CHAPTER FORTY-SIX 



Discovery 

 of the land 

 by verte- 

 brates 



Amphibians 



AMPHIBIANS 



1. IN Palaeozoic times, many millions of years ago, 

 certain vertebrates learned to live upon the land. Al- 

 ready the land was populated with many kinds of plants 

 and multitudes of insects. A vertebrate, entering upon 

 aerial existence, was necessarily subject to certain dis- 

 advantages, especially the chance of desiccation. This 

 danger has not yet ceased to menace the lowest land 

 vertebrates. Thus, on one occasion, it was noticed that 

 little toads were leaving a roadside ditch in which they 

 had lived as tadpoles, and, trying to cross the road, were 

 perrshing in great numbers in the thick dust. The tran- 

 sition from water to land could not be abrupt. The 

 eggs were still laid in the water, and the young stages 

 passed therein ; but emergence on the land opened up a 

 great new territory, with warmth and food in abundance. 

 On land, gills were no longer suitable for breathing, and 

 so their place was taken by internal sacs that is to 

 say, by lungs. The lungfishes already have such organs, 

 so here the transition must have been gradual ; indeed, 

 it could otherwise hardly have taken place. We know 

 least about the origin of the legs for terrestrial locomo- 

 tion, with typically five digits. They are certainly 

 modified from the paired fins of fishes, but the earliest 

 known four-footed animals have passed beyond the 

 transition stage. 



2. The first land vertebrates, arising in some such 

 manner, were amphibians. The word (from the Greek, 

 meaning "both" and "life") has reference to the life 

 both in water and on land, to the metamorphosis from 

 the tadpole to the frog or toad. As a matter of fact, this 

 metamorphosis does not always occur ; many species 



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