HEREDITY AND EVOLUTION 267 
are discovered it is usually owing to a combination of favorable 
circumstances. I ossils that are formed in deep seas are rarely seen, 
as such areas are not often elevated sufficiently to become dry 
land. When areas of fossil-bearing rocks are exposed to erosion, 
the fossils are uncovered, and in this way many fossils are brought 
to the attention of man; but those that were exposed in this man- 
ner in past ages have been destroyed, and in this way a great 

Fig. 244. Cliff more than a thousand feet high composed of alternately 
bedded sandstone and shales (Mountain Province, Philippine Islands) 
deal of the fossil record has been lost. Fossils have also been de- 
stroyed by being dissolved from rocks, by being subjected to great 
pressure, and by various other means. As previously mentioned, 
the soft parts of organisms are rarely fossilized. Owing to the 
above-mentioned facts, and for other reasons, the fossil record 
is not as complete as we should like to have it, and, moreover, 
the study of such fossils as can be found is very far from com- 
plete; yet, in spite of its imperfections, the fossil record has 
given us considerable knowledge of the past history of floras 
and faunas. 
