410 THE PRINCIPLES OF SCIENCE. 



may meet with specimens of a race which was formerly 

 far more common but is now undergoing extinction, and 

 is nearly the ]ast of its kind. Thus we may 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 A erogenous plants. 

 This doubtless indicates that their genealogical con- 

 nexion with other plants must be sought for in the 

 most distant past 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 Diatomaceae, which are 

 clearly separated from their nearest neighbours the De- 

 midiacese 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. 



Let naturalists, too, form their systems of natural 

 classification with all care they can, yet it will certainly 

 happen from time to time that new and exceptional forms 

 of animals or vegetables will be discovered, and will 

 require the modification of the system. A natural system 

 is directed, as we have seen, to the discovery of empirical 

 laws of correlation, but these laws being purely empirical 

 will frequently be falsified by more extensive investiga- 

 * 'Descent of Man/ vol. i. p. 214. 



