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THE BRITISH NATURALIST. 



[March 



length, came the commencement of what we call the carboniferous 

 epoch. 



During this period there were large tracts of land covered by a vege- 

 tation more luxuriant than any that has since existed ; and this excessive 

 growth is attributable to the presence of a relatively large proportion of 

 carbonic acid in the air which stimulated vegetable life but did not 

 permit the existence of the higher terrestrial animals. The general effect 

 of this more than tropically luxuriant vegetation would be to diminish the 

 proportion of carbonic acid in the air, and as a consequence we find 

 towards the close of this epoch, that the field thus laid open was speedily 

 occupied by lung-breathing vertebrates, adaptations for terrestrial life of 

 what had previously been an exclusively gill-breathing order of animals. 



The subsequent geological record supports that theory of the line of 

 descent of man from this earliest lung-breather which has been already 

 indicated. Some may ask : What has all this to do with animals breathing ? 

 I would reply with one of old : " Much every way." At the beginning of 

 this digression I protested against the unscientific notion that some 

 animals higher than ourselves should succeed us. Palaeontology teaches 

 us that as the earth has become fit for higher animals ; higher animals 

 have been developed. All animals, even the lowest, breathe ; but the 

 simpler forms of animal life can exist and even flourish where the higher 

 ones would die. It would be much more scientific, that is it would be 

 more in accordance with what we know as distinct from what we only 

 think, to say, that as our globe cools down it will gradually become 

 unfit for the higher forms of life, and that instead of being succeeded by 

 higher forms there will be a time when, owing to the steadily changing 

 conditions, the higher animals will gradually cease to exist, and instead 

 of a tendency to increased complexity amongst living organisms the 

 tendency will be in the opposite direction. Excluding from our consider- 

 ation any idea of a catastrophic end of the earth and its inhabitants, and 

 assuming only the continuous operation of existing forces, we may safely 

 say that the great back-boned family, of which we are the highest mem- 

 bers, has already existed for much more than half of its allotted time; 

 millions of years have elapsed since the first vertebrate appeared, but it is 

 extremely doubtful if vertebrate life will be possible on earth as far in the 

 future as the coal period is in the past. Just as some fishes live at the 

 bottom of the sea, we are living at the bottom of a great sea of air ; a 

 very slight change in the composition of either sea would suffice to an- 

 nihilate the animals which inhabit it. We have reason to believe that 

 the composition of both seas has varied in the past, and that their 

 inhabitants have exhibited corresponding variations, those which failed 

 to adapt themselves to their altered surroundings perishing. As in the 

 past many race 5 of animals have become extinct, so, at last, all now 

 existing will die out. Instead of idly dreaming of imaginary higher forms 



