722 THE PRINCIPLES OF SCIENCE. [CHAP. 



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- 

 tion. From time to time the notions of naturalists have 

 been greatly widened, especially in the case of Australian 

 animals and plants, by the discovery of unexpected com- 

 binations of organs, and such events must often happen 

 in the future. If indeed the time shall come when all 

 the forms of plants are discovered and accurately de- 

 scribed, the science of Systematic Botany will then be 

 placed in a new and more favourable position, as remarked 

 by Alphonse Decandolle. 1 



It ought to be remembered that though the genealogical 

 classification of plants or animals is doubtless the most in- 

 structive of all, it is not necessarily the best for all purposes. 

 There may be correlations of properties important for 

 medicinal, or other practical purposes, which do not cor- 

 respond to the correlations of descent. We must regard 

 the bamboo as a tree rather than a grass, although it is 

 botanically a grass. For legal purposes we may continue 

 with advantage to treat the whale, seal, and other cetacean 

 as fish. "We must also class plants according as they 

 belong to arctic, alpine, temperate, sub-tropical or tropical 

 regions. There are causes of likeness apart from hereditary 

 relationship, and we must not attribute exclusive excellence 

 to any one method of classification. 



Classification by Types. 



Perplexed by the difficulties arising in natural history 

 from the discovery of intermediate forms, naturalists have 

 resorted to what they call classification by types. Instead 

 of forming one distinct class defined by the invariable 

 possession of certain assigned properties, and rigidly in- 

 cluding or excluding objects according as 1.1 icy do or do not 



1 Laws of Botanical Nomenclature, p. 16. 



