526 THE EVOLUTION OP LIFE. 



of an animal as are occupied in its mechanical actions on the 

 environment. Of the one class of cases, the darkening of 

 the skin which follows exposure to one or other extreme of 

 temperature, may be taken as an instance; and with the 

 other class of cases we are made familiar by the increase and 

 decrease which use and disuse cause in the organs of motion. 

 It is needless here to exemplify these: they were treated of 

 in the Second Part of this work. 



But in animals, as in plants, there are many indispensable 

 offices fulfilled by parts between which and the external con- 

 ditions they respond to, there is no such action and reaction 

 as can directly produce an equilibrium. This is especially 

 manifest with dermal appendages. Some ground exists for 

 the conclusion that the greater or less development of hairs, 

 is in part immediately due to increase or decrease of demand 

 on the passive function, as forming a non-conducting coat; 

 but be this as it may, it is impossible that there can exist 

 any such cause for those immense developments of hairs which 

 we see in the quills of the porcupine, or those complex de- 

 velopments of them known as feathers. Such an enamelled 

 armour as is worn by Lcpidosteus, is inexplicable as a direct 

 result of any functionally-worked change. For purposes of 

 defence, such an armour is as needful, or more needful, for 

 hosts of other fishes; and did it result from any direct re- 

 action of the organism against any offensive actions it was 

 subject to, there seems no reason why other fishes should not 

 have developed similar protective coverings. Of 



sundry reproductive appliances the like may be said. The 

 secretion of an egg-shell round the substance of an egg, in 

 the oviduct of a bird, is quite inexplicable as a consequence 

 of some functionally-wrought modification of structure, im- 

 mediately caused by some modification of external con- 

 ditions. The end fulfilled by the egg-shell, is that of 

 protecting the contained mass against certain slight pres- 

 sures and collisions, to which it is liable during incubation. 

 How, by any process of direct equilibration, could it come to 



