600 GEOLOGICAL HISTORY. 



actually distributed on the plan here indicated (p. 526). These facts 

 suggest again migration and subsequent alteration, under the new 

 regional influences as the cause, as urged by Professors Asa Gray 

 and Heer. 



10. The existence of Representative Species not always a consequence 

 of migration. — On antipodal continents, — as, for example, North 

 America and Australia, — there were in early time both identical and 

 representative species. And now, in insular New Zealand, there are 

 Crustaceans closely representative of some in its antipodes, insular 

 Britain, in the case of which migration cannot be shown to be probable, 

 or hardly in any way possible. Such facts suggest that the succession, 

 in the species of different continents, may have been carried forward 

 independently, even to the introduction of closely similar species. 



11. The transitions between Species, Genera, Tribes, etc., in geological 

 history, are, with rare exceptions, abrupt. — Geological history being 

 prominently a history of the world's life, it is naturally looked to for 

 facts respecting the first appearance of species, or the relations of 

 species, by transitions or otherwise, to one another. A survey of the 

 history finds little that is positive with regard to these transitions. It 

 discovers, as all writers admit, almost no cases of the gradual passage 

 of one species into another, not nearly as many or as close as exist in 

 the present world. At the same time, the truth is apparent that the 

 geological record is very imperfect, so much so as to greatly weaken 

 ell its testimony, with regard to abrupt transitions. It is imperfect, 

 (1) because, under the most favorable circumstances, only a small part 

 of the existing species could have been fossilized ; (2) because in all 

 lands there are great breaks in the series of rocks, as we know from 

 comparing the rocks of different continents, and even different regions 

 on the same continent; (3) because fossiliferous rocks are almost 

 solely of aqueous origin, and consequently they contain exceedingly 

 little of the terrestrial life of the ancient world ; (4) because, when- 

 ever the land was at a higher level than the present, the marine strata 

 then formed around it are now buried in the ocean and are therefore 

 inaccessible ; (5) because only a small part of the rocks of a country 

 are open to view ; and (6) because the continents have not been all 

 thoroughly explored. 



For example : (a) in North America, east of the Mississippi, there 

 is not a trace of the life of the seas of the Triassic and Jurassic peri- 

 ods, two thirds of all Mesozoic time — the Triassic and what there 

 is of Jurassic beds being of brackish- water or fresh-water origin, (b.) 

 In the American Triassic and Jurassic beds, the jaw-bones of two mar- 

 supial Mammals have been found ; and these two are the only relics 

 of Mammals from the whole Mesozoic of the continent, when the 



