282 THE EVOLUTIONARY CAREER 



and occurrences of fossil records in the light of morphological 

 results, believing that structural resemblances between different 

 kinds of organisms are indicative of genetic affinities. Then we 

 have to display the results of such investigations in the forms of 

 the sequences of races of organisms that, we believe, inhabited 

 the earth in the past. 



//. ANIMAL AFFINITIES 



Rational classifications are based on structural likenesses and 

 unlikenesses and not on merely superficial appearances of similarity 

 and dissimilarity in animals regarded as wholes. 



Superficial characters. Superficial resemblances may be illus- 

 trated by (i) a porpoise and a large fish ; (2) a blindworm (Anguis) 

 and a snake ; (3) the tails of a fish and a whale. Although a 

 similarity exists between the members of each pair noted, these 

 members belong to different classes of vertebrates. Superficial 

 dissimilarities may be illustrated by the cases of a garden snail, 

 a limpet and a whelk. These animals are very different in 

 appearance and habit, yet they all belong to the class of molluscs 

 — gasteropoda. 



Trivial Characters. Mendelian races, local races, varieties of 

 species, species and even genera are characterized, in the classifica- 

 tions by differences of structure that are said to be trivial. Such 

 characters are colours and colour patterns ; shell markings and 

 " ornamentation " ; arrangements of feathers, hairs, scales, spines, 

 etc. ; numbers of repetitional parts such as the scales, or finrays 

 in a fish, or the joints in the antennae of a copepod ; relative sizes 

 of bodily parts, etc. These are structural characters, but they 

 may be of the same general nature in widely different classes of 

 animals. Thus the teeth-patterns are important (though 

 " trivial," in the special sense) characters whereby the mammals 

 are classified into families, genera and species, but much the same 

 patterns are also utilized in the classification of the marsupials. 

 Relative sizes of bodily parts may be employed both in the 

 separation of species of fish and nematode worms. Numbers of 

 repetitional parts are used to make fish-species (scales and finrays) 

 and also copepod-species (the antennal jointing) and so on. 



Tectonic Characters. These express the fundamental bodily 

 structure. Thus absence or presence of a notochord (inverte- 



