September 2, 1909] 



NATURE 



291 



of theii- smalltr representatives (the Microsauria) becoiiic 

 lizard-like, or even snake-like in form and habit ; and then 

 there suddenly arose the true reptiles. Still, these reptiles 

 did not iminediately replace the Stegoccphala in the 

 economy of Nature ; they remained quite secondary in 

 importance at least until the Upper Permian, 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 direc- 

 tions. Some of them were almost exactly like the little 

 Sphenodon, which still survives in some islands off New 

 Zealand, only retaining more traces of their marsh-dwelling 

 ancestors. The majority (the Anomodonts or Theroniorphs) 

 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, however, the new race did not imme- 

 diately replace the old, or exterminate it by unequal 

 competition. Reptiles held their own on all lands through- 

 out the Jurassic and Cretaceous periods, and it was not 

 until the Tertiary that mammals began to predominate. 



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, exhibiting 

 many features suggestive of the avian skeleton ; so it is 

 probable that this higher group also originated from an 

 intensely 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 response to their surroundings, or 

 degenerated, or gradually grew to a large size. There was 

 no transition, for instance, between 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 discovered, waiting to burst 

 forth in due time. During this retirement of the higher 

 race the reptiles themselves had enjoyed an extraordiriary 

 development and adaptation to every possible mode of life 

 in nearly all parts of the globe. We do not understand 

 the phenomenon — we cannot explain it; but it is as notice- 

 able in the geological history of fishes as in that of the 

 land animals just considered. It seems to have been first 

 clearly observed by the distinguished American naturalist, 

 the late Prof. Edward D. Cope, who termed the sudden 

 fundamental advances " expression points " and saw in 

 them a manifestation of some inscrutable inherent " bath- 

 ■mic 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 multi- 

 plied possibilities of adaptation to different modes of life. 

 In the first lung-breathers (Stegoccphala), 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. Several also assumed 

 a shape of body and limbs enabling them to live in the open 

 sea. Nearly all were carnivorous at first, and most of them 

 remained so to the end : but many of the Dinosaurs even- 

 tually became practically hoofed animals, with a sharp 

 beak for cropping herbage, and with powerful grinding 

 teeth. In none of these animals, however, were the toes 

 reduced to less than three in number, and in none of them 

 were the basal toe-bones fused together as they are in 

 cattle and deer. It is also noteworthy that the brain in 

 all of them remained very small and simple. In the final 

 grade of backboned life, that of the mammals, each of the 

 NO. 2079, VOL. Si] 



adaptive modifications just mentioned began to arise again 

 in a more nearly perfected manner, and now survival 

 depended not so much on an effective body as on a develop- 

 ing brain. The mammals began as little carnivorous or 

 mixed-feeding animals with a small brain and five toes, 

 and during the Tertiary period they gradually differentiateci 

 into the several familiar groups as we now known them, 

 eventually culminating in man. 



The demonstration by fossils that many aniinals of the 

 same general shape and habit have originated two or three 

 times, at two or three successive periods, from two or three 

 continually higher grades of life, is very interesting. To- 

 have proved, for example, that flying reptiles did not pass 

 into birds or bats, that hoofed Dinosaurs did not change 

 into hoofed mammals, and that Ichthyosaurs did not become 

 porpoises ; and to have shown that all these later animals 

 were mere mimics of their predecessors, originating in- 

 dependently from a higher yet generalised stock, is a re- 

 markable achievement. Still more significant, however, is 

 the discovery that towards the end of their career through 

 geological time totally different races of animals repeatedly 

 exhibit certain peculiar features, which can only be de- 

 scribed as infallible marks of old age. 



The growth to a relatively large size is one of these 

 marks, as we observe in the giant Pterodactyls of the 

 Cretaceous period, the colossal Dinosaurs of the Upper 

 Jurassic and Cretaceous, and the large mammals of the 

 Pleistocene and the present day. It is not, of course, all 

 the members of a race that increase in size ; some remain 

 small until the end, and they generally survive long after 

 the others are extinct ; but it is nevertheless a common, 

 rule that the prosperous and typical representatives are 

 successively larger and larger, as we see them in the 

 familiar cases of the horses and elephants of the northern 

 hemisphere, and the hoofed animals and armadillos of 

 South America. 



Another frequent mark of old age in races was first 

 discussed and clearly pointed out by the late Prof. C. E. 

 Beecher, of Yale. It is the tendency in all animals with 

 skeletons to produce a superfluity of dead matter, which 

 accumulates in the form of spines or bosses as soon as the • 

 race they represent has reached its prime and begins to be 

 on the down-grade. .Among familiar instances ma)' be men- 

 tioned the curiously spiny Graptolites at the end of the Silurian 

 period, the horned Pariasaurians at the beginning of the 

 Trias, the armour-plated and horned Dinosaurs at the end 

 of the Cretaceous, and the cattle or deer of modern Ter- 

 tiary times. The latter case — that of the deer — is specially 

 interesting, because fossils reveal practically all the stages 

 in the gradual development of the horns or antlers, from 

 the hornless condition of the Oligocene species, through the 

 simply forked small antlers of the Miocene species, to the 

 largest and most complex of all antlers seen in Ccrvus 

 scdgivicki from the Upper Pliocene and the Irish deer (C. 

 giganteiis) of still later times. The growth of these excres- 

 cences, both in relative size and complication, was con- 

 tinual and persistent until the clima.x was reached and the 

 extreme forms died out. At the same time, although the 

 pateontologist must regard this as a natural and normal 

 phenomenon not directly correlated with the habits of the 

 race of animals in which it occurs, and although he does 

 not agree with the oft-repeated statement that deer may 

 have " perfected " their antlers through the survival of 

 those individuals which could fight most effectively, there 

 mav nevertheless be some truth in the idea that the growths 

 originally began where the head was subject to irritating 

 impacts, and that they so happened to become of utility. 

 Fossils merely prove that such skeletal outgrowths appear 

 over and over again in the prime and approaching old age 

 of races ; they can suggest no reasons for the particular 

 positions and' shapes these outgrowths assume in each 

 species of animal. 



It appears, indeed, that when some part of an animal 

 (whether an excrescence or a normal structure) bcgan_ to 

 grow relatively large in successive generations during 

 geological time, it often acquired some mysterious impetus 

 by which it continued to increase long after it had reached ' 

 the serviceable limit. The unwieldy antlers of the extinct 

 Sedgwick's deer and Irish deer just mentioned, for example, 

 must have been impediments rather than useful weapons. 

 The excessive enlargement of the upper canine teeth in the 

 so-called sabre-toothed tigers (Machsrodus and its allies) ' 



