^6 THE DOCTRINE OF DESCENT. 



of a low type. By varying and combining comparisons of 

 this sort, and taking account of the points of connection 

 between the various types, to which we shall immediately 

 refer, the figure of the systematic trees completes itself 

 into one vast tree, of which the main branches are re- 

 presented by the types. 



Had the systematizers of the old school been familiar 

 with the construction of plants and animals, they would 

 have first established the diagnoses and distinctive cha- 

 racters, and then called to life the types and their species; 

 for their chief torment has been, that the diagnoses are 

 liable to so many exceptions, and that the characters 

 of the fundamental forms are without any absolute value. 

 Roughly and generally speaking, polypes are radiate in 

 form, but not a few are bilateral, or symmetric on two 

 sides. Most snails possess well-marked mantle-folds, but 

 we can scarcely speak of the testa of many thoroughly 

 worm-like slugs. 



Head and skull seem an inalienable mark of the 

 vertebrata, yet the lancelet has no such head, but merely 

 an anterior end. Nevertheless, it may be objected, it 

 has a vertebral column ; yet this, the special badge of 

 nobility of the vertebrate animals, like the auditory appa- 

 ratus, and the notochord, is, even if only transiently, a 

 possession of the Ascidians, a class of animals which 

 in their mature condition do not bear the remotest re- 

 semblance to the Vertebrata. When we become aware 

 of these deviations from so-called laws of form and 

 structure, seemingly well established, we are prepared 

 for a manifest failure of the system, in regard to con- 

 necting forms, and forms of uncertain position .in the 

 system. 



