Ixviii REPORT — 1866. 



But it is quito possible that these may mean nothing more than -wovld be due 

 to some change in the conditions of existence. Except between the Marl- 

 stone and the Upper Lias there is really no pala^ontological break, in the 

 proper sense of the words ; alterations of form and size consequent on the 

 occurrence of circumstances more or less favourable, migration of species, and 

 other well-known causes sufficiently account for many of those modifications 

 of the form of the shell that have been taken as specific marks. This view is 

 strengthened by the fact that other shells and other organisms generally 

 show no proof of a break of any importance except at the point already 

 aUudedto." 



But, irrespectively of another deficiency in the geological record, which will 

 be noticed presently, the physical breaks in the stratification make it next to 

 impossible to fairly trace the order of succession of organisms by the evidence 

 afforded by their fossil remains. Thus there are nine great breaks in the 

 PalfBozoic series, four in the Secondary, and one in the Tertiary, besides those 

 between Palajozoic and Secondary and Secondary and Tertiary respectively. 

 Thus in England there are sixteen important breaks in the succession of strata, 

 together with a number of less important interruptions. But although these 

 breaks exist, we find pervading the worlis of many geologists a belief, re- 

 sulting from the evidence presented to their minds, sometimes avowed, 

 sometimes unconsciously imphcd, that the succession of species bears some 

 definite relation to the succession of strata. Thus Prof. Eamsay says 

 that " in cases of superposition of fossiliferous strata, in proportion as the 

 species are more or less continuous, that is to say, as the break in the suc- 

 cession of life is partial or complete, so was the time that elapsed between 

 the close of the lower and the commencement of the upper strata a shorter or a 

 longer interval. The break in life may be indicated not only by a difference 

 in species, but yet more importantly by the absence of older and appearance 

 of newer allied or unallied genera." 



Indications of the connexion between cosmical studies and geological re- 

 searches are dawning on irs : there is, for instance, some reason to believe that 

 we can trace many geological phenomena to our varying rotation round the 

 Sim ; thus more than thirty years ago Sir J. Herschel proposed an explanation 

 of the changes of climate on the earth's surface as evidenced by geological 

 phenomena, founded on the changes of excentricity in the earth's orbit. 



He said he had entered on the subject " impressed with the magnificence 

 of that view of geological revolutions which regards them rather as regular 

 and necessary efforts of great and general causes, than as resulting from a 

 series of convulsions and catastrophes regulated by no laws and reducible to 

 no fixed principles." 



As the mean distance of the earth from the sun is nearly invariable, 

 it would seem at first sight that the mean animal supply of light and heat 

 received by the earth would also be invariable ; but according to his calcula- 

 tions it is inversely proportional to the minor axis of the orbit : this would 

 give less heat when the excentricity of the earth's orbit is approaching to- 

 wards or at its minimum, ilr. CroU has recently shown reason to believe 

 that the climate, at aU events in the circumpolar and temperate zones of 

 the earth, would depend on whether the winter of a given region occurred 

 when the earth at its period of greatest excentricity was in aplielion or 

 perihelion — if the former, the annual average of temperature would be lower ; 

 if the latter, it would be higher than when the excentricity of the eartli's 

 orbit were less or approached more nearly to a circle. He calculates the 

 diffei'ence in the amount of heat at the period of maximum excentricity of the 



