300 THE EVOLUTION THEORY 



at different places. One of the longest of these chains of forms is 

 that of Xanina cincta, which runs across the island from east to 

 west, and, beginning with the smallest and most delicate forms, ascends 

 through many intermediate stages to the giant form N. livibifem. 

 Such chains of forms have been previously recognized ; thus Kobelt 

 described one in the case of the Sicilian land-snails of the genus 

 Ihcru!:i, and other cases are recorded in literature, but in all instances 

 they refer to areas which must be regarded as isolated for the snails, 

 and which have been colonized from a single starting-point. 



Wo have now to inquire whether and how we can explain the 

 origin of these chains of forms. The cousins Sarasin tell us how 

 they at first attempted to refer the differences between the individual 

 links of such a chain to the diverse influence of the external con- 

 ditions of life, but in vain ; neither the height above sea-level nor 

 the character of the soil was sufficient, and natural selection was 

 no more so ; ' for why should a high Obha-iorm twisted like a beehive 

 be either better or worse equipped for the struggle for exist- 

 ence than a smaller and flatter one ? ' It is true that we do not 

 understand why, but this does not seem to me any reason to doubt 

 that natural selection should be regarded as one of the causes of 

 the divergence of these species, for we could not answer the same 

 question in regard to any of the other structural differences between 

 two species of snail, for the simple reason that we have far too little 

 knowledge of the biological value of the parts of a snail. Or could 

 any one tell of what use it would be to a snail-species to have 

 the horns slightly longer, the foot somewhat narrower, the radula 

 beset with rather larger or more numerous teeth 1 We might indeed 

 imagine many ways in which it might be of advantage, but we are 

 not in a position to say definitely why, for instance, longer horns 

 should be better for one species than for another, and yet we do 

 not believe that the structure of snails is less well adapted to the 

 life of each species than that of any other animals. The snail's 

 structure -is certainly built up of hundreds and thousands of adapta- 

 tions, like that of every other animal species, but while in many 

 others we can, at least in part, recognize the adaptations as such, 

 we cannot do so at all in regard to the snail. Siraroth has pointed 

 out that the spiral asymmetrical shell bears a relation to the one- 

 sided opening of the genital organs, but that only states the general 

 reason for the coiling of the shell. In studying the differences in the 

 shell one is apt to think of its external appearance alone, of the pro- 

 tection which it affords to the soft internal organs of the easily 

 wounded animal; perhaps also of the distribution of weight, which 



Digitized by Microsoft® 



