SCIENCE IN THE LAST HALF CENTURY. 95 



history of science, physiologists are now in the position to say that they 

 have arrived at clear and distinct, though by no means complete, con- 

 ceptions of the manner in which the great functions of assimilation, 

 respiration, secretion, distributioti of nutriment, removal of waste prod- 

 ucts, motion, sensation, and reproduction are performed ; while the op- 

 eration of the nervous system, as a regulative apparatus, which in- 

 fluences the origination and the transmission of manifestations of ac- 

 tivity, either witbiu itself or in other organs, has been largely elucidated. 

 I have pointed out, in an earlier part of this chapter, that the history 

 of all branches of science proves that they must attain a considerable 

 stage of development before they yield practical "fruits;" and this is 

 eminently true of physiology. It is only within the present ei)0cli that 

 physiology and chemistry have reached the point at which they could 

 offer a scientific fouudatiou to agriculture, and it is only within the 

 present epoch that zoology and physiology have yielded any very great 

 aid to pathology and hygiene. But within that time they have already 

 rendered highly important services by the exploration of the phe- 

 nomena of parasitism. Not only have the history of the animal para 

 sites, such as the tapeworms and the trichina, which infest men and 

 animals, with deadly results, been cleared up by means of experimental 

 investigations, and efficient modes of prevention deduced from the data 

 so obtained, but the terrible agency of the parasitic fungi and of the 

 influitesimally minute microbes, which work far greater havoc among 

 plants and animals, has been brought to light. The "particulate" or 

 "germ" theory of disease, as it is called, long since suggested, has ob- 

 tained a firm foundation, in so far as it has been proved to be true in 

 respect of sundry epidemic disorders. Moreover, it has theoretically 

 justified prophylactic measures, such as vaccination, which formerly 

 rested on a merely empirical basis; and it has been extended to other 

 diseases with excellent results. Further, jnst as the discovery of the 

 cause of scabies proved the absurdity of many of the old prescriptions 

 for the prevention and treatment of that disease, so the discovery of 

 the cause of splenic fever, and other such maladies, has given a new 

 direction to prophylactic and curative measures against the worst 

 scourges of humanity. Unless the fanaticism of philozoic sentiment 

 overpowers the voice of philanthropy, and the love of dogs and cats 

 supersedes that of one's neighbor, the progress of experimental physi- 

 ology and pathology will indubitably, in course of time, place medi- 

 cine and hygiene upon a rational basis. Two centuries ago England 

 was devastated by the plague; cleanliness and common sense were 

 enough to free us from its ravages. One century since smallpox was 

 almost as great a scourge ; science, though working emi)irically, and 

 almost in the dark, has reduced that evil to relative insignificance. At 

 the present time, science working in the light of clear knowledge, has 

 attacked splenic fever and has beaten it. It is attacking hydrophobia 

 with no mean promise of success; sooner or later it will deal iu the 



