[ 



XXII.] CLASSIFICATION. 287 



chaine d'etres." The doctrine fhat natura non facit saltum 

 is now almost become an axiom, and is, I think, suffi- 

 ciently proved by the fact that modern researches among 

 extinct as well as among living organisms have not made 

 known a single type of form fundamentally different from 

 those which have been familiar since the dawn of the 

 science ; while they have made known a vast number of 

 intermediate forms, and in some cases (as, notably, in that 

 of the Cirrhipedes) they have filled up a gap by the dis- 

 covery of a larval form. As it has been truly expressed, 

 "all newly discovered forms can be arranged either in 

 known groups or between them." ^ 



But such expressions as " la cliaine cVetrcs" or " the organic 

 scaled' are inaccurate. A little familiarity with organic 

 classification is enough to show that no single series is 

 possible which shall represent organic affinities. The 

 possibility of such a series is excluded by the fact that 

 different groups generally approximate by their lower, and 

 not by their higher members. Thus, for instance, among 

 the Vertebrata, if we were to write the names of all the 

 orders and genera of fishes in a series from the lowest to 

 the highest (though not even a single class can be truly 

 arranged in such a series), we could not go on straight to 

 the aii'-breathing classes ; on the contrary, it would be 

 necessary to go back far down in the series of fishes, in 

 order to begin that remarkable series of Perennibranchiate 

 Batrachians which constitutes the transition from the fishes 

 to the air-breathers. It is the same generally. When The whole 

 groups are capable of being compared, though one may be °^°^^. 

 higher than the other on the whole, it is seldom or never seldom 

 that all the members of one group are higher than all the af u^the 

 members of another. Thus, though animals are on the ^^o}^ of 

 whole very much higher than vegetables, the higher group, 

 vegetables are very much more highly organized than the 

 Protozoa, or lowest animals. 



^Vlien the student has become convinced of this fact, Organic 

 that the order of organic affinities is not a single series, his i^'uke a 



Unless the pseudembrj'onic forms of the Echinodermata are an excep- ^®*^°^^- 

 tion to this. See Note to last chapter. 



