ORIGIN OF LIFE ON THE EARTH. 133 



and Yertebrata are probably derived from some 

 primeval form of which the Molluscoida are 

 the modern representatives. 



Haeckel has promulgated the theory that the 

 simplest existing group of Protista, the Monera, 

 are continually being produced by spontaneous 

 generation.* But he offers no proof beyond 

 mere conjecture ; and as he has himself observed 

 various modes of reproduction taking place in 

 different species, it is not certain that we have 

 yet actually reached the lowest substratum of 

 organic life. The very lowest forms of Monera, 

 however, possibly multiply by simple fission 

 alone, which is the simplest conceivable form 

 of reproduction. 



It has been suggested that the semi-organised 

 mud at the bottom of the deep sea may be the 

 transitional stage between inorganic and organic 

 matter. As life must have originated de novb 

 once, it is quite possible that it may do so con- 

 tinually ; but it is equally possible that life 

 may have originated only at one particular 

 stage of the earth's development.-}* Even sup- 



* Herbert Spencer has explained how simple marine forms may remain 

 unchanged for long geological periods. " Biology," vol. i. pp. 428, 429. 

 t Compare Huxley's " Critiques and Addresses," pp. 238, 239. 



