ORIGIN OF SPECIES. 441 



placing the course of ancient rivers." * But Mr. Prestwich, 

 in respect to the same valley-deposits, remarks, that, "although 

 often indicating considerable age, they show rates of growth, 

 which, though variable, appear, upon the whole, to have been 

 comparatively rapid." And I fully concur with him in the 

 conclusion, that the present evidence does not necessitate the 

 carrying back the date of man, in past time, so much as 

 bringing forward of the extinct post-glacial animals towards 

 our own time. 



As to the successive appearance of new species in the 

 course of geological time, it is first requisite to avoid the 

 common mistake of confounding the propositions, of species 

 being the result of a continuously operating secondary cause, 

 and of the mode of operation of such creative cause. Biolo- 

 gists may entertain the first without accepting any current 

 hypothesis as to the second. That the species of the miner- 

 alogist and the botanist should be owing — the one to a 

 natural, the other to a supernatural force — the one to the 

 operation of a second cause, the other to the direct inter- 

 ference of a first cause, is not probable. The nature of the 

 forces operating in the production of the cells of a lichen may 

 not be so clearly understood as those which arranged the atoms 

 of the crystal on which the lichen spreads. " Whether an inde- 

 pendent, free-moving, and assimilating organism, of the grade of 

 structure of a germ-cell, may not arise by a collocation of par- 

 ticles through the operation of a force analogous to that which 

 originally formed the germ-cell in the ovarian stroma, is a 

 question worthy all care and pains in its solution." f Pouchet 

 lias contributed valuable evidence of such production, under 

 external influences, of species of Protozoa, t With regard to 



* " Address," etc., on opening the Section of Geology at the Meeting of the 

 British Association at Aberdeen, September 15, 1859. 



■f President's Address on the opening of the Meeting of the British Asso- 

 ciation at Leeds, 1858. 



| Heterogenie, 8vo, 1859. 



