230 PROCEEDINGS OF THE MAL^COLOGICAL SOCIETY. 



as of any other concept of evolution, must be reinforced by field 

 observations planned in advance to satisfy the many questions in 

 which they may be employed as evidence. Why is a certain variety 

 of Helix nemoralis found in locality A and never in locality B ? Do 

 its specific characters appear to be of advantage to it or not ? Do 

 the other snails in locality A tend to show analogous characters or 

 are they different ? If they are different, in what respect are they 

 different ? Do intermediates occur ? If so, what are the offspring 

 of the latter like when they can be bred from known parentage ? 

 These and similar questions the field naturalist should always be 

 asking himself ; and his note-book should be a treasury of informa- 

 tion upon food, soil, enemies, habits, and other bionomic data. 



Field observations are particularly needful in a special group of 

 cases Avhich in a general way are of considerable importance in 

 genetic studies. Every malacologist has at one time or another been 

 puzzled by certain groups in which structural modifications of an 

 exuberant or bizarre form have been developed. For example, 

 among the Lamellibranchia Malleus, Brechites, Tridacna (e.g. 

 T. squamosus), and S'pondylus are genera in which bizarrerie of 

 form or sculpture reaches a maximum. Among Gastropoda Murex 

 and Delphinula have a fantastic exuberance of spines, certain 

 apparently closely allied species of Ennea show a prolific variety of 

 oral armature, while Opisthostoma and Anostoma exhibit a remarkable 

 abnormality hitherto unexplained. Some of these cases seem to 

 transcend the limits of functional adaptation and to illustrate the 

 principle of momentum discussed by Dendy (8), and attributed 

 provisionally by him to the failure or elimination of growth-con- 

 trolling secretions. Others seem either to be adapted to very 

 exceptional bionomic conditions or to have become subject to non- 

 adaptive influences diverging very abruptly and eccentrically from 

 the main tendencies of their groups. 



Now some sort of adaptive explanation of such cases may be 

 forthcoming. But an investigation in the field is most urgently 

 needed. The elaborate spines of the various species of Murex, for 

 example, are at present only explained on an assumption that they 

 are " protective ". If that is the case, what is the enemy that 

 evokes such an elaborate defence absent from, some species of the 

 genus and from allied groups ? Are the spines " protective " as 

 a barbed-wire entanglement or do they serve to entangle seaweed 

 and bottom debris so as to impart some sort of " protective 

 resemblance " ? Or can no such factor of special danger be dis- 

 covered in the environment ? Is it " momentum " or the result of 

 some non-adaptive factorial change ? We cannot dogmatize on 

 such matters. They constitute a lacuna in our knowledge, and a 

 complete and satisfactory account of evolutionary processes cannot 

 be obtained while such cases remain unexplained. 



