74 SCIENCE PROGRESS 



which are formed when explosions or fires occur in mines, 

 neither affects the sense organs nor produces pain, miners may 

 remain unaware of its existence and so do nothing to avoid 

 it, until they suddenly succum-b. Had they only with them a 

 mouse or a sparrow in a cage, forming as much a part of their 

 equipment as a safety lamp, they would have sufficient time to 

 escape from a place which is dangerous, by leaving as soon 

 as the animal showed symptoms, long before they themselves 

 had absorbed a sufficient quantity to be incapacitated. If they 

 are quick the animal will live to aid them in finding a safe place 

 of retreat. As it takes fourteen to fifteen times as long when 

 at rest and seven to eight times as long when at work, for a 

 man to be disabled as for a mouse, the miner, even if working, 

 would have one or two hours for escape with such percentages 

 of carbon-monoxide in the air as usually occur in mines (14). 



The frequency of beat, as we have seen, has not become 

 adapted by itself to regulate the supply of oxygen to the 

 demands of the different warm-blooded animals, but other 

 factors also play their part. We have shown that of these 

 the principal one is the volume of blood expelled per beat. 

 We have now to inquire what significance is to be attached 

 to the fact that now the one and now the other of the two 

 main regulating factors plays the more important part. 



Parrot's observations on the relative heart-weights of over 

 fifty different species of birds and those others of birds and 

 mammals referred to in our tables show that the relatively 

 large heart is found in the more active animals. This is so 

 not only in warm-blooded animals but also, as we have already 

 noticed, in fish, flat fish having a relative heart-weight less 

 than half that of more active fish. It is probably also the 

 case in amphibians and reptiles, although we have not yet 

 the determining data ; for, however small the demand for 

 oxygen, all animals when active must consume more than when 

 at rest and those that are habitually active must have some 

 means of obtaining more. Moreover, since a large heart works 

 more economically than a small one in that it spends less of its 

 time in overcoming inertia, it would for that reason also be 

 favoured when much work has to be done. The range of 

 variation in relative heart-size is fairly large in all species 

 in which it has been determined in several specimens, but more 

 so in some species than in others. Thus in four specimens 



