xviii INTRODUCTION. 



group, great or small, there are certain types of structure, offering 

 fixed characteristic marks, and that analogies are, simply, the repre- 

 sentation in one group of a certain type in another or, to put it in 

 other words, that analogous groups or species simply occupy a cor- 

 responding place in their respective classes, orders, or families. 

 This theory of representation has, perhaps, been carried out, to 

 too great an extent, by certain writers, but, nevertheless, it appears 

 to be founded on nature ; and the existence of these, often unex- 

 pected analogies between distant groups and species, clearly 

 manifests the unity of the plan of the animal creation. According 

 to Mr. Darwin's views, such analogies might be explained on the 

 supposition that the resemblances were due to some remote ancestral 

 origin. The colours and markings of some birds appear to be repeated 

 in other groups ; and, in most natural divisions, great variety 

 of form of bills, and also of other parts is exhibited, representing 

 several distinct types ; and, in some, more distantly related groups, 

 analogy is shown by habits, by the colour of the eggs, by seasonal 

 change of plumage, &c., &c. Many examples of analogy will be 

 pointed out in the present work. 



On beginning at any point in any series of beings, and tracing step 

 by step, the scale of affinities, we soon find that the supposed chain 

 is interrupted, and that branches strike off in various directions. 

 That a linear arrangement is quite impossible has long been conceded 

 universally ; but what directions the divergencies take, is not agreed 

 on ; nor, indeed, have Zoologists of the present day decided, that 

 there is a fixed plan for any one class, still less that the same system 

 extends through all. Strickland, and quite recently, Wallace, have 

 attempted to show the affinites of some families and orders of birds 

 by means of diagrams. 



Certain English Naturalists, and simultaneously, one or more German 

 Botanists, have maintained that, in arranging any series of animated 

 beings, according to their affinities, the tendency is to revert to the 

 point whence they set out, not indeed in an unbroken line, but in a 

 series of circles. Thus, the circular system, as it has been termed, 

 has been strained, perhaps, too far by its exponents, but there is no 

 doubt that in many instances this tendency to a quasi-circular 



