THE WORM-LIKE ANIMALS 347 



worms is caused by flooding of their burrows, and that they 

 do not rain down ; and that crowds of toads, frogs, mice, grass- 

 hoppers, etc., are due to very favorable conditions for the 

 developing eggs and migrations into new territory. All 

 other such cases which puzzled even the scientific people of a 

 few hundred years ago have been explained so well that for 

 more than a hundred years no scientific man has believed in 

 the existence of spontaneous generation of any organisms 

 higher than the bacteria. But until the studies by Pasteur, 

 supported by the work of the English physicist Tyndall, 

 between 1850 and 1870, it was believed even by men of 

 science, that certain bacteria may develop spontaneously in 

 sterile bouillon and other foods. Pasteur showed that if 

 proper precautions are taken to make the foods perfectly 

 sterile, that is, to kill all life in the test-tubes used, and to 

 prevent entrance of other germs, no organisms will develop. 

 In short, Pasteur showed that there is no evidence that liv- 

 ing matter originates spontaneously from not-living matter. 

 He did not show that it could not happen, for there may be 

 conditions of which we know nothing, as perhaps existed at 

 the first appearance of living matter on the earth; but he 

 showed that the few cases in which some scientific men of 

 his time still believed had not been sufficiently tested by accu- 

 rate experiments. So far as 

 concerns the origin of new in- 

 dividuals of all known species 



Of Organisms nOW existing, We FIG. IIO. Plan of annelid's body, 

 may summarize the Studies & . " brain," dorsal to the esoph- 



which culminated with those of ' ^0*^ 



Pasteur in the Statement that alimentary canal ; a, anus or 



all living things come from liv- 

 ing things, or all life from life. 



300. The Segmented Worms : Annelids. The common 

 earthworms and the leeches are members of a group char- 

 acterized by division of the body into rings or segments, 



