514 THE WONDER OF LIFE 



the body-guard of phagocytes, and it is often a life-saving 

 one. 



In many cases the modifications are markedly beneficial. 

 When a mammal is taken to a colder climate it often 

 acquires a thicker coat of hair, which is obviously advan- 

 tageous. When a plant is moved from the plain to the 

 plateau it often acquires a thicker epidermis, and Professor 

 MacDougal has furnished numerous illustrations of useful 

 modifications exhibited by plants when transferred to 

 desert conditions. Every one knows that an area of skin 

 much pressed upon becomes hard and callous, and that this 

 is often of protective value. Many other instances might 

 be given of functionally and environmentally induced 

 modifications which are useful, effective, fit, and may even 

 make for the preservation of the individual, when the 

 struggle for existence is keen. These are adaptive modifica- 

 tions. 



Nature of Adaptations. It tends to clearness of think- 

 ing to keep the term adaptations (used to denote the results 

 of an evolutionary process) for features and qualities and 

 arrangements which are inborn, not individually acquired. 

 An accommodation is the transient expression of plasticity ; 

 a modification is permanent but individually acquired ; 

 an adaptation is racial, the expression of the natural inherit- 

 ance, not an individual gain or loss. It goes without saying 

 that though these adaptations are potentially implicit in the 

 germinal material in the fertilized ovum they cannot be 

 expressed without the appropriate nurture. But this does 

 not bring them in the least within the category of ' acquired 

 characters ' or modifications, which result from changes 

 in the ordinary nurture. In the same way, it is mere 

 word-splitting to find any difficulty in the fact that ac- 



