62 ELEMENTS OF BIOLOGY. 



may include a single species only, in cases where the com- 

 bination of characters which make up the species are so 

 peculiar that 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 families 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 (Agas- 

 siz). 



.' 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 degree to which specialisation of functions 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, however, the status of any given 

 animal depends upon two conditions one its morphologi- 

 cal type, the other the degree to which specialisation of 

 functions 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 functions, however great, will 

 place the former above the latter, as far as its type of struc- 

 ture 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 con- 



