142 THE GENESIS OF SPECIES. [CHAP. 



CHAPTER VI. 



SPECIES AND TIME. 



Two Relations of Species to Time. No Evidence of Past Existence of Minutely- 

 intermediate Forms when such might be expected a priori. Bats, Pterodac- 

 tyls, Dinosauria, and Birds. Ichthyosauria, Chelonia, and Anoura. Horse An- 

 cestry. Labyrinthodonts and Trilobites. Two Subdivisions of the Second Eela- 

 tion of Species to Time. Sir William Thomson's Views. Probable Period re- 

 quired for Ultimate Specific Evolution from Primitive Ancestral Forms. Geo- 

 metrical Increase of Time required for Eapidly-multiplying Increase of Structural 

 Differences. Proboscis Monkey. Time required for Deposition of Strata neces- 

 sary for Darwinian Evolution. High Organization of Silurian Forms of Life. 

 Absence of Fossils in Oldest Eocks. Summary and Conclusion. 



Two considerations present themselves with regard to 

 the necessary relation of species to time if the theory of 

 "Natural Selection" is valid and sufficient. 



The first is with regard to the evidences of the past ex- 

 istence of intermediate forms, their duration and succession. 



The second is with regard to the total amount of time 

 required for the evolution of all organic forms from a few 

 original ones, and the bearing of other sciences on this 

 question of time. 



As to the first consideration, evidence is as yet against 

 the modification of species by " Natural Selection " alone, 

 because not only are minutely transitional forms generally 

 absent, but they are absent in cases where w T e might cer- 

 tainly a priori have expected them to be present. 



Now it has been said : 1 "If Mr. Darwin's theory be 

 true, the number of varieties differing one from another a 



1 North British Review, New Series, vol. vii., March, 1867, p. 317. 



