40 IMPERFECTION OF THE GEOLOGICAL RECORD 



far as we know, intermittent ; but it has been so often repeated 

 that we have no reason to doubt that the wasting continents 

 afford a complete series of aqueous deposits, since the time 

 when the dry land first appeared. 



In recent years the Challenger expedition and similar dredg- 

 ings have informed us of still another continuity of deposition 

 in the depths of the ocean. There, where no detritus from 

 the land, or only a very little fine volcanic ash or pumice has 

 ever reached, we have, going on from age to age, a deposit of 

 the hard parts of abyssal animals and of those that swim in 

 the open sea ; so that if it were possible to bore or sink a shaft 

 in some parts of the ocean, we should find not only a continu- 

 ous bed, but a continuous series of pelagic life from the 

 Laurentian to the present day. Thus we have continuous 

 physical recprds, could we but reach or completely put them 

 together, and eliminate the disturbing influence of merely local 

 vicissitudes. It is when we begin to search the geological 

 formations for fossils, that imperfection in our record first 

 becomes painfully manifest. 



In the case of many groups of marine animals, as, for 

 example, the shell-fish and the corals, and I may add the 

 bivalve crustaceans, so admirably worked up by my friend 

 Prof. Rupert Jones, we have very complete series. With the 

 and snails the case is altogether different. As stated in an- 

 other paper of this series, a few species of these animals appear 

 in the later Palaeozoic age, and after that they have no suc- 

 cessors known to us in all the great periods covered by the 

 Permian, the Trias, and the earlier Jurassic. A few air-breath- 

 ing water-snails appear in the upper Jurassic, and true land 

 snails are not met with again until the Tertiary. Were there 

 no land snails in this vast lapse of time? Have we two suc- 

 cessive creations, so to speak, of these creatures at distant 

 intervals ? Were they only diminished in numbers and distri- 

 bution in the intervening time ? Is the hiatus owing merely 



