38 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



birds, that storms may raise eggs or individual animals or plants high in 

 the air and drop them at great distances; that seeds ingested by birds and 

 other animals pass unharmed through the digestive tract; that animals 

 may be carried upon drifting vegetable material across great bodies of water 

 and that some seeds and nuts endure long immersion in salt water and may 

 be carried great distances. 



(g) EXTINCTION OF A FLORA OR FAUNA. 



Extinction of a flora or fauna may be caused by a variety of conditions. 

 It is especially necessary that the student of stratigraphy recognize that 

 it is frequently caused by entirely organic conditions, as the introduction 

 of disease or the advent of powerful enemies which either attack and destroy 

 the victims or preempt their food-supply or natural habitat. These things 

 are generally effective upon only a portion of the fauna or flora, but may 

 attack forms so dominant as to apparently alter the whole biota. 



It would be totally unwarranted to assume that the sudden disappearance 

 of the giant reptiles at the close of the Mesozoic or opening of the Tertiary, 

 of the horse, mastodon, and elephant from North America in the Pleistocene 

 was due entirely to inorganic changes, as climate, physiography, etc., which 

 altered radically the conditions of all life. There is no evidence of a com- 

 petent change in the inorganic world, and similar catastrophes have been 

 traced to disease among living forms. 



(h) SURVIVALS AND PRECIPITATE DEVELOPMENT. 



Untoward conditions permit archaic forms to survive, as when regions 

 are long free from disturbances of the inorganic conditions and are pro- 

 tected by barriers from the advent of destructive or competing forms. The 

 faunas of the continents of Australia and South America are pertinent 

 examples. 



Other cases of long-lived groups, such as the genera Lingula, Atrypa, 

 and Leptcena, are apparently due to a peculiar hardihood inherent in the 

 group and extraordinary powers of adaptation. It is obvious that such 

 forms are as little adapted to use in determining the environment as they 

 are in determining stratigraphic units. 1 



Precipitate development is supposed to occur in the youth of a group, 

 but this is not an invariable principle. Exuberant growth, as has been 

 demonstrated, is caused by many factors, especially those which in some 

 manner disturb the phylum. Adverse conditions produce such effects at 

 times cross-breeding such as might easily occur in regions crowded with 

 plants or animals, the approaching extinction of a group all these and others 

 induce the appearance of new forms. In paleontology, where the determina- 



1 Ruedemann, Rudolf, The Paleontology of Arrested Evolution, N. Y. State Museum Bull. 

 196, 1916. 



