IMPOSSIBILITY OF A LINEAR CLASSIFICATION. 21 



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. 

 (Agassiz.) 



tiub-ldngdoms 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 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, however, 

 the status of any given animal depends upon two conditions 

 one its morphological type, the other the degree to which spe- 

 cialisation 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 struc- 

 ture is concerned, though it may make the former a more 

 highly organised animal. Every vertebrate animal, for ex- 

 ample, belongs to a higher morphological type than every Mol- 

 lusc; 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 classification, 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 pos- 

 sible, since the higher members of each sub-kingdom are more 

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

 dom in the series, at the same time that they are constructed 

 upon a lower morphological type. 



10. 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 



