ORGANIC EVOLUTION—-MACNAMARA, je 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 foie: 
together with the facility with which they can be cleared of their 
Gale 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 belem- 
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.? 
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.’”’* 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 
~ 1 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. 
