77 



readjustment." Cockerell, in i8g6, wrote. "The essential 

 distinctions between species are physiological, the morpho- 

 logical ones being only valid for diagnostic purposes just so 

 far as they happen to coincide with the physiological." 



Close stud}' of certain groups of Lepidoptera in which 

 evolution is at present at work in the direction of species 

 formation — Anthrocera, Erchia, and Melitcea- — incline me 

 strongly to accept Professor Cockerell's view with very 

 slight limitations. It appears to me certain that there is an 

 essential difference between the development of morpho- 

 logical characters and physiological characters, the former 

 tending to maintain the species in statu qito as a species, 

 whilst its colour or superficial appearance is altered, e. g. 

 Gnophos (in many species), Amphidasys hetnlaria, &c., whilst 

 the latter sets up at once a distinct isolation between the 

 form and the parent stock, preventing any general ten- 

 dency to cross-breeding, and eliminating the progeny when 

 it occurs. Thus the two extreme forms of Amphidasys 

 bettdaj'ia, in spite of their external differences, cross freely in 

 a state of nature; whilst Anthrocera trifolii and A. loniccrce 

 (though equally able to cross and produce fertile progeny) 

 are probably specialised in nature to some particular food- 

 plant, habit, and habitat, and are maintained distinct. 

 AgsAn, Anthrocera palustris d.Y>^edLrstohe maintained in nature 

 as distinct from A. trifolii, owing to its specialisation in the 

 larval stage to Lotus nliginosus, the localisation of its food- 

 plant to a marshy habitat restricting the species to the 

 same habitat, the latter reacting on the species by making it 

 later in its time of appearance than is A. trifolii. There 

 must be some physiological peculiarity, some difference in 

 the physiological processes between two allied species, that 

 limits them to different food-plants, and allows them to die 

 rather than to live on any other. Twelve months ago I 

 pointed out how specialisation to food-plant, specialisation 

 by habit, specialisation in the method of hybernation, &c., 

 also resulting from a difference of physiological function, 

 would insure isolation of any particular form, and cut it off 

 from any connection with its nearest allies ; and whilst quite 

 agreeing with Professor Meldola that " any change of environ- 

 ment requiring a modification of structure of sufficient 

 magnitude to rank as diagnostic in the systematic sense " 

 would also " be accompanied by a greater or less amount of 

 physiological readjustment," yet at the same time one can 

 understand selection to work on some variable factor in the 

 organisation of an insect in such a way as to allow it to take 



