o20 THE INDUCTIONS OF BIOLOGY. 



anites them with various other facts, is a totally-different one. 

 When we see that the similar areas peopled by dissimilai 

 forms, are those between which there are impassable barriers ; 

 while the dissimilar areas peopled by similar forms, are those 

 between which there are no such barriers ; we are at once re- 

 minded of the general truth exemplified in the last section : — 

 the truth that each species of organism, tends ever to expand 

 its sphere of existence — to intrude on other areas, other 

 modes of life, other media ; and through these perpetually- 

 recurring attempts to thrust itself into every accessible habitat, 

 spreads until it reaches limits that are for the time insur- 

 mountable. 



§ 107. We pass now to the distribution of organic forma 

 in Time. Geological inquiry has established the truth, that 

 during a Past of immeasurable duration, plants and animals 

 have existed on the Earth. In all countries their buried 

 remains are found in greater or less abundance. From com- 

 paratively small areas, multitudinous different forms have been 

 exhumed. Every exploration of new areas, and every closer 

 inspection of areas already explored, brings more such forms 

 to light. And beyond question, an exhaustive exataination of 

 all exposed strata, and of all strata now covered by the sea, 

 would disclose forms immensely out-numbering all those at 

 present known. Further, it is now becoming manifest to 

 geologists, that even had we before us every kind of fossil 

 which exists, we should still have nothing like a complete 

 index to the past inhabitants of our globe. It has been long 

 known that many sedimentary deposits have been so altered 

 by the heat of adjacent molten matter, as greatly to obscure 

 the organic remains contained in them. The extensive form- 

 ations once called " transition," and now re-named " meta- 

 morphic," are acknowledged to be formations of sedimentary 

 origin, from which all traces of such fossil as they probably 

 included, have been obliterated by igneous action. And the 

 conclusion forcing itself into acceptance, is, that igneous rock 



