﻿2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 59 



In spite of their minute size, these visible parasites possess charac- 

 ters that permit in most instances of their ready identification by the 

 microscope. The small number that lack distinguishing characters 

 of form may be identified by their physiological properties when 

 cultivated in broth or otherwise outside the body ; a very few demand 

 for identification the test of disease-producing capacity in animals. 



But we are beginning to learn that not all minute organisms can 

 be rendered visible by our most powerful microscopes, and a number 

 of serious diseases of the higher animals, including man, and one 

 disease of plants (the mosaic disease of tobacco) are caused by sub- 

 microscopic parasites. It is, indeed, not remarkable that the present 

 microscopes should have failed to define the limits of organized 

 nature. Whether we shall ever invent instruments capable of resolv- 

 ing and rendering visible these minute particles of living matter 

 is a question impossible to answer. Even doubling the potential 

 power of the microscope, by the device of employing for photo- 

 graphic purposes the ultra-violet rays of the spectrum, has failed to 

 bring them into view. Their place in nature is not accurately es- 

 tablished. Some, as the parasite causing yellow fever, which passes 

 a stage of its existence in mosquitoes, probably are protozoal ; others, 

 as the parasite of pleuropneumonia of cattle, which can be propagated 

 in artificial cultures, probably are bacterial. It can hardly be doubted 

 that they are living organisms, since they are capable of transmission 

 from animal to animal, in which they produce infection, through an 

 indefinite series. The last disease to be discovered to be caused by a 

 submicroscopic parasite is epidemic poliomyelitis, or infantile paraly- 

 sis. Because the submicroscopic parasites are too small to be retained 

 by, but pass through fine earthenware filters, they have also been 

 termed filterable organisms or viruses. No member of this class of 

 organisms is now known, except such as cause disease. 



It is a matter of common knowledge that all individuals of an 

 animal species are not equally subject to disease. This observation 

 can be made on every hand among the human species especially. 

 Careful analysis has indicated that the condition of susceptibility to 

 infection varies not only with the individual but also with the infec- 

 tion itself. Thus measles and small-pox are diseases to which every 

 human being may be regarded as subject, while scarlet fever, diph- 

 theria, epidemic meningitis, and poliomyelitis can claim far fewer 

 victims. So among animals, a certain few diseases secure an almost 

 universal dissemination once they are introduced; while others, the 

 efTects of which, after they become established, may be equally or even 



