1030 HISTORICAL GEOLOGY. 



tological study, having therefore the successional lines which such study 

 ascertains ; but different in method, the change in species being dependent, 

 in his view, on creative acts, and not on natural variation. All students 

 of nature, with a rare exception, then believed in permanence ; for Lyell's 

 chapter against the transmutation of species, in the successive editions of his 

 Principles of Geology, had seemingly settled the question against Lamarck 

 by scientific argument. It was not till 1859 that Darwin's work was pub- 

 lished on the Origin of Species by means of Natural Selection, or the Preser- 

 vation of Favored Races in the Struggle for Life. 



The principles above stated are all in accord with a theory of evolution ; 

 and, through the added facts of later years, they favor the view of evolution 

 by natural variation. Some of these added facts are the following : Botan- 

 ists find numerous cases among existing species in which, owing to the many 

 varieties, no line can be drawn between allied species ; and other cases in 

 which modern species of plants are but slight modifications of fossil Tertiary 

 species, some too slight to be called distinct species, and others more diver- 

 gent up to those of good distinctive characters. Similar facts occur as a 

 consequence of migrations, among animals as well as plants. Arctic America 

 contained, in Tertiary time, plants so much like species existing in the 

 forests of both temperate iSTorth America and Japan (page 939), that the 

 former have been pronounced the undoubted progenitors of the latter. Along 

 the Pacific coast and Gulf coast of Central America there are so many iden- 

 tical and nearly related species of aquatic animals that migration during a 

 time of submergence of the narrow strip of land, with subsequent variation, 

 is regarded as the only reasonable explanation. These and other observa- 

 tions have proved sufficient to make all zoologists of the present day, like 

 the botanists, believers in a system of Evolution by variation. 



It is admitted that in the geological record cases of unbroken gradation 

 between species are of rare occurrence. But the geological record bears 

 evidence, in all parts, of imperfections. 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 is known from comparing the rocks of different conti- 

 nents, and even different regions on the same continent : (3) because fossil- 

 iferous rocks are almost solely of aqueous origin, and consequently they 

 contain exceedingly little of the terrestrial life of the ancient world — one 

 species of Bird being all yet discovered in the world's rocks of the Jurassic 

 period, and two species of Mammals all that are known from the American 

 Triassic beds ; (4) because marine strata that were formed around the land 

 when it was at a higher level than the present are now buried in the ocean, 

 and are therefore inaccessible, a condition that has affected half the borders 

 of a continent for several successive periods; (5) because only a small part 

 of the rocks of a continent are open to view. This subject has been abun- 

 dantly illustrated in the preceding history of the formations and their life. 



But transitions have been nearly filled in so many cases, and are indi- 



