THE NATURALIST S OCCUPATION. 4/ 



and generally so in definition, the addition of new- 

 segments taking place from behind. TJiere is not the 

 least ground for supposing that a single segment has 

 ever been, or can ever be, added to the anteiHor e7id. 

 If, in the course of development, segments disappear, 

 the loss is borne by the posterior end, as we see when 

 the tadpole lays aside its fish-like tail in rising to the 

 estate of frogdom. The direction of loss is the reverse 

 of that of acquisition, the one travelling away from, the 

 other towards, the head. Thus the point of maximum 

 variability in number is always most remote from the 

 head. Both the lav/s of development and the condi- 

 tions of continued existence tend to strengthen the dis- 

 tinction. The head segments developing first, have the 

 advantage in the struggle for existence, and their su- 

 preme importance is the guarantee of their perma- 

 nence. 



Although there is not the least probability, and 

 scarcely a possibility, of adding or interpolating entirely 

 new segments in the head region, and although the 

 chances of loss appear to come to a vanishing point a 

 long way behind this region, still the shadow of un- 

 certainty is not dispelled, and we have to acknowledge 

 that we do not yet know how far the transforming 

 influence of functional changes and substitution of 

 organs has here been felt. 



In looking around us for a possible foothold, we in- 

 quire, first of all, if there are not some structures con- 

 nected with the fore-brain on which the seal of metam- 

 erism has left an indelible impression. One pair of 

 these so-called cerebral nerves, the olfactory, have fast 

 been losing their high claims to a position of isolation, 



