PALEONTOLOGY, ITS AIMS AND METHOD 215 



much obscured, partly by the thrusting back into them 

 of structures which really belong to much later times, but 

 which by their actual physical bulk require a lengthy develop- 

 ment, and partly by modifications imposed by functional 

 needs of the growing organism. 



The palaeontologist uses this recapitulation only to bridge 

 over slight gaps ; the stages he uses, living under similar con- 

 ditions to the adult, are free from special embryonic features. 

 In most cases the characters used are not in all probability of 

 much functional importance to the animals, being generally 

 ornamental structures. 



The study of phylogeny is beset by many difficulties and 

 pitfalls. The determination of the actual relationship of the two 

 animals, whether they be closely allied or remotely related, is 

 seldom easy, resemblances of many distinct kinds having to 

 be sorted out and differences evaluated. The experience of a 

 century of study has shown that in the case of distantly allied 

 forms gross resemblances, those which are obvious to all, are 

 usually misleading, because owing to their very patency they 

 have come into direct contact with the organism's environment 

 and have been modified in adaptation to it in similar ways in 

 diverse phyla. Palaeontologists have thus been thrust into a 

 thorough study of detail, an investigation far more precise 

 and deep-seated than any undertaken in pre-Darwinian days ; 

 they now look for significant resemblances in those incon- 

 spicuous structures often of vital importance to the animal, 

 which by their deep-seated nature may be supposed to be 

 protected from adaptational change. These characters, how- 

 ever, by their very nature are common to all individuals of 

 great groups and cannot serve for the finer separation of the 

 members of the smaller twigs of a phylogenetic tree ; for such 

 distinctions other characters, also not likely to be affected by 

 adaptation, must be chosen, and the very fineness of the divi- 

 sions they are to be used in making renders it necessary that 

 they should not be of any great importance in the animal's life. 



The study of phylogeny, therefore, demands the searching 

 out of deep-seated structures essential to the animal's life for 

 the distinction of the greater groups, of the patent features 

 of the animal and the investigation of its adaptations for 

 the further sub-division of these groups, and finally of the 

 small, non-adaptive details of its make-up for the disentangling 



