MULTIPLICATION OF THE HUMjVN RACE. 491 



1 ndivlduation and Genesis which different types of organisms 

 show us. Again, there may be a functional change of pro- 

 portion. If the engine has to draw a considerable load, the 

 abstraction of steam by the cylinders greatly reduces the 

 discharge by the safety-valve ; and if a high velocity is kept 

 up, the discharge from the safety-valve entirely ceases. Con- 

 versely, if the velocity is low, the escape-steam bears a large 

 ratio to the steam consumed by the motor apparatus ; and if 

 the engine becomes stationary the whole of the steam escapes 

 by the safety-valve. This inverse variation answers to that 

 which we have traced between Expenditure and Genesis, as 

 displayed in the contrasts between species of the same type 

 but unlike activities, and in the contrasts between active and 

 inactive individuals of the same species. But now beyond 

 these inverse variations between the quantities of consumed 

 steam and escape-steam, that are structurally and functionallj' 

 caused, there are coincident variations, producible in both by 

 changes in the quantity of steam supplied — changes that 

 may be caused in several ways. In the first place, the fuel 

 thrown into the furnace may be increased or made better. 

 Other things equal, there will result a more active locomo- 

 tion as well as a greater escape ; and this will answer to that 

 simultaneous addition to its individual vigour and its repro- 

 ductive activity, caused in an animal by a larger quantity, or 

 a superior quality, of food. In the second place, the steam 

 generated may be economized. Loss by radiation from the 

 boiler may be lessened by a covering of non-conducting sub- 

 stances ; and part of the steam thus prevented from con- 

 densing, will go to increase the working power of the engine, 

 while part will be added to the quantity blowing off. Thia 

 variation corresponds to that simultaneous addition ^o bodily 

 vigour and propagative power, which results in animals that 

 have to expend less in keeping up their temperatures. In 

 the third place, by improvement of the steam-generating 

 apparatus, more steam may be obtained from a given weight 

 of fuel, A better-formed evaporating surface, or boiler plates 



