96 AN INTRODUCTION TO ZOOLOGY, 



life, or whether they are in some mysterious way 

 developed from the material of inorganic nature. 

 This last theory, — namely that of the spontaneouf! 

 generation of life, — has been held by some inquirers ; 

 but their best efforts have not suflSced to prove that 

 it has any ground at all. The organisms whose ■ 

 insidious presence manifests itself in solutions which 

 the experimenter has believed to be clear of all traces 

 of life, and secured from the entrance of any, are 

 organisms closely allied to the germs which modern 

 research has shown to play so important a part in 

 the history of many diseases and septic conditions. 

 On account of their extreme minuteness the study 

 of these forms is attended by great difficulties, and, 

 for the same reason, their growth may sometimes seem 

 to be spontaneous. Yet it may safely be said that it 

 is at least as likely that they present a type already 

 highly specialized and of a retrograde character, as 

 that their apparent simplicity is that of a primitive 

 type, such as we might expect life in its first begin- 

 nings to present. History repeats itself; and pro- 

 bably those who have looked among these " germs " 

 for the initial form of the living organism have made 

 the same mistake as those who looked to the Ascidians, 

 — a highly specialized though perhaps retrograde type, 

 — for the initial form of the vertebrate organism. 



Whatever may be the outcome of further research, 

 it must at present be said that we know nothing what- 

 ever regarding the origin of life. So far as we know, 

 all living forms take their origin from previously 



