NATURAL SELECTION 119 



They are far behind (I regret to say it of them) the more 

 intelligent mammals. Their cleverness is mainly of the nature 

 of instinct ; only to a very limited extent are flight and nest- 

 building acquired accomplishments. The young house-martin 

 (Chelidon urbica) plucks up courage, trusts himself to the sea 

 of air and finds with delight that he is able to fly though he has 

 never learnt. 



True, this instinctive accomplishment requires perfecting, 

 there must be intelligence working on the instinctive basis. 

 The young swallow, though quite capable of flight from the 

 moment of the first plunge, has to learn by practice all the 

 niceties of muscular co-ordination, all the subtleties of adjust- 

 ment of his wings to sudden gusts and all the quick turns that 

 are necessary if he is not to let half the gnats escape. He learns 

 a good deal, yet his versatility and intelligence are far below 

 what we find in those mammals whose play is varied and 

 prolonged. 



I wish now to discuss parenthetically an interesting question to Notending 

 which the subject in hand draws our attention ; why do " water- Q ^ 

 breathing " animals, those that for respiration make use of the among 

 oxygen dissolved in water, all allow the direct and pitiless in- b ' ( 

 cidence of Natural Selection upon their young ? It is, I believe, 

 because they have never risen to a high enough pitch of vitality 

 to permit the adoption of a better system. The only warm- 

 blooded animals in the sea, such as whales and porpoises, are 

 not, properly speaking, children of the waters, for in order to 

 breathe they rise at intervals to the surface. But if in the true 

 children of the sea and river the pulsation of life is weaker, 

 why is it so ? Here the answer is not difficult. The oxygen 

 at their disposal is so limited in amount that their breathing is 

 comparatively ineffective. According to M. Milne Edwards 

 from T \ to ^j of the volume of sea water consists of air, and 

 though as much as 32 or even 39 per cent, of this is oxygen, yet 

 the amount available for a fish to oxidise his blood with is 

 obviously very small. The deficiency of sunlight and the con- 

 sequent poverty of vegetable life may also be hindrances, but 

 the scarcity of oxygen must rank before them. The whales and 



