THE ORIGIN OF LIFE 169 



soap is used as the emulsifier) will cause the external or 

 continuous phase (water) to become the internal or discon- 

 tinuous phase. Vice versa, a water-in-oil emulsion can be 

 reversed into an oil-in-water emulsion, under the same con- 

 ditions, by the addition of the salt of a monovalent cation 

 {e.g. NaOH). Solutions of hydrophilic colloids, like gelatine 

 or agar-agar, can be made to "set" from the semifluid state 

 of a hydrosol into the semisolid state of a hydrogel, by low- 

 ering the temperature, after which the opposite effect can 

 be brought about by again raising the temperature. In white 

 of egg, however, once gelation has taken place, through the 

 agency of heat, it is impossible to reconvert the "gel" into a 

 "sol" (solution). In such phenomena, it is, perhaps, possible 

 to see a certain parallelism with some processes taking place 

 in the cell, e.g. the osmotic processes of absorption and excre- 

 tion, but to construe them as evidence of propagation by 

 growth and division would be preposterous. 



Nor is the subterfuge of relegating the question to the 

 obscurity of the submicroscopic world of any avail; for, as 

 a matter of fact, submicroscopic organisms actually do exist, 

 and manage, precisely by virtue of this uniquely vital power 

 of multiplication or reproductivity, to give indirect testimony 

 of their invisible existence. The microorganisms, for exam- 

 ple, which cause the disease known as Measles are so minute 

 that they pass through the pores of a porcelain filter, and 

 are invisible to the highest powers of the microscope. Never- 

 theless, they can be bred in the test tube cultures of the 

 bacteriologist, where they propagate themselves for genera- 

 tions without losing the definite specificity, which make them 

 capable of producing distinctive pathological effects in the or- 

 ganisms of higher animals, including man. Each of these 

 invisible disease germs communicates but one disease, with 

 symptoms that are perfectly characteristic and definite. 

 Moreover, they are specific in their choice of a host, and 

 will not infect any and every organism promiscuously. 

 Finally, they never arise de novo in a healthy host, but must 



