VARIATION IN SPACE AT PRESENT EPOCH 129 



habitat, a preponderant part in the determination 

 of the local races. 



An ingenious French conchologist, M. G. Coutagne, 

 seems to me to have brought us in the study of 

 the variation of land Molluscs some very interest- 

 ing and exact data. The shells of these animals, 

 particularly those of the innumerable family of the 

 Helicidce commonly called snails, lend themselves 

 marvellously to a precise study of this variation, 

 owing to their easy preservation, their abundance 

 in any given place, and lastly, and especially, to 

 their slight aptitude for even limited displacements. 

 Their areas of dispersion, or in other words their 

 domains, are small in extent, and the important 

 study of the limits and forms of these domains can 

 be easily effected. The method followed by Cou- 

 tagne consists in collecting and examining the 

 greatest possible number of shells belonging to what 

 he calls a colony, that is to say, a gathering of indi- 

 viduals of the same type dwelling in the same 

 limited locality, and so little different from each 

 other that the crossing of these individuals among 

 themselves may be considered possibleTand may 

 give fertile results. If we find, for instance, on 

 the same rock Helix alpina and Helix lapicida, 

 we may say tHat these Helices constitute two 

 colonies ; for it will never enter the mind of a 

 malacologist that there can be fertile crossing 

 between these two groups. It is possible thus to 

 appreciate at first sight the variation undergone by 

 the closely related individuals of the same colony. 



But then the different colonies of the same species 

 from more or less distant stations must be compared. 



