1907. T.aylor. — Vitrina elongata in Ireland. 227 



diverse species, but is especially keen among individuals of 

 the same kind and markedly so between local varieties or 

 races when brought into competition. Thus all the individuals 

 composing a species are not even approximately equal in 

 dominancy, but, as Darwin has pointed out, certain mountain 

 sheep will starve out other mountain varieties so that they 

 cannot be kept together, and the same result occurs with other 

 organisms, and there is no doubt this applies to mollusca 

 also. These stronger forms will eventually survive, and we 

 may regard them as the latest evolved forms aud consequently 

 nearest to the evolutional area within which they arose. 



The reality and keenness of this struggle for existence 

 becomes evident when we realise that the normal increase of 

 any organism is so rapid, that if unchecked, the progeny 

 would in a very few years fill the whole earth; so that when 

 we consider the enormous number of species existent on the 

 globe, each one capable of quickly overcrowding the earth, we 

 are able to conceive the enormous destruction and selective 

 influence that continually goes on. 



Bearing all these points in mind we can understand that 

 although certain species may at the present day in some 

 countries be strictly alpine in habit, yet in others, where the 

 struggle for existence is not so keen and deadly, the same 

 species may be found inhabiting the more fertile lowlands, and 

 we have an exemplification of this in the occurrence in Ireland 

 of the species I now bring forward. 



The genus Vitrina, to which this species is allocated, is con- 

 chologically characterised by a vestigial shell of great tenuity 

 and glassy transparency, but this general resemblance is 

 superficial, as the internal organisation shows disparities 

 of structure so great, that we may reasonab^^ conclude that 

 the component members of the group have descended from 

 very diverse ancestors, for although still usually classified 

 together, this is merely an expression of their arrival at a 

 similar stage in the progress of the degeneration, leading 

 eventually to the total loss of the shell. 



Dr. Simroth, the great German limacologist, regards the 

 Vitrinae as a primitive group which has retained its simple 

 structure and has been the stem from which many other groups 

 may have sprung. From a study of the various species 



A 2 



