THE DEVONIAN AGE. 101 



and its possession of gill -cleaners, correspond with 

 this view. It is possible that the Devonian fishes 

 possessed this semi- reptilian respiration; and if so, 

 they would be better adapted than other fishes tc 

 live in water contaminated with organic matter in a 

 state of decay, or in waters rich in carbonic acid oj 

 deficient in oxygen. Possibly the palaeozoic waters, 

 as well as the palaeozoic atmosphere, were less rich in 

 pure oxygen than those of the present world; and it 

 is certain that, in many of the beds in which the 

 smaller Devonian fishes abound, there was so much 

 decaying vegetable matter as to make it probable 

 that the water was unfit for the ordinary fishes. 

 Thus, though at first sight the possession of external 

 armour and means to respire air, in the case of these 

 peculiar fishes, may seem to have no direct connection 

 with each other, their obvious correlation in some 

 modern ganoids may have had its parallel on a more 

 extensive scale among their ancient relatives. Just 

 as the modern gar-fish, by virtue of its lungs, can 

 live in stagnant shallows and hunt frogs, but on that 

 account needs strong armour to defend it against the 

 foes that assail it in such places; so in the Devonian 

 the capacity to inhabit unaerated water and defensive 

 plates and scales may have been alike necessary, 

 especially to the feebler tribes of fishes. We shall 

 find that in the succeeding carboniferous period there 

 is equally good evidence of this. 



We have reserved little space for the Devonian 

 plants and insects ; but we may notice both in a walk 



