12 INTRODUCTION 



of the fossil forms that the race-history or phylogeny of 

 animals and plants can be traced with certainty ; but in 

 attempting such investigations a great difficulty is pre- 

 sented by the imperfection of the record of the life of past 

 ages, since only a very small proportion of the animals 

 and plants has been preserved, and these often in a very 

 imperfect manner. We have already seen several reasons 

 why this record must be imperfect ; some animals are with- 

 out hard parts, while others, particularly land animals, 

 do not become covered up with sediment. Further, the 

 remains of animals which were originally present in the 

 rocks have been, in some cases, dissolved by percolating 

 water, or to a great extent obliterated by the meta- 

 morphism which the rock has undergone. Then again 

 the record of life is incomplete because of the breaks in 

 the succession of the stratified rocks ; these breaks have 

 been caused sometimes by denudation having removed 

 a great thickness of rocks, in other cases by an absence 

 of deposition. 



Notwithstanding this imperfection of the record, 

 many groups of animals are found to undergo gradual 

 modification when traced through series of strata or 

 formations. For example, in the Pliocene deposits of 

 Slavonia there are numerous shells of pond-snails (Vivi- 

 parus or Paludina) ; and specimens found at the top and 

 bottom of the formation, and also at certain intervening 

 levels, differ so much from one another that they appear 

 to belong to distinct species. When, however, examples 

 are collected from all the beds of the formation, the 

 apparently distinct species are seen to be connected by 

 intermediate forms, and a series, showing a gradual passage 

 from the species found in the lowest bed to that in the 

 highest, can be obtained. 



In several groups of Tertiary Mammalia there is also 



