ON THE PROGRESS OF SCIENCE. VII 



The old theories of geological convulsions and cataclysms by 

 which the inequalities of the earth's surface and the many breaks 

 in the geological record were explained, are now supplanted by 

 the modern view of Lvell and others, which refers the chan^-es in 

 the past to causes similar to those now in operation. With this, 

 since the researches of Darwin, has become connected the ques- 

 tion, whether, in a geological formation unmistakably continu- 

 ous, the different characters of the fossils represent absolutely 

 permanent varieties, or may be explained by gradual modifying 

 changes. It is quite possible that many modifications of size and 

 form, regarded as permanent, and on which specific diff"erences 

 have been assumed, may be due to changes in the conditions of 

 existence. The opponents of Darwin's theory have a strong 

 point in the fact that, with the i^resent knowledge of fossil forms, the 

 physical breaks in the strata make it impossible to fairly trace the 

 order of succession of organisms ; but, notwithstanding the imper- 

 fection of the geological record, the belief widely pi-e vails among 

 geologists that the succession of species bears a definite relation 

 to the succession of strata. 



Since Sir John Herschel, more than thirty years ago, proposed 

 to explain the climatal perturbations on the earth's surface, with 

 the attendant geological phenomena, by changes in the eccen- 

 tricity of the earth's orbit, cosmical studies have been more inti- 

 mately associated with geology. Mr. Croll has recently shown 

 reason to believe that the climate in the frigid and temperate 

 zones of the eai'th Avould depend on whether the winter of a given 

 region occurred when the earth at its period of greatest eccen- 

 tricity was in aphelion or perihelion — if the former, the annual 

 average of temperatare would be lower, — if the latter, it would 

 be higher than when the eccentricity of the eartli's orbit was less, 

 or approached more nearly to a circle — he calculates tlie differ- 

 ence in the amount of heat, in these two positions, as nineteen to 

 twenty-six. He thus explains the glacial, carboniferous or hot, 

 and the normal or temperate periods, which we observe in geo- 

 logical records; he estimates that it is certainly not less, and 

 pi-obably much moi-e, than one hundred thousand years since the 

 last glacial epoch. 



The progress of physiology during the last two years has been 

 great, ijrincipally owing to microscopical and chemical investiga- 

 tions. The discovery of development by cells, evincing a simple, 

 uniform law, underlying and working out the very different forms 



