io62 LIFE . OUTLINES OF GENERAL BIOLOGY 



Yet it is difficult to draw the line with firmness. Growth is a 

 general characteristic of living creatures, and it shows primary 

 adaptiveness in a way that is not true of the growth of a crystal. 

 For, as we have seen, organic growth is characteristically a regulated 

 process; over-growth, for instance, is unusual. Yet when we pass 

 from growth in general to the particular kind of growth illustrated 

 in the regeneration of lost parts, such as the arm of a starfish or the 

 tail of a lizard, we are face to face with a special adaptation. Still 

 more emphatic is this impression when we find that in many lizards 

 which readily surrender their tail in the spasms of capture, and thus 

 may escape with their life, there is a pre-formed breakage-plane, 

 which extends as a soft gristly disc right through the centrum of a 

 vertebra, or of several vertebrae, near the anterior end of the tail. 

 And our impression of special adaptiveness is further strengthened 

 when we notice that the capacity for surrendering and regrowing the 

 tail, general among lizards, is not exhibited by the chamseleon, 

 which keeps its tail twisted round a branch and out of harm's way, 

 nor by various other lizards, like the South African Zonurus, which 

 habitually use their tails as weapons. 



In the same way there is general adaptiveness in the capacity 

 the leucocytes of vertebrate blood have of manufacturing anti- 

 bodies. These counteract the disintegrative action of strange proteins 

 that may find their way in, either by wounds or by intruding enemies. 

 But when the blood of a particular animal is rapidly and very 

 effectively resistant to a definite poison, as the hedgehog's to the 

 adder's venom, must we not speak of a special adaptation here ? 



The word adaptation is often used not for the result as we are 

 using it here, but for the evolutionary process by which the result 

 is reached. Thus the gizzard of a fowl, the flukes of a whale, the 

 sting of a wasp, and the tentacles of a sundew may all be spoken of 

 as the outcome of long processes of adaptation — more direct accord- 

 ing to the Lamarckians, more indirect according to the Darwinians. 



It has been suggested, usefully we think, that the term adjustment 

 should be kept for processes or results that occur during the 

 individual lifetime. Thus a callosity developing on a much- worn 

 part of the skin may be a protective adjustment; and extreme 

 Lamarckians believe that even such individual adjustments might 

 form the raw materials of racial adaptations. The more general 

 term "modification" is applied to any result that comes about in 

 the individual lifetime as the direct result of some peculiarity in 

 environment, nutrition, use and disuse, and persists after the 

 inducing conditions have ceased to operate. But modifications are 

 not always adjustments; they may be disadvantageous or indifferent. 



Adaptations Classified. — To start with, it is convenient to 

 distinguish functional and structural adaptations. Of the former we 

 may give the following instances. Warm-bloodedness, a preroga- 



