354 BEV. J. T. GDUCK OK 



any disadvantage be experienced by either species ? It is im- 

 possible to conceive of any disadvantage that would follovk^, and 

 therefore I cannot believe that this difference in the two species 

 is primarily due to natural, sexual, or any other form of selection. 



There are many other specific distinctions presented in this 

 family which seem to be of no advantage, though they are not so 

 far removed i'rom all suggestion of the possibility of use as the 

 character we have just been considering. The brilliant colours 

 and varied patterns presented by many of the arboreal species 

 would be of advantage to themselves, if they served as warning 

 of nauseous qualities to creatures that are liable to prey upon 

 them; but no such creatures exist. The birds of the forest- 

 region are exclusively fruit- and nectar-feeding, and the mice 

 which in recent years have made sad havoc with the mountain 

 snails, unfortunately do not spare the highly- coloured species. 



There can be no doubt that when representatives of different 

 groups or subgenera occupy the same trees they remain segre- 

 gated through the influence of sexual instincts, which must be 

 associated with some means of recognizing those of their own 

 group ; but it is not at all probable that the colours and patterns 

 of any species are recognized by their mates, or have been deve- 

 loped under the influence of sexual selection. There is, there- 

 fore, strong reason to doubt whether selection of any kind has 

 been concerned in the production of the beautiful colours and 

 patterns of these species, unless possibly correspondences in colour 

 within the limits of a genus are, in some cases, due to the in- 

 heritance of tendencies produced by selection when conditions 

 were very different from what we now find. But the divergences 

 in colour and pattern in the species of one genus cannot be thus 

 explained. 



5. The average radius of disiribution for species of the same 

 value in different groups of closely-allied species varies in 

 the different groups directly as the power and opportunity 

 for migrating, and inversely as the plasticity and variability 

 of the several groups. 

 Comparing the distribution of the Helices of Europe with 

 that of the AchatinellincB of Oahu, the most striking contrast is 

 found in the size of the areas occupied. Helix pomatia is dis- 

 tributed from England to Turkey, over an area two thousand miles 

 in length, while of the seven genera of Achatinellince on Oahu 



