GENERAL PRINCIPLES OF ZOOLOGICAL CLASSIFICATION 13 



groups or eras : the Archaean or oldest, the Palaeozoic, the 

 Mesozoic, and the Cenozoic or most recent. These are divided 

 into various systems or periods, which may be tabulated as fol- 

 lows, the most recent being at the top of the table, the oldest at 

 the bottom : — 



1 Recent 1 Quaternary or 



I . 



I Pleistocene Post Tertiary 



One of the most significant phenomena met with in any 

 attempt to classify animals lies in the fact that we find at times 

 groups whose affinities are doubtful. We may have, for example, 

 two groups of animals differing strikingly in certain morpho- 

 logical characteristics ; a third group may combine these differ- 

 ences and thus form a sort of connecting link between them, 

 while at the same time it is difficult to say which of the two it 

 resembles the more. So it is a matter of no little interest that 

 in attempting to separate plants from animals we should find 

 groups which show affinities to both kingdoms. 



The most important of these groups consists of the slime- 

 moulds ; they are fully described in most text-books of botany 

 under the name Myxomycetes, but since some zoologists classify 

 them with the unicellular animals as Mycetozoa (Gr. (iv/ci)?, a 

 fungus, and ^coov, animal), we may note briefly their structure 

 and life-history here. They are found on decaying organic 

 matter, such as leaves and bark, where they exist as irregularly 

 shaped masses of protoplasm, containing many nuclei and en- 



