Preliminary Definitions and Illustrations. 1 1 



of behaviour is absolute, and that the instinctive activities 

 are quite invariable. Were this the case, there could be no 

 organic progress so far as instincts are concerned. Evolu- 

 tion would be no longer possible. The constancy is rather 

 to be regarded as analogous to that which we find in those 

 structural characters of animals which are termed specific 

 or generic, as the case may be. When we say that struc- 

 tural characters are constant, we no longer, since the 

 revolution of thought caused by the publication of the 

 " Origin of Species," mean that they are absolutely invari- 

 able. We know, or have reason to believe, that these are 

 often subject to variations, many, if not most, of which 

 are slight and inconspicuous. In the same way, the per- 

 formance of instinctive activities is probably subject to 

 variations of a similar kind. This is, no doubt, difficult to 

 establish on the basis of actual observation, since slight 

 departures from the normal type of activity would be more 

 liable to escape detection than departures from the normal 

 type of structure. We must be guided by analogy, and 

 this certainly does not warrant the assumption that con- 

 stancy is to be interpreted in any absolute sense. 



The next point to notice with regard to the class of 

 activities which we term instinctive is that they are per- 

 formed under special circumstances, which are either of 

 frequent occurrence, or are vitally essential to the welfare 

 or continuance of the race. They are often of protective 

 value, and very frequently correlated with structural 

 characters which are likewise protective. Thus the hedge- 

 hog, whose body is clad in a spiny skin, instinctively 

 coils himself up when molested, and thus gives effect to 

 his protective structure ; so, too, the tortoise instinctively 

 withdraws his head and limbs under the carapace ; and the 

 snail retreats within his shell. And in like manner the 

 instincts of the hermit-crab lead him not only to withdraw 



