INTRODUCTION. 



XN several different departments of research, and from many- 

 different points of view, we are brought to see that the study of 

 Comparative Anatomy, while raising questions of the highest 

 theoretical importance, throws light at the same time upon pro- 

 blems of great practical interest. Of its twofold bearings, the con- 

 troversies carried on at the present moment as to the Origin of 

 Life and as to the Origin of Species, extending, as they do, on the 

 one side into the provinces of Hygiene and Therapeutics, and on 

 the other into yet higher regions than those, furnish two apposite 

 but not isolated illustrations. It is not within the scope of this 

 work to do more than thus glance ^ at such practical questions 



* It may be well here to give a few instances in which light has been thrown upon 

 complex questions of Hygiene and Pathology in the course of investigations ap- 

 parently wholly dissociated from such subjects. Dr. Charlton Bastian has, in the 

 Philosophical Transactions for 1866, vol. 156, pt. ii., pp. 583, 584, put upon record a 

 most instructive account of the production of a spasmodic and catarrhal affection not 

 altogether unlike hay fever, under circumstances, however, which appear to preclude 

 the possibility of any living organisms having been, as has recently been suggested, 

 the cause of it. This affection was invariably produced from the emanations inhaled 

 during the dissection of a particular Nematoid worm, the Ascarismtgalocephala, from 

 the Horse, and this not only when the animal was fresh, but 'after it had been 

 preserved in methylated spirit for two years, and even then macerated in a solution 

 of chloride of lime for several hours before it was submitted to examination.^ It would 

 seem certain that no morphological unit, nor even any cell-like or ' cy toid ' body, can 

 have been at work under circumstances such as these. On the other hand, we have 

 no less an authority than Helmholtz for the coexistence of vibrios in the nasal 

 passages with the presence of hay fever (Virchow's Archiv. xlvi., p. loi, Feb. 1869), 

 and indeed for the causal relation of the former to the latter condition. Chauveau's 

 experiments, again (Revue des CoursScientifiques, Jan. i, 1870, p. 77), shew that in the 



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