ORGANIC EVOLUTION — MACNAMARA. 367 



an almost unique field for observation, on account of its thickness 

 (considerably over 1,000 feet), its slow, uniform, and continuous 

 deposit in a sea of moderate depth, with no closely adjacent land, 

 the abundance and wonderful state of preservation of its fossils, 

 together with the facility with which they can be cleared of their 

 chalky covering." ' 



Among the most common chalk fossils is the flattened, heart- 

 shaped sea-urchin. These* are first found in their shelled, sparsely 

 ornamented forms, from which spring, as we ascend the zone, all 

 the other species of the genus. The progression is unbroken and 

 minute in the last degree. We can connect together into continuous 

 series each minute variation and each species of gradation of struc- 

 ture so insensible that not a link in the chain of evidence is wanting. 

 In the other common sea-urchin of the chalk, although evidence 

 derived from the details of structure is not equally available, that 

 afforded by the gradual variation in shape as we ascend through 

 the zones of formation is convincing and complete. Equally clear 

 proof of continuous evolution is provided by the study of the beiem- 

 nite Actinocamax. Although this genus reaches at definite zonal 

 levels a sufficiently accentuated degree of variation in its intrinsic 

 character to warrant, for purely stratigraphical purposes, the use of 

 trivial titles, the fact remains that these so-called species are but 

 landmarks in the progressive and unbroken evolution of a single 

 though somewhat plastic genus. The bearing of this evidence upon 

 the question of continuity or discontinuity in evolution is of para- 

 mount importance. Nowhere has evidence been collected so fully as 

 in the case of the white chalk; nowhere have such conclusive proofs 

 of continuity in evolution been established. 2 



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, all parts of the structure participating in the 

 advance — which, it should be emphasized, the changes are gradual 

 and uninterrupted." 3 This series of fossils points to the fact that 

 existing species of horses are derived from individuals less highly 

 capable of evading enemies, and obtaining food ; that is, they point 

 to progressive improvement through long periods of time in structural 

 arrangement of this species of animals. 



Prof. E. B. Poulton was much impressed by the series of mammalian 

 skulls from the Lower Tertiary beds of North America, arranged in 



' Darwin and his Modern Critics, by E. B. Poulton. The Quarterly Review, July 1909, p. 19. 

 2 Prof. E. B. Poulton. The Quarterly Review, p. 20, July, 1909. 

 8 The Cambridge Darwin Memorial Volume, p. 190. 



