MACKIE— FIRST TKAOES OP TUE SUCCESSION OF LIFE, 345 
allow themselves to get confused with the vanety of terms used by 
different authors to designate the same or nearly the same classes of 
animals or plants. Such differences are no more than the natural 
results of the attempts to found a perfect system of classification 
while necessarily beginning to make that effort with imperfect 
materials, which tlrrough many deficiencies in our knowledge present 
also great gaps and voids in the desired continuity of the order of 
arrangement of organized beings. By degrees such gaps are filled 
up in the progress of our investigations, and by the acquirement of 
additional knowledge of the structm'al character of known species, 
or the discovery of new forms. 
Under the scientific classification presented in further detail in 
Table II., existing forms of animals can be more or less harmoniously 
arranged. It may be regarded as that generally received, the terms 
being those ordinarily in use by the principal writers ; and in it we 
have included the latest revisions and amendments in the arrange- 
ment of the Mammalia by Professor Owen, whose indefatigable 
researches, skilful observation, and perspicuous deductions have long 
since placed him in the foremost rank of naturalists, whether past or 
present. 
By him this most important class has been primarily grouped, 
according to the characters of the brain, into four principal divisions, 
which thus displayed also exliibit their comparative intellectual 
capacities. 
For the complete grouping and arrangement of the animal and 
vegetable kingdoms it is necessary that all those fossil forms, often so 
vddely different from those existant, which palaeontology has added, 
and is still daily adding, to the fauna and flora of our planet in its 
completeness, should be included and brought into one and the same 
harmonious grouping. Thus does every new form exhumed fi'om the 
great cemetery of the Past add some new link to or produce some fresh 
de'saation from our latest and most complete results of airangements. 
It is, however, not a little curious to find the relics of past ages sup- 
plying the gaps and deficiencies of the creation around us, and form- 
ing the links between what were previously considered aberrant and 
abnormal conditions ; every step of progress adding to the beauty, 
harmony, consistency, and unity of the great plan of creation. 
