INSTINCT AND STRUCTURE. 267 



all instincts, is an original impulse, a tendency 

 forming part of its constitution. On the other 

 hand, the structure of the uncovered portion of 

 its body is such as to prove beyond question that 

 it is expressly adapted to the act to which it is in- 

 stinctively impelled. The tail or hinder part of 

 its body is not merely capable of assuming a 

 spiral form, so as to fit itself into the chambers of 

 the shell, but it is terminated by certain hard, 

 movable pieces, worked by a powerful muscle, by 

 which it can easily fix its tail in the interior of 

 the shell and draw itself into its retreat at will. 

 The instinct could not have given origin to the 

 structure, nor could the structure have suggested 

 or inspired the instinct which the little creature 

 acts upon long before it has had either the expe- 

 rience of its danger or of the facility its structure 

 affords for the end in view. We must regard this 

 adaptation as a beautiful and striking evidence of 

 the intention of Him to whose divine skill and 

 purpose alone we can attribute at once the struc- 

 ture and the instinct of living beings and such 

 adaptations as that to which we now refer. 



One of the most marvellous things in th& his- 

 tory of the crab is the strange metamorphosis it 

 undergoes before assuming its permanent form. 

 It was long believed by naturalists that the young 

 of the crab, on being excluded from the ova, pre- 

 sented a minute but perfect resemblance to the 

 species they belong to ; and this belief gained 

 support from the circumstance that vast numbers 

 of very minute crabs are frequently to be found 



