ADAPTATION 355 



great distances, so that they can find a suitable environment 

 and need not stay passively subjected to unfavourable con- 

 ditions. The influences which might be expected to act as 

 selective agencies may merely induce migration. 



A simple metaphor may be of some assistance in contrasting 

 this idea of adaptation with that put forward by Fisher (p. 351). 

 If we imagine the environment into which the animal has to 

 fit as an irregular cavity in a hard substance, then on Fisher's 

 view living organisms would resemble a liquid of relatively 

 low viscosity which would soon, by mere force of gravity, come 

 to fill every crevice. On our view the organism would 

 resemble more a tennis ball, which would fill the cavity com- 

 pletely only if subjected to very extreme pressure. Except 

 after prolonged and extreme exposure, it would be sufficiently 

 elastic to regain its shape if the pressure were released, while 

 if the pressure was not very carefully applied the ball would 

 shoot out and leave that particular environment altogether. 



We do not believe that the view that animals are very 

 accurately adapted to the environment is now nearly so 

 generally held by naturalists as Fisher supposes. As he 

 admits [I.e. p. 41), the more adapted an animal is, the 

 greater is its danger from deterioration of* the environment. 

 If an animal is too well adapted to one set of conditions, it 

 must necessarily be proportionately less well adapted if the 

 conditions change. This principle is highly important when 

 we remember the marked environmental fluctuations experi- 

 enced by nearly all animals {cf. Elton, 1930, pp. 19-28). 

 The phylogeny of such a group as the Vertebrata, as revealed 

 in their fossil history, suggests that it is the unspecialised 

 and, therefore, the relatively less well adapted that have 

 survived. Forms which ' dated ' met with no approval in 

 later periods. 



But as Bateson (1894, p. 12) has said, 'We, animals, live 

 not only by virtue of, but also in spite of what we are,' and it is 

 not difficult to find instances of highly specialised animals 

 which live successfully in habitats to which they are quite 

 unadapted. Thus Hudson (1892, p. 18) describes an opossum 

 {Didelphys azarae) which lives on the plains of La Plata, yet 

 still retains the specialisations which adapted it for life in the 

 forests further north. The grasping hand, so necessary for 

 tree-climbing, is a positive hindrance to walking on the earth, 



