142 ELEMENTS OF BIOLOGY. 



invariably sharply separated from the forms which they 

 connect ; and no case is yet known to us, even taking the 

 Tertiary period alone, in which we can point to a graduated 

 series of intermediate forms, by which one well-marked 

 species can be shown to pass into another equally well- 

 marked species. 



7. The changes in the life of the globe revealed to us by 

 geology are so vast and so numerous that the imagination is 

 utterly powerless to grasp the inconceivable lapse of time 

 required for the bringing about of these changes by the 

 tardy action of natural selection alone. Physical geology 

 teaches us that geological time is something as inconceiv- 

 ably vast as astronomical space ; but it may fairly be 

 doubted if the utmost lapse of time required by the phe- 

 nomena of physical geology can be regarded as more than 

 a mere drop in the ocean, as compared with the time re- 

 quired for the zoological revolutions indicated by the study 

 of Palaeontology — if these revolutions have been brought 

 about by the action of natural selection. It can hardly be 

 reasonably asserted that the time necessary for such biolo- 

 gical changes is fixed by physical geology alone ; and that 

 if this latter informs us that the geological changes of the 

 earth have taken place within a given limited period, then 

 we must simply change our beliefs as to the time required 

 for the conversion of one species into another by natural 

 selection. This certainly appears to be a species of reason- 

 ing in a circle. The very essence of the theory of." Evolu- 

 tion by Natural Selection " is the almost entire impossibility 

 of one species being converted into another otherwise than 

 by an extremely slow process, during which a vast number 

 of generations lived and died. AVe have also, upon the 

 doctrine of " the adequacy of existing causes," certain 

 definite data as to the duration of species. For we know 

 that many existing species have lived without change during 

 what may justly be considered a very vast period of time. 

 It is therefore for Evolution to say how long a period is 



