vii.] DISCUSSION OF THE DATA OF STATURE. 123 



that law is expressed, that the relation between yy r and 

 x is also governed by it. The value of P of course 

 remains the same throughout, but the Q in the system 

 of yy' values is necessarily less than that in the system 

 of y values. 



It might well be that natural selection would favour 

 the indefinite increase of numerous separate faculties, if 

 their improvement could be effected without detriment 

 to the rest ; then, mediocrity in that faculty would 

 not be the safest condition. Thus an increase of 

 fleetness would be a clear gain to an animal liable to 

 be hunted by beasts of prey, if no other useful faculty 

 was thereby diminished. 



But a too free use of this "if" would show a 

 jaunty disregard of a real difficulty. Organisms are 

 so knit together that change in one direction involves 

 change in many others ; these may not attract atten- 

 tion, but they are none the less existent. Organisms are 

 like ships of war constructed for a particular purpose 

 in warfare, as cruisers, line of battle ships, &c., on the 

 principle of obtaining the utmost efficiency for their 

 special purpose. The result is a compromise between 

 a variety of conflicting desiderata, such as cost, speed, 

 accommodation, stability, weight of guns, thickness of 

 armour, quick steering power, and so on. It is hardly 

 possible in a ship of any long established type to make 

 an improvement in any one of these respects, without a 

 sacrifice in other directions. If the fleetness is increased, 

 the engines must be larger, and more space must be 

 given up to coal, and this diminishes the remaining 



