102 * The Scottish Naturalist. 



developments of the life-bearing earth summed up; and is thus 

 still the starting point for all future developments. By that time, 

 according to Darwin, in the same section of his work on the 

 "Origin of species," there is some reason to conclude that a com- 

 plete revolution of the earth's surface had been effected, where- 

 by oceans have since spread themselves out where continents 

 before lay, and continents have arisen where clear open oceans 

 flowed before. This is given as a hypothesis that may possibly 

 be found yet to explain an otherwise inexplicable difficulty — 

 " the difficulty of assigning any good reason for the absence of 

 vast piles of strata rich in fossils beneath the Cambrian system." 

 The earth, it appears, on her march of evolution, having reached 

 the Cambrian stage, then shifted step, as it were, and began to 

 move on along a new line, inaugurating thus a new cycle — the 

 one to which we and the theory in hand belong, the only one 

 that has left clear traces of its course and its character. The 

 old solid earth had become submerged under our present 

 oceans ; and the old oceans had their bottoms elevated to be 

 the foundation of the new ten-a firma. This may have been. 

 It explains the loss to observation of the course of previous 

 evolution. But still the earth's known history lies all between 

 the era of that extreme exchange and this : so that if, at the 

 worst, Cambria and Siluria are the goal of a primeval develop- 

 ment and the starting point of a succeeding one, at the best 

 they may be the initiation of the only cycle of evolution through 

 which the earth as yet has run. However it may be, it is in the 

 stretch of time between the base of the known geological series 

 and its highest strata that there is, as we have seen, some ground 

 for holding a law of polarity in the distribution of genera of 

 organic beings. 



I now pass to the question of a physical theory to account 

 for the profusion of genera at the geological poles. And here 

 let me at once go the length of asking, may it not be possible 

 that natural science during the last decade or so, working 

 entirely in oblivion of the ideas of this Forbesian theory, and 

 even notwithstanding the obstacles in the way of these ideas 

 it seemed in its advance to be raising, has nevertheless actually 

 fallen on results that are capable of being regarded as demand- 

 ing, and therefore as applicable to account for, the very state of 

 things which polarity describes ? Would not the doctrine of 



