324 



SCIENCE 



[N. S. Vol. XXX. No. 767 



these reptiles did not immediately replace 

 the Stegocephala in the economy of na- 

 ture; they remained quite secondary in 

 importance at least until the Upper Per- 

 mian, in most parts even until the dawn of 

 the Triassic period. Then they began their 

 flourishing career. 



At this time the reptiles rapidly diverged 

 in two directions. Some of them were al- 

 most exactly like the little Sphenodon, 

 which stiU survives in some islands off New 

 Zealand, only retaining more traces of their 

 marsh-dwelling ancestors. The majority 

 (the Anomodonts or Theromorphs) very 

 quickly became so closely similar to the 

 mammals that they can only be interpreted 

 as indicating an intense struggle towards 

 the attainment of the higher warm-blooded 

 grade; and there is not much doubt that 

 true mammals actually arose about the end 

 of the Triassic period. Here, again, how- 

 ever, the new race did not immediately re- 

 place the old, or exterminate it by un- 

 equal competition. Reptiles held their own 

 on all lands throughout the Jurassic and 

 Cretaceous periods, and it was not until 

 the Tertiary that mammals began to pre- 

 dominate. 



As to the beginning of the birds, it can 

 only be said that towards the end of the 

 Triassic period there arose a race of small 

 Dinosaurs of the lightest possible build, ex- 

 hibiting many features suggestive of the 

 avian skeleton; so it is probable that this 

 higher group also originated from an in- 

 tensely restless early community of reptiles, 

 in which all the variations were more or 

 less in the right direction for advancement. 



In short, it is evident that the progress of 

 the backboned land animals during the 

 successive periods of geological time has 

 not been uniform and gradual, but has 

 proceeded in a rhythmic manner. There 

 have been alternations of restless episodes 

 which meant real advance, with periods of 

 comparative stability, during which the 



predominant animals merely varied in re- 

 sponse to their surroundings, or degener- 

 ated, or gradually grew to a large size. 

 There was no transition, for instance, be- 

 tween the reptiles of the Cretaceous period 

 and the mammals which immediately took 

 their place in the succeeding Eocene 

 period; those mammals, as we have seen, 

 had actually originated long ages before, 

 and had remained practically dormant in 

 some region which we have not yet discov- 

 ered, waiting to burst forth in due time. 

 During this retirement of the higher race 

 the reptiles themselves had enjoyed an ex- 

 traordinary development and adaptation 

 to every possible mode of life in nearly all 

 parts of the globe. We do not understand 

 the phenomenon — we can not explain it; 

 but it is as noticeable in the geological his- 

 tory of fishes as in that of the land animals, 

 just considered. It seems to have been first 

 clearly observed by the distinguished Amer- 

 ican naturalist, the late Professor Edward 

 D. Cope, who termed the sudden funda- 

 mental advances "expression points" and 

 saw in them a manifestation of some in- 

 scrutable inherent "bathmic force." 



Perhaps the most striking feature to be 

 noticed in each of these ' ' expression 

 points" is the definite establishment of 

 some important structural character which 

 had been imperfect or variable before, thus 

 affording new and multiplied possibilities 

 of adaptation to different modes of life. 

 In the first lung-breathers (Stegocephala), 

 for example, the indefinite paddle of the 

 mud fishes became the definite five-toed 

 limb; while the incomplete backbone 

 reached completeness. Still these animals 

 must have been confined almost entirely 

 to marshes, and they seem to have been all 

 carnivorous. In the next grade, that of 

 the reptiles, it became possible to leave the 

 marshes; and some of them were soon 

 adapted not only for life on hard ground 

 or in forests, but even for flight in the air. 



