** X -J CLASSIFICATION. 721 



to other groups, there has been little extinction. In order 

 that the naturalist may be enabled to give precise and 

 clear definitions of his divisions, whole ranks of inter- 

 mediate or gradational forms must have been utterly swept 

 away : if here and there a member of the intermediate 

 ranks has escaped annihilation, it puts an effectual bar to 

 any absolutely distinct definition." 



In other cases a particular plant or animal may perhaps 

 have transmitted its form from generation to generation 

 almost unchanged, or, what comes to the same result, those 

 forms which diverged in character from the parent stock 

 may have proved unsuitable to their circumstances, and 

 perished. We shall then find a particular form standing 

 apart from all others, and marked by many distinct 

 characters. Occasionally we may meet with specimens of 

 a race which was formerly far more common but is now 

 undergoing extinction, and is nearly the last of its kind. 

 Thus we explain the occurrence of exceptional forms such 

 as are found in the Amphioxus. The Equisetaceae perplex 

 botanists by their want of affinity to other orders of Acro- 

 genous plants. This doubtless indicates that their genea- 

 logical connection with other plants must be sought for in 

 the most distant ages of geological development. 



Constancy of character, as Mr. Darwin has said, 1 is 

 what is chiefly valued and sought after by naturalists ; 

 that is to say, naturalists wish to find some distinct family 

 mark, or group of characters, by which they may clearly 

 recognise the relationship of descent between a large 

 group of living forms. It is accordingly a great relief to 

 the mind of the naturalist when he comes upon a defi- 

 nitely marked group, such as the Diatomacese, which are 

 clearly separated from their nearest neighbours the Des- 

 midiaceas by their siliceous framework and the absence of 

 chlorophyll. But we must no longer think that because 

 we fail in detecting constancy of character the fault is 

 in our classificatory sciences. Where gradation of charac- 

 ter really exists, we must devote ourselves to defining and 

 registering the degrees and limits of that gradation. The 

 ultimate natural arrangement will often be devoid of strong 

 lines of demarcation. 



1 Descent of Man, vol. i. p. 2 1 4. 



3 A 



