Chapter 8 



MOLLUSCA 



LAND SNAILS 



We do not eat snails in this country, nor are they so destruc- 

 tive in gardens as in Europe, so that on the whole they receive 

 scant attention from the public. This is to be regretted, for they 

 are interesting animals, and the study of their distribution will 

 teach us many things about the past condition of this continent. 

 Insects, birds and mammals are relatively mobile; the seeds of 

 plants are often scattered far and wide. But the drama of snail 

 life moves slowly, and where other creatures have departed or 

 changed, land snails remain to testify concerning earlier days. 

 Not all snails are equally significant in this respect. The collector 

 who hunts shells in our western and southwestern states, will 

 find many kinds, large and small. He may naturally suppose 

 that the large species travel faster than the small ones, and hence 

 will be more widely distributed. The exact contrary is true; 

 many of the small forms are scattered all over the country, but 

 the larger ones are restricted to particular mountain ranges, or 

 even parts of ranges. Any conchologist can tell you where to 

 find new species of snails. It is only necessary to take a map of 

 the southwest, note the regions which have been searched, and 

 then investigate such mountain ranges in Arizona and New Mexico 

 as have not yet been explored by any student of snails. The new 

 species thus found will nearly all be large, as large as a coat button 

 or a marble. It seems that the little ones, in a dormant condition, 

 are easily blown from place to place by the high winds. The 

 larger sorts cannot travel that way, and half a mile of sandy 

 desert is just as impassable as the broad ocean. Thus colonies 

 are isolated on mountain peaks, and gradually come to possess 

 distinctive features. This local diversity is very much more 

 pronounced in New Mexico and Arizona than in Colorado, for 

 in our State the valleys are often passable for snails, and there is 

 every reason to believe that during the glacial period the present 

 mountain fauna inhabited much lower levels. Further south 

 desert conditions must have persisted for ages, as shown by the 

 large number of highly specialized desert loving animals and plants. 



127 



