l64 THE FOUNDATIONS OF ZOOLOGY 



circumstances it would be found that animals of some particular 

 stature had on the whole a better chance of escape than any 

 other ; and if their race is closely adapted to these circumstances 

 in respect to stature, the most favored stature would be identical 

 with the mean of the race. Though the impediments to flight are 

 less unfavorable to this (stature) than to any other, they will differ in 

 different experiences. The course of an animal might chance to 

 pass through denser foliage than usual, or the obstacles in his 

 way may be higher. In that case the animal whose stature 

 exceeded the mean would have an advantage over mediocrities. 

 Conversely, the circumstances might be more favorable to a small 

 animal. Each particular line of escape might be most favorable 

 to some particular stature, and, whatever this might be, it might 

 in some cases be more favored than any other. But the acci- 

 dents of foliage and soil in a country are characteristic and per- 

 sistent, and may fairly be considered as approximating to a typical 

 kind. Therefore those which most favor the animals of the mean 

 stature will be more frequently met with than those which favor 

 any other stature, and the frequency of the latter occurrence will 

 diminish rapidly as the stature departs from the mean. 



" It might well be that natural selection would favor the 

 indefinite increase of numerous separate faculties if their improve- 

 ment could be effected without detriment to the rest; then medi- 

 ocrity 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 were 

 thereby diminished. 



" But a too free use of this ' if ' would show a jaunty disre- 

 gard of a real difficulty. Organisms are so knit together that 

 change in one direction involves change in many others; these 

 may not attract attention, but they are none the less existent. 

 Organisms are like ships of war, constructed for a particular pur- 

 pose in warfare, as cruisers, line-of-battle ships, etc., on the prin- 

 ciple of obtaining the utmost efficiency for their special purpose. 

 The result is a compromise between a variety of conflicting de^ 

 siderata, such as cost, speed, accommodation, stability, weight of 

 guns, thickness of armor, quick steering power, and so on. It is 

 hardly possible in a ship of any established type to make an 



