134 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



other methods, viz : a physiological method, which 

 studies the special reaction of each species with 

 regard to the circumstances of its environment ; 

 a geographical method, if the domains of the two 

 species are entirely distinct or overlap each other ; 

 finally, and above all, a mixiological method, when 

 the two species form common colonies. The boun- 

 daries are then established by a sort of genetic 

 barrier, due either to the impossibility of cross- 

 breeding or to an instinctive repulsion of the two 

 species from each other, or, again, to the infertility 

 of the cross. Observation shows us, in this case, 

 the complete absence of intermediate forms in 

 common colonies. This is the case with the Helix 

 acuta and ventricosa. In other species, it is true, 

 for example in the Helix Jiortensis and nemoralis, 

 so like each other in many points, in a few sparse 

 localities there may be observed a certain number 

 of intermediate examples which are probably 

 hybrids ; but even the very small number of these 

 crosses is an indication of a veritable genealogical 

 barrier between the two species. By the analytical 

 application of these two criterions it seemed possible 

 to Coutagne to distinguish between the colonies of 

 the different species and the different species of 

 the same locality ; in a word, to separate neigh- 

 bouring species. This was only possible, however, 

 by ignoring those innumerable pseudo-species which 

 modern authors have described from simple and 

 insignificant morphological variations, often pe- 

 culiar to individuals, which bring into the system 

 of land shells an almost inextricable confusion. 

 The facts that Coutagne so well observed in this 



