168 THE PATH OF SCIENCE 



Soon after the turn of the century it was found by P. Rem- 

 linger in Constantinople that the virus of rabies will pass 

 through one filter but not through another. This gave the 

 clue that made it possible to estimate the size of virus par- 

 ticles, although the microscope could not reveal them. Ex- 

 traordinarily fine filters were made, in which the size of the 

 holes, though ultramicroscopic, could be determined indi- 

 rectly. In the twenties virus particles were already known 

 to be minute. The virus of foot-and-mouth disease is particu- 

 larly small, not many times larger, in fact, than certain large 

 molecules, such as the molecule of hemoglobin. A compli- 

 cated building cannot be constructed from a few bricks, and 

 it is clear that the viruses must be extremely simple in struc- 

 ture: they seem to stand halfway between living and non- 

 living matter. We cannot regard them, however, as the 

 forms in which life first appeared on this planet, for they 

 seem remarkably dependent on the living cells of organisms, 

 and they do not multiply in profusion outside the body as 

 bacteria do. The invention of the electron microscope is 

 already beginning to help in the elucidation of the nature 

 of viruses. The resolving power of this new instrument with 

 suitable objects is much higher than that of the ordinary light 

 microscope, and actual micrographs of virus particles have 

 been obtained. 



Again and again in the history of science we see new devel- 

 opments foreshadowed in old writings. In 1656 the London 

 physician Thomas Wharton had claimed that the secretion of 

 the pineal gland, in the brain, passed into the blood stream; 

 but no one followed up this idea. It had only recently been 

 discovered that glands have ducts, and the contrary idea— 

 that some of them have not— was unattractive. It was not 

 until the nineteenth century that people began to understand 

 how hormones or chemical messengers originate in ductless 

 glands, pass into the blood stream, and exert powerful in- 

 fluences on the action or growth of distant parts of the body. 

 In our own times it has been discovered that plants too have 

 their hormones. 



