406 RETROSPECT OF THE OOLITIC SYSTEM. CHAP. 



district sure data to be combined with others in general reasoning-, 

 such speculations cannot be discussed with advantage, and are not 

 worth discussion at all unless they have the quality of pointing to 

 trains of useful further research, or serve to exhibit to inexperienced 

 labourers the hopelessness of attempting the solution of problems 

 for which there are no data. 



The speculation of Darwin and many who preceded him, of 

 indefinite change in animal and vegetable forms through length 

 of time and variation of physical conditions, is of the highest 

 theoretical and, at the same time, practical interest to the palaeon- 

 tologist. Given a primary, or what, for the purpose of this reason- 

 ing, may be called a primary form, can we trace it varying in any 

 case, with time and circumstance, in such directions and to such 

 an extent, that, if time and change of physical accompaniments 

 were taken at a maximum, one generic type could be changed to 

 another? Out of one original lamellibranchiate mollusk, placed 

 at the limits of the monomyarian and dimyarian divisions, could 

 avicula, perna, and crenatula &c. go forth as colonies in one direc- 

 tion ; ctenodonta, cucullsea, area in another ; cypricardia, orthonota, 

 and the mytilacese in a third d ? This may not be beyond the range 

 of research, but the scope of this treatise admits of only one 

 suggestion regarding it. 



If the amount of change which can certainly be recognized in 

 natural groups extend only to specific distinctions in the course 

 of all assignable time, and yet genera have given birth to others 

 unlike themselves, how vast must have been the pre-Cambrian 

 periods, to have allowed of this change from some one supposed 

 primary into the many definite genera which the Cambrian rocks 

 contain ! Many times one hundred millions of years would be 

 required if the slow process now observable in nature be taken as 

 the measure of effect : we have no trace of such periods, and perhaps 

 Astronomy and the Mathematical Theory of Heat will not allow 

 of such vast duration to the habitable condition of the earth e . 



Was, then, the measure of change greater in early times ? were 

 these epochs 'rich in generic ideas,' as E. Forbes, an admirable 



d See remarks bearing on this subject in Memoirs of the Geological Survey, vol. ii. 

 p. 264, 1848. 



e Sir W. Thomson on Secular Cooling of the Earth, in Treatise on Nat. Phil., by 

 Thomson and Tait, 1867. 



