D.— ZOOLOGY. 89 



Both Lamark and Darwin based their theories on the assumption that 

 every structure in an animal had a definite use in the animal's daily life or 

 at some stage of its life history. They understood by adaptation a change 

 in the structure, and by implication also in the habits of an animal which 

 rendered it better fitted to its " organic or inorganic conditions of life." 

 Thus, for Darwin at any rate, a general increase in the efficiency of an 

 animal was an adaptation. But amongst his followers the term came to 

 imply a definite structural change of a part or parts by which an animal 

 became better suited to some special and characteristic mode of life. 

 The adaptation of flowers to ensure fertilisation by definite species of insects 

 is a characteristic case. Such definite adaptations can only be shown to 

 exist by very long continued observation of the animal under its natural 

 conditions of life. In the post-Darwinian literature the suggestion that 

 such and such a structure could be used for some definite function is too 

 often regarded as evidence that in fact it is actually so used. My 

 colleagues amongst the palaeontologists are, I am afraid, offenders in 

 this way. 



But even if it can be shown that the structure of an animal is such 

 that it is specially fitted for the life which it in fact pursues, it does not 

 necessarily follow that this structure has arisen as a definite adaptation to 

 such habits. It is always conceivable, and often probable, that after the 

 structure had arisen casually the animal possessing it was driven to the 

 appropriate mode of life. 



The only cases in which we can be certain that adaptation in this true 

 sense has occurred are those, unfortunately rare, in which we can trace in 

 fossil material the history of a phylogenetic series, and at the same time 

 establish that throughout the period of development of the adaptation its 

 members lived under similar conditions. 



It is not unusual for a student of fossils to discuss the habits of an 

 extinct animal on the basis of a structural resemblance of its ' adaptive 

 features ' with those of a living animal and then to pass on to make use of 

 his conclusions as if they were facts in the discussion of an evolutionary 

 history or of the mode of origin of a series of sediments. 

 , In extreme cases such evidence may be absolutely reliable : no man 

 faced with an ichthyosaur so perfectly preserved that the outlines of its 

 fins are visible can possibly doubt that it is an aquatic animal, and such a 

 conclusion based on structure is supported by the entire absence of 

 ichthyosaurs in continental deposits of appropriate ages and their abund- 

 ance in marine beds. But if extremes give good evidence, ordinary cases 

 are always disputable. For example, there is, so far as I know, not the 

 least evidence in the post-cranial skeleton that the hippopotamus is 

 aquatic : its limbs show no swimming modification whatsoever, and the 

 dorsal position of the eyes would be a small point on which to base assump- 

 tions. 



Most paleontologists believe that the dentition of a mammal, and by 

 inference also that of a reptile or fish, is highly adaptive, that its character 

 will be closely correlated with the animal's food, and that from it the 

 habits of an extinct animal can be inferred with safety. 



Here again the extreme cases are justified, the flesh-eating teeth of a 

 cat and the grinding battery of the horse are clearly related to diet. 



