LIVING MAMMALIA AND THEIK ANCESTORS. 83 



by scarcely noticeable deviations raised them- 

 selves above their cousins by making use of the 

 advantageous changes and adaptations in their 

 organisations. And yet the probability is that the 

 lowest animal forms were, in general, also the 

 oldest geologically. 



Although even Cuvier had prepared the down- 

 fall of Buffon's indefinite idea of arranging the 

 animal kingdom into one series, still it was not till 

 our own day that it became generally admitted that 

 the conception of a figurative expression for the 

 system could only take the form of an immense 

 tree, with hundreds of branches and thousands of 

 twigs. The animals we see living to-day are the 

 tips of the twigs and shoots ; those that preceded 

 them must be looked for down towards the branches, 

 and from these again down towards the trunk. The 

 comparison of a tree, however, serves only as a 

 means of arrangement. On our grand tree of life, 

 the branchings are all unlike one another, and 

 show resemblances only where they are in close 

 proximity ; the farther they have branched off from 

 the tree the more different do they become. 



From another point also the figure of a tree 

 will give us a distinct idea of the actual state of 

 things. The farther we go back into the history 



