134 



MACLEAN SYSTEM OF 



phically detached, these lines perhaps consisting of par- 

 ticular typical groups placed in an independent succession, 

 or of two or more of these groups. And for this idea there 

 is, even in the present imperfect state of our knowledge 

 of animated nature, some countenance in ascertained facts, 

 the birds of Australia, for example, being chiefly of the 

 suctorial type, while it may be presumed that the obser- 

 vation as to the predominance of the useful animals in the 

 Old World, is not much different from saying that the ra- 

 sorial type is there peculiarly abundant. It does not ap- 

 pear that the idea of independent lines, consisting of par- 

 ticular types, or sets of types, is necessarily inconsistent 

 with the general hypothesis, as nothing yet ascertained of 

 the Macleay system forbids their having an independent 

 set of affinities. On this subject, however, there is as yet 

 much obscurity, and it must be left to future inquirers to 

 clear it up. 



We must now call to mind that the geographical distri- 

 bution of plants and animals was very different in the 

 geological ages from what it is now. Down to a time not 

 long antecedent to man, the same vegetation overspread 

 every clime, and a similar uniformity marked the zoology. 

 This is conceived by M. Brogniart, with great plausibility 

 to have been the result of a uniformity of climate, pro- 

 duced by the as yet unexhausted effect of the internal heat 

 of the earth upon its surface ; whereas climate has since 

 depended chiefly on external sources of heat, as modified 

 by the various meteorological influences. However the 

 early uniform climate was produced, certain it is that, from 

 about the close of the geological epoch, plants and animals 

 have been dispersed over the globe with a regard to their 

 particular characters, and specimens of both are found so 

 isolated in particular situations, as utterly to exclude the 

 idea that they came thither from any common centre. It 

 may be asked — Considering that, in the geological epoch, 

 species are not limited to particular regions, and that since 

 the close of that epoch, they are very peculiarly limited, 

 are we to presume the present^prganisms of the world to 

 have been created ab initio after that time ? To this it 

 may be answered — Not necessarily, as it so happens that 

 animals begin to be much varied, or to appear in a consi - 

 derable variety of species, towards the close of the geolo- 

 gical history. It may have been that the multitudes of 

 locally peculiar species only came into being after the 



