XIX.] WHAT EVOLUTION MEANS : ORGANIC EVOLUTION. 155 



the first air-breathers ; the wing of a Cockroach, and several entire and 

 undoubted Scorpions ! Thus in addition we get vertebrates as 

 opposed to invertebrates, and the -first traces of the fishes. In the 

 advance to the Devonian the fishes (associated with giant Crustacea) 

 predominate ; it has been called the age of fishes. In the next series, 

 the Carboniferous, we find the first certain traces of amphibians, of 

 which the early existence is like that of a fish : a state of things illus- 

 trated by the frog, which the majority of us in our early days have, 

 I am sure, studied as a tadpole in its early stages ; and some of these 

 amphibians still retain fish-like characters. It is not until we arrive 

 at the Permian that the true reptiles are met with, but in the next 

 great series, the Triassic, we meet with a remarkable evolutionary 

 group of Keptiles, the Theriodontia, or beast-toothed animals, because 

 (unique among reptiles) they possess a dentition like a dog or a lion, 

 with incisors, canines and cheek-teeth ; the precursors, doubtless, of the 

 succeeding mammalian type. We pass easily thus from the reptiles 

 to mammals which are related to them ; for instance, the ornitho- 

 rhyrichus and the echidna are both Australian mammals which bring 

 forth their young within the egg as do the reptiles. After that we 

 begin to deal with birds. The early birds were strikingly reptilian in 

 some of their characters ; and the pterodactyle, remains of which exist 

 in many museums, was really a winged reptile and not a bird. From 

 that we gather that mammals and birds are variants of reptiles. When 

 we progress from the Jurassic to the Recent, we find man making his 

 appearance as a direct descendant of all those early forms. 



When we come to study the life-history of the various forms 

 brought before us by the geological beds, we find it to vary consider- 

 ably, a fact indicated by the presence or absence of the different 

 genera in the various strata. We find that the trilobites, for instance, 

 only appear in the very early geological formations ; there is no trace of 

 them in the recent, but of the annelids and Brachiopods we note that they 

 are continuous from the earliest to the latest formations ; we still have 

 our worms. Again we learn that certain other organic forms made 

 their appearance very low down in the time scale, forms which were 

 not represented at all in the earlier Cambrian and Silurian, and that 

 some of these are continuous to the present day. 



Let us take the story of the fishes. A great many fishes made 

 their appearance at the Devonian stage, there were few in the Silurian ; 

 some of these stopped there, whereas others have been continued from 

 the Devonian times to our own. Take, for instance, the Australian 

 mudfish Ceratodus ; to judge from the teeth this fish might well have 

 lived on unchanged from late Palaeozoic times until the present day ! 

 We see there is a tremendous variation of possible life-range, so to 



