76 EVOLUTION, ORIGIN OF LIFE. 
been the similarity amongst those which have been 
produced in different ages—just as the lowest forms 
are now similar in all regions of the earth. On the 
other hand, the longer any particular ‘tree of life’ 
has lived (of which there have been countless multi- 
tudes born in each. age), the wider may be the diver- 
gence of form presented by the ultimate outgrowths 
of any two of them, or of outgrowths of similar rank 
produced from trees which have developed during 
different ages—especially when the assemblages of 
organisms constituting one of these ideal trees, have 
lived under the influence of any unusual set of telluric 
conditions. : 
The ‘vertebrate’ grade of organization may have 
been many times attained by ultimate branches of 
_ different ‘trees of life’ But how long or when the 
particular ‘tree of life, from one of the branches of 
: which Man was developed, appeared upon the earth, 
it is quite impossible to-say. 
