INTRODUCTION 3 



tree, whose trunk is buried in the past, and many of whose 

 twigs and branches have decayed. 



Every large phylum, again, can be resolved into a number 

 of smaller groups all of which differ from one another, though 

 they agree in fundamental points of structure. Such large 

 groups within the phylum are known as Classes. 



Every large Class, again, is composed of a number of 

 smaller groups, or Orders, all of which, beyond their common 

 class-features, have their own distinctive peculiarities. 



Every large Order, again, can be split into smaller groups, 

 or Families, and every large Family into Genera, and every 

 large Genus into Species, which are the ultimate units of 

 ordinary classification. 



As with the phylum, so with its subdivisions ; on the 

 theory of Evolution the successive subdivisions are supposed 

 to represent minor branches and shoots of the great 

 genealogical tree, and the constituent members of each sub- 

 division are regarded as the descendants of a common ancestor 

 in their own degree, the final series, or species, being the 

 collateral descendants of the supposed grand-ancestor of the 

 phylum. 



The successive subdivisions of the animal kingdom are 

 not to be taken as concrete objects, but as abstractions 

 representing a narrowing series of general concepts. What 

 the zoologist deals with are individual animals, and these 

 after due examination he labels as species, and pigeon- 

 holes in genus, family, order, and so on ; and since every 

 man's concepts are his own, it is inevitable that there 

 must be considerable difference of opinion as to the com- 

 pass of a species and genus, and even of an order, or of a 

 phylum. 



As regards the term "species," the ordinary zoologist 

 applies it to a group of individuals, existing in a state of 

 nature, that he cannot distinguish one from another, by any 

 ^•o«j/ff;2/ characters that are not sexual. The 'individuals of 

 the group may — leaving sexual characters out of account — 

 differ one from another in little points ; but if the amount of 

 difference be so fluctuating and inconstant that it cannot be 

 readily and precisely defined, or if it can be shown to depend 

 upon certain periodically-recurring or seasonal conditions. 



