Adaptability of Organisms 211 



progression is unbroken and minute in the last 

 degree. We can connect together into con- 

 tinuous series each minute variation, and each 

 species of a gradation of structure so insensible 

 that not a link in the chain of evidence is want- 

 ing. The bearing of this evidence upon the 

 question of continuity or discontinuity in evolu- 

 tion is of paramount importance. Nowhere has 

 evidence been collected so fully as in the case 

 of the white chalk; nowhere have such con- 

 clusive proofs of continuity in evolution been 

 established. 



Prof. W. B. Scott, referring to the evolution 

 of the existing species of horses, states that 

 in the Lower Tertiary deposits of North 

 America each one of the different Eocene and 

 Oligocene horizons has its characteristic genus 

 of horses, showing a slow, steady progress in 

 a definite direction. This series of fossils 

 points to the fact that existing species of horses 

 are derived from individuals less highly cap- 

 able of evading enemies and obtaining food; 

 that is, they point to progressive improvement 

 through long periods of time in the structural 

 arrangement of this species of animals. 



