48 CHAPTERS ON EVOLUTION. 



Having thus endeavoured to show the chief types of animal life, 

 we may now glance at the conclusions bearing upon the constitution 

 of the animal world to which our researches may legitimately be 

 presumed to lead. It thus seems clear, so far as our studies have 

 led us, that the constitution of the animal world is one in which the 

 development of its included units has followed a series of definite 

 plans or types, leading to the construction of the six or seven 

 primary groups into which the naturalist is accustomed to divide the 

 hosts of animal life. These " types," it must further be noted, are 

 not in any sense theoretical groups, but are founded, as we have 

 seen, on exact and fundamental likenesses in structure. Nor must 

 we lose sight of the exact meaning of the word " fundamental " as 

 thus employed. The use of this term implies that the likeness and 

 similarity in the plan admits of variation in the carrying out of its 

 details. The lobster and the butterfly, for example, are fundamentally 

 alike ; their bodies are constructed on an essentially similar plan ; 

 and the positions of their organs are identical. But whilst these are 

 fundamental likenesses, they do not imply that of necessity the two 

 bodies should be similar throughout. The tissues of a butterfly may 

 be more complex than those of a lobster, or vice versa ; just as the 

 heart or brain of a frog is a more complex organ than that of a fish, 

 and as each organ of a bird or a man shows, in turn, an advance 

 upon that of the frog. The variations, however, are all more or less 

 plain and evident elaborations of One type. As Cuvier put it, " the 

 ulterior divisions " of each type, or, in other words, its arrangement 

 into subordinate groups, drawn upon differences in the included 

 animals, are founded upon " slight modifications " of the type, or by 

 " the development or addition of certain parts " which parts, it may 

 be added, can, as a rule, be shown to be represented in one form or 

 another in the original constitution of the type. 



A second consideration of importance in discussing the constitu- 

 tion of the animal world consists in the emphatic declaration of the 

 modern naturalist that it is impossible to arrange animals in a linear 

 series, beginning with the lowest form and ending with man. The 

 nature of the constitution of the animal world, in short, does not 

 admit of any such arrangement ; since it would be manifestly im- 

 possible to determine in very many instances which, of two animals 

 or of two groups, should be ranked the higher. It would be a 

 puzzling, if not an impossible task for any naturalist to determine,, 

 for example, whether a cockroach or a cuttle-fish should be ranked 

 highest in the scale. The fact that the body of the one is constructed 

 on an utterly different type from that of the other, constitutes a primary 

 difficulty of no mean order ; and there intervenes a second considera- 

 tion, namely, that of the impossibility of settling any standard whereby 

 the organisation of the one might be legitimately compared with that of 



