140 THE EVOLUTION OF LIFE. 



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, perhaps, 

 exists for the conclusion that the greater or less development 

 of hairs, is in part immediately due to increase or decrease of 

 demand on their passive function, as non-conductors of heat ; 

 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 the Lejridosteus, 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 pressures 

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

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

 the required thickness ? or, indeed, how could it come to 

 exist at all? Suppose this protective envelope to be too 

 weak, so that some of the eggs a bird lays are broken or 

 cracked. In the first place, the breakages or crackings are 

 actions of a kind which cannot react on the maternal organ- 

 ism, in such way as to cause the secretion of thicker shells 

 for the future : to suppose that they can, is to suppose that 

 the bird understands the cause of the evil, and that the 

 secretion of thicker or thinner shells can be controlled by its 



