166 THE PATH OF SCIENCE 



tion can be enormously increased by subjecting organisms to 

 x-rays; and ultraviolet light and radium have since then been 

 shown to act in the same way. These agencies act on the 

 chromosomes of the germ cells. We may look for great ad- 

 vances in this line when someone has discovered how to con- 

 trol the process. At present it is a hit-or-miss affair; there 

 is no known way of producing one ne^v gene rather than 

 another. 



It is strange to recall that the controversy on spontaneous 

 generation was only laid to rest in the middle of the nine- 

 teenth century. We have already seen (page 149) that Spal- 

 lanzani had disproved spontaneous generation by careful ex- 

 periments in the sixties of the century before, but people 

 were not easily convinced. The great Swedish chemist Ber- 

 zelius (page 121) still believed in the spontaneous generation 

 of some of the lower animals at the beginning of the nine- 

 teenth century; so, later still, did that restless genius of physi- 

 ology and marine zoology, Johannes Miiller. The most 

 ardent supporter of spontaneous generation, however, was 

 the Rouen professor Felix Pouchet, who thought that the 

 fermentation of decaying substances was actually the process 

 by which the micro-organisms found in such substances orig- 

 inate. This cart-before-the-horse opinion was opposed by 

 Louis Pasteur, whose critical experiments finally convinced 

 the scientific world in 1861. 



Pasteur went straight on to the study of micro-organisms as 

 the causes of disease. In 1835 an Italian amateur microscop- 

 ist, Agostino Bassi, had shown that a disease of silkworms was 

 caused by a microscopic fungus. Not much attention had 

 been attracted by this discovery; and now, strangely enough, 

 Pasteur started his investigation of germs by studying another 

 disease of the same insect. Things moved quickly in the six- 

 ties. Another Frenchman, Casimir Davaine, discovered bac- 

 teria in the blood of animals suffering from anthrax and 

 showed that one-millionth of a drop of infected blood was 

 sufficient to carry the disease into a previously healthy indi- 

 vidual. Pasteur's final proof that micro-organisms are not 



