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CONTENTS 
Introduction ......... 5 
What is Disease? ........ 8 
What is Normal Living? ....... 9 
The Meaning of Abnormal Living ..... 10 
Protective Covering a Basic Factor in Dependence . . 15 
Proper Understanding of the Shell or External Skeleton 20 
Stabilization, Longevity and Dissolution .... 23 
Divisions of Geological History ..... 26 
Independence of the First Fauna ..... 28 
General Survey of the Cambrian Fauna of North America . 29 
The Cambrian Fauna Generally . . . . . 32 
Precambrian Life ........ 32 
The Beginnings of Symbiosis and Parasitism ... 35 
Complex Character of Parasitism .... 37 
Beginnings of Symbiosis ...... 37 
Relation of Symbiosis to Parasitism .... 38 
Illustrations of Primitive Parasitism .... 39 
The Case of the Annelids ...... 39 
The Barnacles . . . . . . . .61 
Early Parasitism of the Snails upon the Crinoids . . 64 
Symbiotic Conjunction of Crinoids and Starfishes . 78 
The Work of Pseudoparasitic Boring Organisms . . 80 
The Distinction between Protozoan and Metazoan Parasitism 104 
Sporozoan and Bacterial Parasitism in Geological His- 
tory ......... 107 
Summary and Conclusions ...... 109 
