36 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, 
