GREAT STEPS IN ORGANIC EVOLUTION 829 



is to be seen in the Peabody Museum at Yale. It is the oldest known 

 fossil footprint, and, so far as we know, it marks the adventure of 

 the first backboned animal upon the dry land his progeny were to 

 colonise. There are marks of two well-developed toes, of a budding 

 third, and a hint of a fourth ; and the whole imprint shows a close 

 resemblance to that of a stage in the development of a newt's foot. 



The freshwater fishes of the Devonian Period were either Lung- 

 fishes (Dipnoi) or well-armoured Ganoids, of which a few genera 

 still survive, especially of the Crossopterygian or "f ringed-fin" 

 order, such as the well-known fossil Holoptychius. The order has 

 representatives to-day in Polypterus and Calamoichthys, and it is 

 interesting to notice that these best of all point the way, and this 

 very distinctly, towards Amphibians. Thus the air-bladder of 

 Polypterus is a paired structure and is filled with dry air gulped in 

 at the surface of the stream; and although this quasi-lung lies 

 dorsally, it arises, as in all lunged animals, from an outgrowth on 

 the ventral surface of the gullet, with which its air tube (answering 

 to our trachea) remains connected. The larva of Polypterus is 

 remarkably tadpole-like, for it has feathery external gills, and also 

 a glandular attaching organ just behind the mouth — both very 

 familiar features of the young tadpole. But our present point is 

 simply that there were in the Devonian period various freshwater 

 fishes allied to Polypterus, which most zoologists regard as ancestral 

 to Amphibia, though a few still hold by the Dipnoans. Also character- 

 istic of freshwater sedimentary rocks of L^pper Silurian and 

 Devonian Age are certain very primitive limbless types included 

 with difficulty among fishes, such as Pterichthys and Pteraspis. 



Some of these Ostracoderms or Hypostomes, such as Pteraspis 

 and Cephalaspis, are so beautifully preserved in Norwegian and 

 Spitzbergen strata that there are indications of even the principal 

 nerves and blood-vessels of the head ! They were pioneer Vertebrates, 

 below the level of true fishes, probably more nearly related to our 

 hags and lampreys. 



The marine fishes of this Devonian Period were chieflj'' of the 

 shark type, and these were so dominant that the descriptive title 

 "The Age of Fishes", often applied to the Devonian Period, is well 

 justified by them, as well as by the numerous freshwater forms, 

 which seem to have been even more successfully progressive. In 

 connection with Devonian fishes it is of interest to recall Hugh 

 MDler's Old Red Sandstone, which was one of the rare instances of 

 a technical book winning a large audience by the force and pictur- 

 esqueness of its vividly visualising style. It dealt largely with the 

 author's discoveries of "Old Red" fishes of the Cephalaspid order 

 — pre-fishes as many have come to regard them ; and it was for one 

 of these strange types, limbless, sometimes jawless, that Agassiz, 

 proposed at the British Association Meeting in 1840, the technical 



