THE OLDEST AIR-BREATHERS 303 



occur in the lower Carboniferous beds below the marine lime- 

 stones, in the middle coal measures, and in the upper coal 

 formation, separated by a thickness of beds which may be 

 estimated at 15,000 feet, and certainly representing a vast lapse 

 of time. Did we know the creature by these impressions 

 alone, we might infer its continued existence for all this great 

 length of time ; but when we also find its bones in the princi- 

 pal repositories of reptile remains, and in company with the 

 other creatures found with it, we satisfy ourselves that of them 

 all it was the most likely to have left its trail in the mud flats. 

 We thus have reason to conclude that it existed alone during 

 this period, in so far as its especial kind of habitat was con- 

 cerned ; though there lived with it other reptiles, some of 

 which, haunting principally the woods, and others the water, 

 were less likely to leave impressions of their footprints. These 

 may be but slight indications of truth, but they convey strong 

 impressions of the persistence of species, and also of the pau- 

 city of species belonging to these tribes at the time. 



If we could affirm that the Air-breathers of the coal period 

 were really the first species of their families, they might acquire 

 additional interest by their bearing on this question of origin 

 of species. We cannot affirm this ; but it may be a harmless 

 and not uninstructive play of fancy to suppose for a moment 

 that they actually are so, and to inquire on this supposition as 

 to the mode of their introduction. Looking at them from this 

 point of view, we shall first be struck with the fact that they 

 belong to all of the three great leading types of animals which 

 include our modern Air-breathers the Vertebrates, the Arthro- 

 pods, and the Mollusks. We have besides to consider in this 

 connection that the breathing organs of an insect are air tubes 

 opening laterally (trachece), those of a land snail merely a 

 modification of the chamber which in marine species holds the 

 gills, while those of the reptiles represent the air bladder of the 

 fishes. Thus, in the three groups the breathing organs are 



