INTRODUCTION OF NEW LIFE- FORMS. 193 



seas to have prevented the continuance of trilobites ; 

 nothing in the geography or climate of the coal period to 

 have prevented the huge terrestrial reptiles of the Weald 

 from browsing on its vegetation, or marine species, like 

 those of the lias, from preying on its fishes. The appear- 

 ance and preponderance of certain races during certain 

 geological epochs is a problem which lies as yet beyond the 

 solution of science. That this succession occurs regularly 

 as regards time, space, and biological sequence, we clearly 

 perceive; but how, or by what means of causation, we are 

 altogether unable to determine. We can often trace the 

 extinction of races to a change of external condition ; and 

 as vitality is endowed with a certain amount of elasticity 

 and adaptability, we may account for modifications within 

 the limits of what naturalists term varieties; but we appeal 

 in vain to physical conditions for the first introduction or 

 creation of species. 



In the PAST LIFE of the globe we only see dimly and 

 broadly the outline of a great scheme of gradation and pro- 

 gress a progress on which we may rest as a matter of 

 FAITH, but the terms of whose LAW lie far, as yet, beyond 

 the grasp of exact scientific demonstration. In vain we 

 turn to "external conditions" and "unlimited time;" to 

 the doctrines of "embryology" and " morphology;" or to 

 ''natural selection in the struggle for existence." These 

 are oracles to which theorists have often appealed, but they 

 fail, as yet, to utter an intelligible response. That each of 

 them has some portion of the mystery in keeping, all the 

 tendencies of modern science do, no doubt, appear to indi- 

 cate, but how much, and in what order of connection, our 

 highest determinations are little better than a train of inge- 

 nious guess-work. As far as geological evidence goes, all 

 the great types of life began simultaneously and inde- 

 pendently. All the subsequent introductions of new genera 



