1 78 CLUES AND SUGGESTIONS CHAP. 



sumptions that no organic structure 

 has ever come into existence except by 

 ordinary generation we cannot even 

 conceive it to be true. But putting this 

 aside, of the second of these two assump- 

 tions, namely, that organic structures 

 can never have been developed except 

 by actual use, it may be confidently said 

 that it is certainly unfounded. We can- 

 not be sure that the calling into existence 

 of new germs a process in which the 

 whole animal world must confessedly 

 have begun is a process which was 

 adopted only once, and has never been 

 repeated in the whole course of time. 

 We cannot, therefore, be certain that 

 the Cetacea, which constitute a very 

 distinct division in the animal kingdom, 

 have not been thus begun, with pre- 

 determined lines and laws of growth 

 which stand in close relation to the 

 development of all the terrestrial Mam- 



