24 THE ANATOMY OF INVERTEBRATED ANIMALS. 



bone or shell is associated with such and such structural feat- 

 ures in the rest of the body, and no others. And it is these 

 empirical laws which are embodied and expressed in a natural 

 classification. 



II. Distribution. 



Living beings occupy certain portions of the surface of 

 the earth, inhabiting either the dry land, or the fresh or salt 

 waters; or being competent to maintain their existence in 

 either. In any given locality, it is found that these different 

 media are inhabited by different kinds of living beings ; and 

 that the same medium, at different heights in the air and at 

 different depths in the water, has different living inhabitants. 



Moreover, the living populations of localities which differ 

 considerably in latitude, and hence in climate, always present 

 considerable differences. But the converse proposition is not 

 true — that is to say, localities which differ in longitude, even 

 if they resemble one another in climate, often have very dis- 

 similar Faunae and Florae. 



It has been discovered, by careful comparison of local fau- 

 nae and floras, that certain areas of the earth's surface are 

 inhabited by groups of animals and plants which are not found 

 elsewhere, and which thus characterize each of these areas. 

 Such areas are termed Provinces of Distribution. There is 

 no parit3>- between these provinces in extent, nor in the phys- 

 ical configuration of their boundaries ; and, in reference to 

 existing conditions, nothing can appear to be more arbitrary 

 and capricious than the distribution of living beings. 



The study of distribution is not confined to the present 

 order of Nature ; but, by the help of geology, the naturalist is 

 enabled to obtain clear, though too fragmentary, evidence of the 

 characters of the faunae and florae of antecedent epochs. The re- 

 mains of organisms which are contained in the stratified rocks 

 prove that, in any given part of the earth's surface, the living 

 population of earlier epochs was different from that which now 

 exists in the locality ; and that, on the whole, the difference 

 becomes greater the farther we go back in time. The organic 

 remains which are found in the later Cainozoic deposits of any 

 district are always closely allied to those now found in the 

 province of distribution in which that locality is included ; 

 while in the older Cainozoic the resemblance is less ; and in 

 the Mesozoic, and the Palaeozoic strata, the fossils may be 

 similar to creatures at present living in some other province, 

 or may be altogether unlike any which now exist. 



