption. The original, 

 generalised number is five. This becomes reduced to four and even 

 three. An eminent case of the result of absorption, where the two 

 directions join forces, so to speak, is offered by the large, common 

 white butterfly Mancijnum hrassicae. Here the evolution of the 

 primary wing has proceeded in the first direction, so that the 

 upper branch of the media, vein IVj, has become absorbed by the 

 stationary branch of the radius, vein III5, to a point nearly one- 

 third of the length of the latter outside of the cell. At the same 

 time the evolution in the second direction has reduced the radial 

 branches to three from the original five. In consequence we have 

 for vein III5 the formula III 3 + 4 + 5 , III5 being the absorbing 

 vein ; now, if the absorption of IVj by this vein became complete, 

 we should have, for the single resulting vein, the formula : 

 III 3 + 4 + 5 + TV I We may here stay our remarks on the second 

 •direction afforded by the evolution of the forewing with the obser- 

 vation : that it appears sporadically, perhaps in connection with the 

 original eftbrt which apparently transformed the five-branched radius of 

 the secondaries in the primeval lepidopteron into the single branched 

 radius which we find in the vast majority of the lepidoptera of to- 

 day. According to our view it is the normal march of wing evolu- 

 tion in the order, that it proceeds from the hindwings, and that the 

 primaries consequently remain in a generalised condition in having 

 the radius three to five branched, where the same forms show a 

 single-branched radius in the secondaries. It is far from certain, 

 however, that a single-branched radius on the primaries is the ' end ' 

 aimed at by the reduction of the branches. The morphological 

 difficulties have been discussed by us in our original communication.- 

 The second direction appears independently in the Parnassiinae, the 

 Pierinae (except Anthochariui), the Eiodinidae and Lycaenidae and 

 in the Saturniades.^ That this is a specialisation is plainly apparent 

 from the sequence in the Whites from a five- to a three-branched 



1 Those authors, who speak of the 'cubitus becoming four-brauched,' do not appear 

 to have understood the evolution of this portion of the wing, or appreciated the direc- 

 tion of the slow and halting movement which leads to the disappearance of the median 

 system of the veins. 



'^ Compare Mittheilungen aus d. Boemer Museum, No. 8, 6. From the movement 

 of the radial branches no less than the greater absorption of vein IVi, I would dis- 

 agree with Comstock's statement that the Nymphalidae sliow a greater specialisation 

 of the wings than the Pieridae. 



2 In the Agliadae the movement in the fust direction seems to lag behind the 

 movement in the second, the reduction of the radial branches. The Citheroniadae re- 

 semble also the Pieridae in the absorption of I Vi. 



