PRINCIPLES AND METHODS OF PALAEONTOLOGY. 373 



■we should find that a gradual change would take place iu the fauna* 

 and flora of the countries traversed, their inhabitants diftering' more and 

 more widely from those of this country, the more nearly they approxima- 

 ted either the pole or the ecpiator. Xor is this result other than might 

 be naturally exi)ected, for we know how closely dependent the health and 

 strength of animals and plants are upon the amount of heat, light, and 

 moisture to which they are exposed ; and in traveling due north or due 

 south, these climatal conditions necessarily become very greatly altered. 

 A corresponding change iu the flora and fauna is observed when, in a 

 mountainous country, we ascend from the plains to the line of perpet- 

 ual snow; and the animal and vegetable inhabitants of the sea in like 

 manner vary in character and abundance atdilferent depths. But these 

 cases also seem readily intelligible, for elevation has much the same 

 effect on climate as northing; and every fathom of increased depth iu 

 the sea corresponds with a certain diminution in the amount of light 

 and a certain alteration in temperature. 



Again there seems to be no difliculty in understanding why, as we 

 find to be the case, terrestrial animals and plants differ from those whose 

 existence is spent iu the water; nor why, among purely aquatic crea- 

 tures, the inhabitants of fresh water are usually widely different from 

 those of the sea. The discrepancy in form seems quite iu harmony with 

 the discrepancy in external circumstances. 



But there are some other facts connected with distribution, the cause 

 of which is by no, means so obvious. If the traveler, instead of moving 

 to the north or to the south of this country, journeyed east or west, keep- 

 ing as nearly as possible within similar climatal conditions, he would, 

 nevertheless, still find that the successive faunas and floras through 

 which he passed were widely different ; and if a voyager were to cir- 

 cumnavigate the globe between the parallels of 40° and 00° S., touch- 

 ing at ports in the continents of Africa, Australia, and America, the 

 differences between the indigenous animals of each country would be 

 immense, and altogether out of i)roi)ortiou to the changes in climatal 

 conditions. 



The globe, then, maybe marked out by boundary lines, some of which 

 ruu northerly and southerly, and others easterly and westerly, into a 

 number of districts or •'provinces," each of which is characterized by a 

 peculiar assemblage of animals and plants. And again, each district 

 might be subdivided by lines parallel ^vith the horizon, into zones of 

 depth and of height, iu each of which a certain group of this assem- 

 blage would flourish. It nuist be remembered, however, that neither 

 zones nor provinces are capable of a strict limitation, there being always 

 a border-land between every two, iu which the inhabitants of both are 

 mixed. 



The phenomeua of distribution in depth are i^articularly worthy of 

 attention, from their bearing on geology; for it is obvious that if we 

 are enabled to lay down certain rules with regard to the depth at which 

 particular forms live, we shall be able, when we find these forms in an 

 ancient sea-bed, to form a judgment as to the depth of that sea-bed, and 

 hence, in many cases, to gather valuable indications as to the proximity 

 or distance of dry laud. Every one who has walked along the sea shore 

 is familiar with certain forms of life — barnacles, limpets, periwinkles, 



•The term "Fauna" is applied to the whole of the animal inhabitants, "i^^/ora" to the 

 ■whole of the plants, of a district or country. Thus, the fauna of Africa means all the 

 animals found in Africa ; the Hora of India, the tlora of Kent, means all the planta 

 f(mnd in India and Kent respectively. In speaking thus it will be understood that the 

 "indigenous " animals and i>lants, or those which natiually exist in a country, are alone 

 referred to. 



