GEOLOG Y AND E VOLUTION. 371 



periods are further "blank," and unrepresented by any tangible 

 details in the earth's history, is also plain. It is matter of geological 

 detail, for example, to show how the phenomena known as " uncon- 

 formability," wherein one rock system lies athwart another, so to 

 speak alone proves the immensity of the period of time which 

 intervened between their formation. Prior to and during this interval, 

 migration of the species to be represented as fossils in the epoch 

 which had just closed, would unquestionably take place. We thus 

 note how rocks formed in widely separated areas may contain like 

 fossils, and also how the animals and plants of one period may 

 have passed onwards to take part in the development of life in the 

 next. Mr. Darwin's own words are highly succinct and convincing 

 on the points just mooted. "I have given my reasons," says Mr. 

 Darwin, "for believing that most of our great formations, rich in 

 fossils, were deposited during periods of subsidence ; and that blank 

 intervals of vast duration, as far\as fossils are concerned, occurred 

 during the periods when the bed of* the sea was either stationary or 

 rising, and likewise when sediment was not thrown down quickly 

 enough to imbed and preserve organic remains. During these long 

 and blank intervals I suppose that the inhabitants of each region 

 underwent a considerable amount of modification and extinction, 

 and that there was much migration from other parts of the world. 

 As we have reason to believe," continues Mr. Darwin, " that large 

 areas are affected by the same movement, it is probable that strictly 

 contemporaneous formations have often been accumulated over 

 very wide spaces in the same quarter of the world ; but we are very 

 far from having any right to conclude that this has invariably been 

 the case, and that large areas have invariably been affected by the 

 same movements. When two formations have been deposited in two 

 regions during nearly, but not exactly, the same period, we should 

 find in both, from the causes explained in the forgegoing paragraphs, 

 the same general succession in the forms of lite ; but the species 

 would not exactly correspond ; for there will have been a little more 

 time in the one region than in the other for modification, extinction, 

 and immigration." It thus seems clear, that on grounds connected with 

 the lapse of past time in relation to life- development on our globe, 

 and also for reasons connected with the " blank periods " between the 

 formations comprising its crust, the evolutionist may be content to 

 assume, firstly, that questions connected with the duration of time 

 past need not cause him any perturbation. He may secondly argue, 

 with reason, that even when the question of unrepresented periods 

 is alone considered, the evidence at hand tends to prove how 

 life's succession is in reality continuous beneath the breaks in time, 

 and in the order of rock-formation ; whilst he also notes how 

 aptly the theory .of descent harmonizes with the full consideration 



