CLASSIFICATION OF THE ANIMAL KINGDOM. 85 



our making this assertion absolutely positive. If, however, we were 

 to find a pelvis with marsupial bones, we should not be justified in 

 asserting that the owner of the same must have possessed an in- 

 flected angle to the lower jaw. On the contrary, we know that such 

 an assertion would be erroneous, since the " marsupial bones " are 

 present in the Monotremes, in which the angle of the jaw has its 

 usual form. 



Classification of the Animal Kingdom. 



Vast as is the number of known animals, all, whether living or 

 extinct, may be classed under a limited number of primary divisions 

 or " morphological types," which are technically spoken of as the 

 " sub-kingdoms." There are also certain groups of animals (the 

 Molluscoids and Tunicates) which have not the value of " sub- 

 kingdoms," but which are so far separated by their characters from 

 their nearest relatives that it is expedient in the meanwhile to treat 

 them as constituting distinct divisions of the animal kingdom. All 

 the animals in any one sub-kingdom agree with one another in their 

 structural type, or in the fundamental plan upon which they are con- 

 structed, and they differ from one another simply in the modifications 

 of this common plan. Two animals belonging to different sub-king- 

 doms may be rendered closely similar to one another as the result of 

 similar adaptive modifications, but no amount of physiological like- 

 ness will counterbalance or efface the morphological unlikeness due 

 to the fact that each is constructed upon an essentially different 

 ground-plan. As the animals belonging to any given sub-kingdom 

 are separated solely by the characters due to varying modifications 

 of a common morphological type, it is possible to arrange the mem- 

 bers of each in an approximately linear series, in which the lowest 

 members most closely approach the primitive or ideal form of the 

 sub-kingdom, while the highest exhibit the greatest amount of com- 

 plexity and specialisation of this type. But it is not possible to 

 establish any such linear classification for the animal kingdom as a 

 whole. Given an animal of a lower " sub-kingdom " than another 

 animal, no amount of complexity, no specialisation of organisation, 

 can raise the former above the latter. The one may be the result of 

 the high evolution of a low morphological type, the other may be the 

 result of the low evolution of a higher morphological type, but the 

 superiority of the ground-plan gives the latter the higher place. 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 constructed upon a lower morphological 

 type. 



