44 



EVOLUTION AND ANIMAL LIFE 



speculations that biologists have allowed themselves concerning 

 the origin of living substance on the earth. A speculation that 

 is interesting only because it was suggested by a great scientific 



Fig. 35. — Golden eagle, Aquila chrysaetus. 



man — a physicist, however, not a biologist — is Lord Kelvin's 

 theory that living substance was brought to this earth from 

 celestial regions by meteorites. A more acceptable theory is 

 that at some earlier geologic age the conditions of earth, atmos- 

 phere, temperature, etc., were at one time of such a favorable 



