September 10, 1909] 



SCIENCE 



323 



will be much easier than it is at present to 

 decide where missing links in any particu- 

 lar case are most likely to be found. 



Among these general features which 

 have been made clear by the latest system- 

 atic researches, I wish especially to em- 

 phasize the interest and significance of the 

 persistent progi-ess of life to a higher plane, 

 which we observe during the successive 

 geological periods. For I think paleontol- 

 ogists are now generally agreed that there 

 is some principle underlying this progress 

 much more fundamental than chance- 

 variation or response to enviromnent, how- 

 ever much these phenomena may have con- 

 tributed to certain minor adaptations. 

 Consider the case of the backboned animals, 

 for instance, which I happen to have had 

 special opportunities of studying. 



"We are not likely ever to discover the 

 actual ancestors of animals on the back- 

 boned plan, because they do not seem to 

 have acquired any hard skeleton until the 

 latter part of the Silurian period, when 

 fossils prove them to have been typical and 

 fully developed, though low in the back- 

 boned scale. The ingenious researches and 

 reasoning of Dr. W. H. Gaskell, however, 

 have suggested the possibility that these 

 animals originated from some early rela- 

 tives of the scorpions and crustaceans. It 

 is therefore of great interest to observe that 

 the Eurypterids and their allies, which oc- 

 cupy this zoological position, were most 

 abundant during the Silurian period, were 

 represented by species of the largest size 

 immediately afterwards at the beginning 

 of the Devonian, and then gradually 

 dwindled into insignificance. In other 

 words, there was a great outburst of 

 Eurypterid life just at the time when back- 

 boned animals arose; and if some of the 

 former were actually transformed into the 

 latter, the phenomenon took place when 

 their powers both of variation and of mul- 

 tiplication were at their maximum. 



Pishes were already well established and 

 distributed over perhaps the greater part 

 of the northern hemisphere at the begin- 

 ning of Devonian times; and then there 

 began suddenly a remarkable impulse 

 towards the production of lung-breathers, 

 which is noticeable not only in Europe and 

 North America, but also probably so far 

 away as Australia. In the middle and 

 latter part of the Devonian period, most of 

 the true fishes had paddles, making them 

 crawlers as much as swimmers; many of 

 them differed from typical fishes, while 

 agreeing with lung-breathers, in having the 

 basis of the upper jaw fused with the 

 skull, not suspended; and some of them 

 exhibited both these features. Their few 

 survivors at the present day (the Crossop- 

 terygians and Dipnoans) have also an 

 air-bladder, which might readily become a 

 lung. The characteristic fish-fauna of the 

 Devonian period, therefore, made a nearer 

 approach to the land animals than any 

 group of fishes of later date ; and it is note- 

 worthy that in the Lower Carboniferous of 

 Scotland— perhaps even in the Upper De- 

 vonian of North America, if footprints can 

 be trusted — amphibians first appeared. 

 In Upper Carboniferous times they became 

 firmly established, and between that period 

 and the Trias they seem to have spread all 

 over the world; their remains having been 

 found, indeed, in Europe, Spitzbergen, 

 India, South Africa, North and South 

 America and Australia. 



The Stegocephala or Labyi-inthodonts, as 

 these primitive amphibians are termed, 

 were therefore a vigorous race; but the 

 marish-dwelling habits of the majority did 

 not allow of much variation from the sala- 

 mander-pattern. Only in Upper Carbonif- 

 ei'ous and Lower Permian times did some 

 of their smaller representatives (the Micro- 

 sauria) become lizard-like, or even snake- 

 like, in form and habit; and then there 

 suddenly arose the true reptiles. Still, 



