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to the one referred to, and the absence of unanimity amongst the workers 

 in this science may possibly indicate that no such principle exists. (1) 



Again, looking at rocks and organisms without any regard to Eetiological 

 considerations, there can be no doubt that the individuals and groups of 

 individuals are much better defined among the latter than among the former. 

 Indeed, if we take our idea of the precision which should attach to the term 

 species from the organic world then this term is unquestionably out of 

 place as applied to groups of rocks. (2) 



Consider now the case of the mineralogist. A mineral is a definite 

 chemical compound. Its composition is either constant, or the differences 

 can be explained by introducing the principle of isomorphism. The 

 fundamental law of chemistry the law of definite combining proportions 

 comes in, therefore, and gives precision to mineralogical classification. 

 It furnishes a criterion by which the independent existence of a mineral 

 species may be tested. Now rocks are as a general rule aggregates of 

 minerals, and the relative proportions of the different constituents are not 

 subject to any definite laws. By the appearance or disappearance of 

 certain minerals one variety of rock shades into another by the most 

 imperceptible gradations. It is often as difficult to fix the point at which one 

 variety of rock ends and another begins as it is to fix the point at which one 

 colour ends and another begins, in the continuous spectrum of white light. 



It is impossible to give a distinct name to every tint in the spectrum, 

 and it is equally impossible to give a distinct name to every kind of mineral 

 aggregate. "We can, however, indicate any particular tint by reference to 

 the principal colours ; and so, with rocks, we can often indicate the characters 

 of a particular specimen by referring to well marked types when it is 

 impossible or unadvisable to use a distinctive name. 



These considerations show therefore that the biologist and mineralogist 

 have immense advantages over the petrographer in all matters relating to 

 classification. If we endeavour to define these advantages more precisely 

 we see that they depend, mainly, on the fact that organising and minerals 

 possess an individuality which is much more definite than that of rock 

 specimens. The form and structure of an organism are mainly determined 

 by the properties which it has inherited from its ancestors, and only to a 

 very subordinate extent by its environment. In the same way the form 

 and structure of a mineral are determined mainly by the inherent 

 properties of the chemical compound. Rock specimens, on the other 

 hand, are merely fragments which may or may not be average samples 

 of the larger masses from which they have been broken. They 

 possess individuality only in a very subordinate sense. Their external 

 form is merely an accident depending on the manner in which 

 the fragment has been detached ; their texture is mainly determined 

 by the conditions under which consolidation has been effected, and only to 

 a limited extent by the composition of the magma. 



(1) BUNSBN'S law is the only one that may possibly render a qualification of this 

 sentence necessary. 



(2) J. HOTH. Allgcmeine und chemische Geologic. Band II., p. 41. 



