XXXll INTRODUCTION. 



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, in- 

 deed, 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, Wal- 

 lace, have attempted to show the affinities 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. This, 

 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 

 arrangement appears to exist in nature, and even Wallace's 

 diagrams show this. It appears, however, according to 

 some, that the affinities of the species of any group are 

 various, and cannot be expressed by figures, every natural 

 group and species being connected not with two only, but 

 with several ; and it is possible that any natural group, if 

 we possessed all the forms which it comprised, would pre- 

 sent links of transition towards all the other groups of the 

 same family or order. Many examples might be given to 

 show the tendency to a circular arrangement, but I shall 

 content myself by pointing out to the student this sup- 

 posed feature, to verify, or otherwise, in any group he 

 may be studying. Many gaps of course occur in follow- 

 ing the chain of affinities, some very great, others easily 

 bridged over. These of course are stumbling blocks in 

 the way of such as believe in a complete chain or circle ; 

 and the fossil remains of birds, hitherto discovered, have 



