24 MANUAL OF ZOOLOGY. 



no Other species exhibits similar structural characters ; or, on 

 the other hand, it may contain many hundreds of species. 



Families are groups of genera which agree in their general 

 characters. According to Agassiz, they are divisions founded 

 upon peculiarities of " form as determined by structure." 



Orders are groups of famiHes related to one another by 

 structural characters common to all. 



Classes are larger divisions, comprising animals which are 

 formed upon the same fundamental plan of structure, but differ 

 in the method in which the plan is executed (Agassiz). 



Sub-kingdoms are the primary divisions of the animal king- 

 dom, which include all those animals which are formed upon 

 the same structural or morphological type, irrespective of the 

 degi'ee to which specialisation of function may be carried. 



Impossibility of a Linear Classification. — It has sometimes 

 been thought that the animal kingdom can be arranged in a 

 linear series, every member of the series being higher in point 

 of organisation than the one below it. As we have seen, how- 

 ever, the status of any given animal depends upon two condi- 

 tions — one its morphological type, the other the degree to 

 which specialisation of function is carried. Now, if we take 

 two animals, one of which belongs to a lower morphological 

 type than the other, no degree of specialisation of function, 

 however great, will place the former above the latter, as far as 

 its type of structure is concerned, though it may make the 

 former a more highly organised animal. Every Vertebrate 

 animal, for example, belongs to a higher morphological type 

 than every Mollusc ; but the higher Molluscs, such as cuttle- 

 fishes, are much more highly organised, as far as their type is 

 concerned, than are the lowest Vertebrata. In a linear classi- 

 fication, therefore, the cuttle-fishes should be placed above the 

 lowest fishes — such as the lancelet — in spite of the fact that 

 the type upon which the latter are constructed is by far the 

 highest of the two. 



It is obvious, therefore, that a linear classification is not 

 possible, since the higher members of each sub-kingdom are 

 more highly organised than the lower forms of the next sub- 

 kingdom in the series, at the same time that they are con- 

 structed upon a lower morphological type. 



I p. Reproduction. 



Reproduction is the process whereby new individuals are 

 generated and the perpetuation of the species insured. The 

 methods in which this end may be attained exhibit a good 



