CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 51 



dealing with the origin of these plans of animal life ; and the 

 conclusion that these types have each had an independent origin 

 might seem, at first sight, to possess actual warranty for acceptation 

 and belief. But a fuller consideration of the constitution of the 

 animal world will tend to dispel any such tacit agreement concerning 

 the actual independence and distinctness, or regarding the separate 

 origin of the animal types already noted. On the contrary, a 

 deeper acquaintance with facts as they stand, will inevitably tend to 

 show us, firstly, that the limits of the types are by no means so 

 rigidly circumscribed as the older naturalists supposed. Whilst, 

 secondly, we may discover that evidence exists to show, not merely 

 that the various types are by no means sharply demarcated from 

 each other, but that in the nature of things they exhibit relationships 

 of the highest importance in the attempt to discover the exact nature 

 and constitution of the animal world. 



With regard to these latter contentions it is easy to show, for 

 example, that the great " types " of animal life, whilst remaining 

 distinct enough to constitute divisions of utility in classifying animals, 

 nevertheless often merge into one another, and become con- 

 nected by " intermediate forms " that is, are linked together by 

 animals or by groups which may be termed " transitional " in every 

 respect. If the existence of such links between any of these types 

 be proved, the distinct and utterly separated character of all may 

 logically be denied. The fact that the types are connected in any 

 fashion, must also be held as showing that some form of progression 

 from one to the other must be postulated as an essential part and 

 feature of the animal constitution. In other words, we are led to 

 believe in the continuity of these types, as opposed to the idea 

 of their separate origin. We are led to espouse the idea of an 

 uninterrupted development, as opposed to that of the separate and 

 independent origin of the great plans of animal structure. It is 

 interesting, however, in the first place, to find that there is an 

 unmistakable reflection of such a continuous development to be 

 discovered within the limits of each type ; and to this latter aspect, 

 or that concerning the types themselves, it may now be well to 

 direct our attention. 



If we select any type, from the lowest to the highest, we may 

 readily discover that its included animals exhibit amongst themselves 

 a connected relationship such as the mere fact of their bodies being 

 built upon one and the same plan would of itself be sufficient to 

 suggest. Amongst the Articulate animals, for instance, this relation- 

 ship is plainly seen ; and it is no less evident amongst the Vertebrates 

 and Molluscs, as will be more plainly shown in succeeding chapters. 

 Why, it may be asked, should the segments or joints of the lobster's 

 body (Fig. 2) and of the insect frame be constructed on one and the 



