THEORETICAL BIOLOGY 471 



nevertheless in a number of cases it has been possible to tra 

 the actual descent of living types or of particular organs from 

 prehistoric ancestors ; the evolution of the horse, already de- 

 scribed, is a striking example, and the development of other 

 Ungulata has been worked out in a similar way. The descent 

 of certain types of the Gastropoda has likewise been determined 

 with great completeness, but for the invertebrates in general 

 direct evidence in palaeontology of evolution is rare, since in the 

 earliest fossiliferous formations the larger groups of inverte- 

 brates are already well developed, as we have noted. A further 

 important argument supplied by the palaeontological record lies 

 in the fact that the earlier representatives of a type are much 

 more generalized in structure than the later ; high degrees of 

 specialization which would arise through evolution do not 

 appear until the type has existed for a considerable time. The 

 older fossil fishes and reptiles had the vertebral column cartila- 

 inous or only incompletely ossified. The structure and shape 

 of the skull in most of the older fossil reptiles and mammals 

 were like embryonic stages in those groups to-day. 



The morphological evidence in favor of evolution is in part 

 anatomical and in part embryological. A distinguished German 

 scientist has said, " For the reflecting naturalist the facts of 

 morphology are a single great inductive proof in favor of the 

 theory of descent." The classification of animals is based on 

 morphological resemblances; we distinguish a number of differ- 

 ent kinds of animals, that is to say species, which do not usually 

 interbreed ; they are, however, in many cases connected with 

 one another by numerous varieties. Species can be grouped 

 into genera, they into families, and so on to the largest groups ; 

 and in very many cases, as we have noticed in the second part 

 of this book, there are intermediate individuals or groups which 

 form connecting links as it were between them, even the largest 

 groups, the types, not excepted. And it is further possible to 

 arrange the various groups of animals more or less like a genea- 

 logical tree, the simplest animals at the root and above them 

 branch after branch of greater and greater complexity of differ- 

 entiation given off as we ascend. That such a rational arrange- 

 ment is possible is a strong argument in favor of the theory of 

 descent. 



