xxill.] THE CAUSES OF DEVELOPMENT. 307 



different reason. Take the case of protective structures ; 

 the skin hardens and thickens in any place that is exposed 

 to rough usage ; this is a case of self-adaptation, and it 

 may become hereditary, as in the instance of the knuckles 

 of the gorilla.^ But such cannot be the origin of the shell nor of the 

 that protects the bird's egg. AVithout this protection, no ^^^"^ ^ ' 

 egg could be hatched, and the whole tribe of bii'ds would 

 perish. But the contact of comparatively hard and rough 

 substances, which makes the shell necessary, cannot have 

 had the slightest tendency to produce the shell ; for the 

 shell is formed, and from the necessity of the case must be 

 formed and completed, before any such contact can take 

 place.^ And the law of hereditary habit, universally true 

 as it is, makes no difference here ; for it appears utterly 

 impossible that egg-shells can ever have been formed 

 under different circumstances from the present. The same nor of the 

 is true of the bony or cartilaginous skulls that protect ^ " ' 

 the cephalic ganglia in the Cephalopoda (cuttlefish and 

 nautilus), and in the Vertebrata, and of the hard woody nor of 

 shells that protect the seed in nuts. The fact that there ^^ ® ® 

 is something to protect, is not a jphysical cause of the 

 production of a protective structure. 



In this chapter I have considered how far the facts of Summary. 

 organization can be accounted for by the direct action of 

 inorganic forces on the organism, and by the actions of the 

 organism itself ; and I have come to the conclusion, that 

 such a purely physical account of the origin of organization 

 is probably valid in the case of some of the simpler 

 structures belonging to the vegetative system ; very doubt- 

 ful in the case of the peculiarly animal ones, as muscle 

 and nerve ; and demonstrably, not merely inadequate, but 

 totally inapplicable to the case of the complexities of the 

 organs of special sense, and to many cases, not necessarily 

 complex ones, in which the structure is produced under 

 such circumstances that the peculiar work for which it is 

 adapted can have no tendency to originate it. In the 

 next chapter I shall have to consider how much may be 

 explained by the law of Natural Selection. 



* Spencer's Principles of Biology, vol. ii. p. 295. '' Ihid. vol. i. p. 440. 



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