CLASSIFICATION. 15 
1. No naturalist now believes, as Linnzeus did, in the fixity of species ; 
swe believe, on the contrary, that one form has given rise to another. 
At the same time, the common characteristics on the strength of which 
we deem it warrantable to give a name to a group of individuals, must 
BIRDS SOF 
Snakes Ligand, coylians 
a Ss S cero? 
Rants 
Fishes i. Amphibian 
= é 78 
ancelet ’ cycloto™ 
B wna 
d (< pba opods 
‘ / motu Z A Syerons 
Bos, wy onltgee 
SS Unsectg S25. forms] 
-- fs} a D F 
BORO SE _ | echinodermy 
we = Ter astaceans 
iS 
@eflenfdra 
Mesozoa~y, A Spon ges, 
(nfusonians —, ; w_Gregarines 
do. 
Prrotoza Plants. 
per ee 
Fic. 18.—Diagrammatic expression of classification in a 
genealogical tree. B indicates possible position of Balano- 
glossus, D of Dipnoi, S of Sphenodon or Hatteria. 
not be markedly fluctuating. The specific characters should exhibit 
a certain degree of constancy from one generation to another. 3 
2. Sometimes a minute character, such as the shape of a tooth or the 
marking of a scale, is so constantly characteristic of a group of indi- 
viduals that it may be safely used as the index of more important 
