Chapter 5 

 AMPHIBIA 



The Amphibia, evolving from some fish ancestor, discovered 

 the land. A footprint in the Upper Devonian rocks of Penn- 

 sylvania indicates the remoteness of their origin, but their actual 

 beginnings may have been considerably earlier. They were not, 

 of course, the first land animals; already the Arachnida, scorpions 

 and mites, had established themselves on land surfaces, and were 

 probably abundant. There were also primitive myriapods. Hirst 

 has recently (1923) made known a series of remarkable Arachnida 

 from the Old Red Sandstone, showing that numerous and diver- 

 sified types existed, at a time when, so far as we know, the insects 

 had not come into existence. We may infer that this extensive 

 arachnid fauna furnished the food of the early amphibians, and 

 conditioned their development much as the insects in the Mesozoic 

 apparently conditioned that of our mammalian ancestors. 



Although we speak of the amphibians as land animals, they 

 have a double life, as the name indicates. The early aquatic 

 stage departs little from the fish type, and each individual, as it 

 develops the terrestrial form, may be said to climb its own family 

 tree. But this course of development is not always carried out, 

 some kinds being permanently aquatic. The Tiger Salamander 

 (Ambystoma tigrinum of Green), very common in Colorado, is 

 when adult spotted and blotched with yellow on a black or nearly 

 black ground. It may be found under rocks in damp places. 

 Its aquatic phase, commonly known as the water-dog, may be 

 found in lakes and reservoirs. It is without the yellow markings, 

 and has large tufted external gills on each side of the throat. This 

 is the famous "axolotl", long ago observed in Mexico, and thought 

 to be an adult animal. It does, in fact, under certain conditions 

 reproduce without ever reaching the typical adult condition, and 

 it is easy to imagine that the latter might in the course of time 

 cease to exist. Thus, it has been argued that presumably the 

 permanently aquatic genera of amphibians are not primitive, but 

 derived forms which once had terrestrial ancestors. They are 

 like some flowering plants which no longer flower, but reproduce 

 entirely by tubers or in some other way. The name axolotl has 

 an Aztec form, and according to Gadow means "play in the water". 



Ill 



