464 TRANSACTIONS OF SECTION C. 
cance. In other words, there was a great outburst of Eurypterid life just 
at the time when backboned animals arose; and if some of the former weré 
actually transformed into the latter, the phenomenon took place when their 
powers both of variation and of multiplication were at their maximum. 
Fishes were already well established and distributed over perhaps the 
greater p»rt of the northern hemisphere at the beginning of Devonian 
times ; ani 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 Crosso- 
pterygians and Dipnoans) have also an air-bladder, which might readily 
become a lung. The characteristic fish-fauna of the Devonian period, there- 
fore, made a nearer approach to the land animals than any group of fishes 
of later date ; and it is noteworthy that in the Lower Carboniferous of Scot- 
land—perhaps even in the Upper Devonian of North America, if footprints 
can be trusted—amphibians first appeared. In Upper Carboniferous times 
they became fizmly 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 Labyrinthodonts, as these primitive amphibians are 
termed, were therefore a vigorous race; but the marsh-dwelling habits of 
the majority did not allow of much variation from the salamander pattern. 
Only in Upper Carboniferous and Lower Permian times did some of their 
smaller representatives (the Microsauria) become lizard-like, or even snake- 
like in form and habit; and then there suddenly arose the true reptiles. 
Still, these reptiles did not immediately replace the Stegocephala 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 directions. 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 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, however, 
the new race did not immediately replace the old, or exterminate it by 
unequal competition. Reptiles held their own on all lands throughout the 
Jurassic and Cretaceous periods, and it was not until the Tertiary that 
mammals hegan 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 
