40 IOWA GEOLOGICAL SURVEY 



erly applied), had been proposed by Lamarck in France a score 

 of years earlier, only to be ignominiously rejected. All edu- 

 cated readers are familiar with the example of Darwin's per- 

 sistent, long-continued striving for the truth, how at first it was 

 dimly perceived and at length fully revealed to him after making 

 the most wonderful collection of illuminating and explaining 

 facts which had ever been assembled in biology by any single 

 investigator, and how with utmost intellectual candor he tested 

 it, as we are told, "by applying to it successively fact after fact, 

 group after group, and category after category of facts, until 

 he convinced himself of the theory's consonance with all this vast 

 array of observed biological actuality." * Thanks in part to his 

 masterly presentation of the theory, it gained almost imme- 

 diately a wide acceptance, and is now held to be as thoroughly 

 demonstrated a part of natural science as is Newton's law of 

 gravitation in physics, or the heliocentric system in cosmography. 

 Palaeontology in particular received a profound stimulus under 

 the influence of evolutionary ideas, and its whole aspect, method 

 and outlook were revolutionized in consequence. It is now uni- 

 versally admitted that "the facts revealed by the study of 

 palaeontology are explicable wholly satisfactorily by the theory 

 of descent and in no single instance do they contradict it." Con- 

 sider for a moment what this means. Naturalists are acquainted 

 nowadays with about 400,000 species of living animals and half 

 as many species of existing plants. A computation based on the 

 number of new species being found and described from year to 

 year, and the extent of biologically unexplored areas of the 

 earth's surface, shows that the total number of species constitut- 

 ing the modern fauna must number at least several millions. For 

 the insects alone, entomologists hold that a total of two million 

 species is not an excessive estimate. And these all belong to 

 but a single geological epoch, the present. But in the case of 

 extinct speeies, "those hosts of strange denizens of our changing 

 earth in the ages gone," it is evident that the variety of forms 

 preserved for us in the rocks is but an insignificant fraction of 



* See the interesting recent work by Professor V. L. Kellogg, "Darwinism To- 

 day", and his joint production with President Jordan, "Evolution and Animal 

 Life", 1907. 



