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Annals of Medical History 



recent forms are highly parasitized and are 

 occasionally subject to disease. It appears 

 probable that vertebrates have been more 

 liable to diseases which afflict the hard 

 parts than have the invertebrates, and this 

 liability to pathologic processes has been 

 increased with the passage of geologic time. 



IMMUNITY IN MODERN INVERTEBRATES 



The greater immunity of early Paleozoic 

 animals to disease, based on the evidences 

 of paleontological material, is probably not 

 a true index to actual conditions, though it 

 may be so. It is probably not safe to con- 

 clude from present-day conditions what the 

 state of Paleozoic animals may have been 

 as regards disease. At any rate the paleon- 

 tological evidences are not wholly sub- 

 stantiated by conditions found in modern 

 forms. Metchnikoff 24 has called attention 

 to the occurrence of epidemics of a severe 

 nature among protozoa, such as diseases 

 in Amoebae caused by the Microsphsera and 

 the disease in Actinopbrys attributed to 

 Fungi allied to the genus Pythium. Pasteur's 

 studies on the pebrine and fldcherie of the 

 silkworms will be remembered as instances 

 of severe epidemics in an invertebrate 

 group. Molluscs, however, are apparently 

 largely immune to infection, and since the 

 molluscous animals formed such a large 

 percentage of the preserved faunas of the 

 early periods of the earth's history we may 

 attribute our ignorance of the presence of 

 disease to this factor, in part at least. The 

 immunity of many intermediate hosts to 



24 Metchnikoff: "Immunity in Infective Diseases," 

 translated from the French by Francis G. Binnie, 

 1905, p. 18; also Chap. iii. 



25 Edward Hindle: "Flies in Relation to Disease 

 (Blood sucking Flies)," 1914. 



G. S. Graham-Smith: "Flies in Relation to 

 Disease (Non-Blood sucking Flies)," 1914. 



26 Elias Metchnikoff: "Die Lehre von den Phy- 

 gocyten und deren experimentelle Grundlagen." 

 In "KoIIe und Wassermann's Handbuch der patho- 

 genen Mikroorganismen," 1913, Bd. ii, erste Halfte, 

 pp. 655-731, with an excellent bibliography. 



infection 25 is well known, and the classical 

 example of the mosquito-borne infections 

 will suffice, although it is well known that 

 insects of many kinds are subject to fatal 

 diseases. Kowalevsky has discussed the 

 anthrax of crickets and many other students 

 have studied the problem. The entire ques- 

 tion of immunity in its relation to all forms 

 of extinct animals is of course a new and 

 unsolved, probably an insolvable, problem. 

 But it seems certain that if the early ani- 

 mals were diseased, the ensuing pathology 

 was of such a nature as to leave no impress 

 upon the fossilized part; or else we have not 

 yet learned to recognize these lesions. 



THE ORIGIN OF DISEASE 



Phagocytosis 26 doubtless began very early 

 in the history of animal life, and it is prob- 

 able that the natural immunity of the 

 early animals was sufficiently strong to 

 resist the invasion by any pathogenic organ- 

 isms in sufficient numbers to produce dis- 

 ease. The breaking down of this immunity 

 may possibly be correlated with the develop- 

 ment of senescence 27 among the early races 

 of animals, which reached a climax in some 

 forms — the trilobites, for instance, — at 

 about the time when we find the first indi- 

 cations of disease among fossil animals. 

 The breaking down of the immunity, due 

 to the development of race senescence and 

 the introduction of disease, doubtless was 

 of very great importance in the extinction 

 of the trilobites and other great groups 



27 The studies of Charles Emerson Beecher (1856- 

 1904), an American paleontologist, upon evolu- 

 tionary phases of the early fossil brachiopods and 

 trilobites are especially important to consider in 

 connection with the question of race senescence and 

 the extinction of animal groups. His papers have 

 been collected into a volume: "Studies in Evolution," 

 New York, 1901. 



The entire subject of senescence in the recent 

 lower animals is discussed by Child in "Senescence 

 and Rejuvenescence," University of Chicago Press, 

 1915. 



