ADAPTATION 365 



from being able to deal satisfactorily with this question, and 

 we shall put forward only certain tentative suggestions. 



The point at issue is how far structures or behaviour 

 patterns originally elaborated in relation to a particular 

 environment may eventually become incorporated in the 

 general organisation of the species. We believe that some evi- 

 dence may be obtained from the so-called ' Law of Irreversi- 

 bility of Evolution.' In so far as this ' law ' is not merely 

 a description of the somewhat imperfectly known geological 

 history of animals, it suggests that animals usually fail to 

 recover from any too detailed or too long extended specialisa- 

 tion. On the other hand, where life in an environment has 

 not entailed too great specialisation, reversal is possible. 

 We have already mentioned one example in the South 

 American opossum, Didelphys azarae. Loss of flight in birds, 

 or the reacquirement of the terrestrial habit by aquatic 

 dipterous larvae, will also be recalled. The process of fcetalisa- 

 tion ' in the evolution of man (Bolk, 191 9) also seems to 

 show a retracement of stages in specialisation, even if not 

 leading back to an adult ancestral type. 



It appears that a distinction must be drawn between 

 detailed specialisation for a restricted habitat and more general 

 specialisation for a broad one. Under the former conditions 

 it is necessarily the environment which to a large extent 

 determines what specialisations are feasible ; under the latter 

 there are so many different methods of successful conquest 

 (e.g. conquest of the air by insects, reptiles, mammals and 

 birds) that the method actually employed depends more on 

 the individuality of the organism than on the peculiarities of 

 the environment. Successful adaptation is mainly dependent 

 on a perfect system of internal relations. In a review of the 

 broad features of evolution, organismal adaptation would 

 stand out as the most characteristic general tendency, but 

 there is also much specialisation, particularly in those nume- 

 rous degenerate lines which have sooner or later become 

 extinct. 



It is relatively easy to make broad generalisations, but 

 very difficult to envisage such a twofold system of adaptation 

 in terms of the actual origin and multiplication of new variants. 

 The suggestion that an elaborate system of internal relations 

 is perpetually being improved by a series of entirely random 



