THE PALEONTOLOGICAL RECORDS 299 



Climates ; glaciations ; ice caps ; swamp, lacustrine and 



desert conditions, etc. ; 

 Earth-crustal disturbances ; mountain building ; vulcanism, 

 etc., and thus the setting up, or the removal of barriers to 

 the migrations and disseminations of organisms ; 

 Past faunas and floras ; the distributions and densities of 



organic populations ; 

 Kinds of organisms and their relative dominances ; 

 Habits of animals that lived in the past ; their anatomy 

 (which can be inferred, by analogy, even if only some of 

 the hard parts are preserved in the fossil form) ; 

 Phylogenies, or formal schemes of evolution ; and so on. 

 The limitations of paleontological evidence of evolution. But 

 it is obvious that the evidence is, and must always be, very 

 defective. Great numbers of animals and plants must die 

 and leave no permanent records for every one that is fossilized. 

 Vast masses of sedimentary rocks have been eroded away and 

 redeposited as new sediments : in such cases the included fossils 

 are destroyed. Stratified rocks have been metamorphosed by 

 heat, pressure, etc., and their included fossils have become 

 changed so that they cannot be recognized. The paleontological 

 record is therefore defective and must remain so in spite of all 

 investigation. 



99«. Paleontological Sequenxes. So far as it goes, 



paleontology presents us with sequences of occurrences of fossil 



remains which are as they would be if a process of evolution had 



occurred. Let A be some morphological types preserved as fossils 



and let the accents of ^ denote progressive morphological changes : 



then we have examples of such series as the following one : 



A — A' — A" — A!" — \"" — A!"" —etc. 



Time — early- --^Late. 



Thus the evolution of the horse is inferred from the series of 



fossils (the particular series is illustrative — there are several on 



the books) : 



Phenacodus — a sniall ungulate mammal with 5 com.plete toes A 

 Pachynolophus — do. do. 4 toes ... A' 



Anchitheriwn — do. do. 3 toes and the 



vestige of a 

 fourth one . A" 

 Anchitherium — do. do. only 3 toes . . A'" 



Hippariofi — a donkey-sized do. i large toe and 



2 small ones . A"" 



