222 



ANIMAL BIOLOGY 



and so we are face to face with the problem of how it came to be 

 established on the Earth in the past — the remote past, since the 

 geological record affords convincing proof that life has existed 

 continuously on the Earth for some hundreds of millions of years. 

 Accordingly, unless one is willing to ascribe life's origin to 

 special creation — which at once removes it from the sphere of 



Fig. 153. — Apparatus used by Tyndall in his experiments on spontaneous 

 generation. The front and side windows (w, w) of the cabinet are made of 

 glass. Air can enter through the two tubes (a, b). The optical test for the purity 

 of the atmosphere within the cabinet was made by passing a powerful beam 

 of light from the lamp (/) through the side windows. When the atmosphere 

 contained no suspended ' dust ' particles, the tubes (c) within were filled, 

 through the pipet (/>) with sterile culture medium suitable for the growth of 

 germs, but none developed. (After Tyndall.) 



science and so beyond the present discussion — we have the fol- 

 lowing alternatives : either life came to this planet from some other 

 part of the universe; or it arose spontaneously from non-living 

 matter at one period at least in the past as a natural result of the 

 evolution of the Earth and its elements. With these in mind, a 

 review may be made of several modern theories of the origin of 

 life. It is nearly, if not quite, as important to define our area of 

 ignorance as to extend our area of knowledge. 



