PREFACE. 



This volume treats of the tree snails of the Hawaiian Islands 

 belonging to the family Achatindlidae ; those of the family Amas- 

 tridas (the genus Laminello) having been dealt with in Volume 

 XXI, and arboreal forms of Tornatettinida (Auriculella) remain- 

 ing to be considered in Volume XXIII of this series. 



The Achatinellidse hold an important place in general biological 

 literature for the reason that a contribution of the first importance 

 to the doctrine of evolution arose from a study of these snails. 

 Gulick's theory of "divergent evolution through cumulative seg- 

 regation ' is now generally recognized as setting forth one of 

 the conditions invariable for the "origin of species,' 1 having no 

 necessary connection with natural selection, Lamarckian factors 

 or mutation theory, but everywhere essential to speciation. For 

 this reason, among others, the account of the group has been 

 made somewhat more elaborate than usual in purely systematic 

 treatises, in order that zoologists may be in a position to form 

 intelligent ideas of the facts of distribution and variation of the 

 group. It will readily be understood that having to cover the 

 whole field, it was impossible to enter upon a detailed study of 

 any single group of forms in a restricted area. The necessary 

 limits of this work permit only brief discussion under each 

 species. For an exposition of the facts bearing on evolution in 

 general, a far more limited field should be chosen, two or three 

 adjacent valleys, where the data could be made nearly complete. 

 Even so, a certain amount of breeding under control would be 

 essential to solid conclusions. As matters now stand, we infer 

 where we should have exact knowledge. 



The name of Dr. C. Montague Cooke is placed upon the title- 

 page of this volume in recognition of his collaboration in eluci- 

 dating the involved synonymy of the genus Achatinella. To the 

 systematist this is perhaps the more important work of the vol- 



v 



