286 



HABIT AND INTELLIGENCE. 



[chap. 



Acari, 

 though the 

 lowest 

 Arthro- 

 pods, do 

 not revert 

 to the 

 worm- 

 type. 



Pygno- 

 gonidse. 



Classifica- 

 tion in a 

 single 

 series is 

 impos- 

 sible. 



" ISTatura' 

 non facit 

 sal turn." 



degree invaKdates the evidence of community of descent ; 

 it is only another instance of the truth, that no morpho- 

 logical character is quite invariable. It is to be observed 

 that the determination of the number of segments in an 

 Arthropod is generally difficult, and sometimes impossible, 

 in consequence of the almost perfect obKteration of the 

 joints between some of the consolidated segments, analo- 

 gous to the consolidation together of some of the vertebrae 

 in all the higher vertebrate forms. But to return to 

 the Acari : these are a family of Arachnids, and, though 

 they are the lowest of all Arthropods, they have not 

 reverted to the worm-like structure ; on the contrary, like 

 the rest of the Arachnid class, they have their segments 

 more completely consolidated and less distinguishable 

 from each other than is common among the rest of the 

 Arthropod classes. The same remarks apply to the Pyg- 

 uogonidae, a crustacean family of organization so low that 

 they have neither a circulatory nor a distinct respiratory 

 system. Their respiration is through the general surface 

 of the body, as it is in all the lower Invertebrates ;^ and 

 yet their forms are not worm-like, but crab-like. These 

 facts concerning the Acari and the Pygnogonidse may be, 

 I think, most easily interpreted by supposing that they are 

 tribes which, instead of advancing in organization, have 

 fallen below the general level of the classes to which they 

 belong, and yet have not in any way reverted to the 

 ancestral worm-structure. 



The first attempts that were made at zoological classifi- 

 cation, generally aimed at the arrangement of all species of 

 animals in a single series, according to their affinities, from 

 the lowest to the highest ; or rather, as the early schools 

 would have expressed it, from the highest to the lowest. 

 " Naturalists," says Agassiz,^ " were bent upon establishing 

 one continuous uniform series to embrace all animals, 

 between the links of which it was supposed there were no 

 unequal intervals. The watchword of their school was 

 Kahira non facit saltum ; they called their system la 



1 Carpenter's Comparative Physiology, p. 403. 

 * Spencer's Principles of Biology, vol. ii. p. 299. 



