156 THE INDUCTIONS OP BIOLOGY. 



In a kindred manner there results a limitation of growth 

 in a land-animal, which does not exist for an animal living 

 in the water. If, after comparing the agile movements of 

 a dog with those of a cow, the great weight of which ob- 

 viously prevents agility; or if, after observing the swaying 

 flesh of an elephant as it walks along, we consider what 

 would happen could there be formed a land-animal equal 

 in mass to the whale (the long Dinosaurs were not propor- 

 tionately massive) it needs no argument to show that such 

 a creature could not stand, much less move about. But 

 in the water the strain put upon its structures by the weights 

 of its various parts is almost if not quite taken away. Prob- 

 ably limitation in the quantity of food obtainable becomes 

 now the chief, if not the sole, restraint. 



And here we may note, before leaving the topic, something 

 like a converse influence which comes into play among 

 creatures inhabiting the water. Up to the point at which 

 muscles tear from over-strain, larger and smaller creatures 

 otherwise alike, remain upon a par in respect of the relative 

 amounts of energy they can evolve. Had they to encounter 

 no resistance from their medium, the implication would be 

 that neither would have an advantage over the other in 

 respect of speed. But resistance of the medium comes into 

 play; and this, other things equal, gives to the larger 

 creature an advantage. It has been found, experimentally, 

 that the forces to be overcome by vessels moving through 

 the water, built as they are with immersed hinder parts 

 which taper as fish taper, are mainly due to what is called 

 " skin-friction." Now in two fish unlike in size but other- 

 wise similar skin-friction bears to the energy that can be 

 generated, a smaller proportion in the larger than in the 

 smaller; and the larger can therefore acquire a greater 

 velocity. Hence the reason why large fish, such as the 

 shark, become possible. In a habitat where there is no am- 

 bush (save in exceptional cases like that of the LopJiius or 

 Angler) everything depends on speed; and if, other things 



