GREAT STEPS IN ORGANIC EVOLUTION 837 



In a sandstone slab dating from the end of the Old Red Sandstone 

 or Devonian Period there is a footprint with three distinct toe-marks 

 and a hint of a fourth — the earliest record of a terrestrial backboned 

 animal. It is one of the treasures of the Museum at Yale, and the 

 creature that made it was called Thinopns antiquus. But while this 

 footprint is eloquent, it does not carry us very far; and it is satis- 

 factory to find in Carboniferous strata many fossil Amphibians. 

 Some were small and newt-like, others huge and heavily armoured. 

 Some wallowed and others crawled sluggishly, and others, having 

 lost their limbs, could only swim. It was a Golden Age of Amphibians ; 

 and our imagination cannot but be thrilled when we look at these 

 Carboniferous skulls and skeletons, the remains of animals that 

 were the first to have fingers and toes, true lungs, a three-chambered 

 heart, a movable tongue, a drum to the ear, and a voice breaking 

 the silence of Nature. There are distinct fossilised traces of a gill 

 apparatus in some of them; and the eggs were doubtless laid in the 

 water, as is the case with most Amphibians to-day. But perhaps the 

 biggest fact is that these ancient Amphibians, following in the foot- 

 steps of Thinopus, were the first vertebrate animals to invade the 

 dry land with success. They made the higher animals possible, for 

 they gave origin, probably in the Carboniferous Period, to Reptiles, 

 whence in the course of time there emerged both Birds and Mammals. 



Permian. — ^The easygoing life of the Carboniferous was certainly 

 not uninterrupted, yet on the whole it was a period of luxuriance as 

 compared with the Permian, during which the life struggle became 

 sterner. In some areas there was uplift and aridity, making it more 

 difficult for the progressive Amphibians to go back to the waters 

 to spawn; and this must have proved a spur to the evolution of 

 terrestrial Reptiles and their foetal membranes. In other areas, 

 especially in the Southern Hemisphere, there was widespread 

 glaciation, even more severe than in the Pleistocene Ice Ages, and 

 this involved an elimination of many of the older spore-bearing 

 plants, e.g. among the ferns and tree-ferns and horsetails. Hardier 

 seed-bearing types began to flourish — such as Conifers, Cycads, and 

 Gingkos. The Lower Permian plants are largely spore-bearers, like 

 those of the Carboniferous Period; the Upper Permian plants show 

 an increasing number of seed-plants. In technical language, the 

 change from the "Palaeophyticum" to the "Mesophyticum" took 

 place in the middle of the Permian Formation. Zoologically we think 

 of the Permian Period as the time of the rise of reptiles and the 

 beginning of a physiological movement towards warm-bloodedness. 

 Associated with the Permian Ice Ages, there was probably an inter- 

 polation of pupa or chrysalid phases in the life-history of many 

 insect types. 

 Triassic. — There seems to have been a prevalence of desert 



