IMPERFECTION OF THE GEOLOGICAL RECORD 4 1 



to the unlikelihood of such shells being preserved ? Or is it 

 owing to the lack of diligence and care in collecting ? 



In this particular case we are, no doubt, disposed to say 

 that the series must have been continuous. But we cannot 

 be sure of this. In whatever way a few species of land snails 

 were so early introduced in the time of the Devonian or of 

 the Coal formation, if from physical vicissitudes or lack of 

 proper pabulum they became extinct, there is no reason known 

 to us why, when circumstances again became favourable, they 

 should not be reintroduced in the same manner as at first, 

 whether by development from allied types or otherwise. The 

 fact that the few Devonian and Carboniferous species are very 

 like those that still exist, perhaps makes against this supposition, 

 but does not exclude it. If we suppose that new forms of life 

 of low grade are introduced from time to time in the course 

 of the geological ages, and if we adopt the Darwinian hypo- 

 thesis of evolution, we arrive, as Naegeli has so well pointed 

 out, at the strange paradox, that the highest forms of life must 

 be the oldest of all, since they will be the descendants of the 

 earliest of the lower animals, whereas the animals now of low 

 grade may have been introduced later, and may not have had 

 time to improve. But all our attempts to reduce nature to 

 one philosophic expression necessarily lead to such paradoxes. 



On the other hand, the chances of the preservation of land 

 snails in aqueous deposits are vastly less than those in favour 

 of the preservation of aquatic species. The first Carboniferous 

 species found ^ had been preserved in the very exceptional 

 circumstances afforded by the existence of hollow trunks of 

 Sigillariae on the borders of the Coal formation flats, and the 

 others subsequently found were in beds no doubt receiving 

 the drainage of neighbouring land areas. Still it is not un- 

 common on the modern sea-shore, anywhere near the mouths 

 of rivers, to find a few freshwater shells here and there. The 

 ^ Pupa vetusia of the Nova Scotia coal formation. 



3* 



