THE NATURALIST S OCCUPATION. 45 



nally to the trunk, but now pressed into the service of 

 the head. This conversion of trunk into head, in answer 

 to the greater and greater demands made upon the brain, 

 as the vertebrate line rose in the scale of development, 

 is just that kind of economy which nature everywhere 

 practises, and which we find exhibited in most instruc- 

 tive grades of elaboration in the nervous system of in- 

 vertebrates. That this has been the history of the mid 

 and hind portions of the vertebrate brain, is a truth 

 resting upon so many convergent lines of evidence that 

 there is no longer room for scepticism. 



The fore-brain, in which the problem culminates, is 

 still enveloped in a dense cloud of uncertainty, pierced 

 by so few and feeble rays of light that we are compelled 

 to accept the lead of conjecture, or to abandon the hope 

 of further advance. We are limited to three hypotheses : 

 We may assume with Balfour, that the fore-brain is the 

 unsegmented brain inherited from an invertebrate ances- 

 tor ; or with Kolliker, that it is a new formation, repre- 

 senting an outgrowth from the unsegmented anterior 

 end of the primitive nervous axis; or with Kleinenberg, 

 that it represents a number of fused trunk segments, in 

 which the ancestral brain — the " head-glanglion " of 

 annelid worms — has either been absorbed beyond the 

 hope of identification, or totally suppressed. 



Balfour's view marks the level of investigation ten 

 years ago. Since that time the progress of discovery 

 has been steadily in the direction of Kleinenberg's view. 

 But we have reached a point where direct, demonstrative 

 evidence appears to vanish, and it is only by the circui- 

 tous route of circumstantial evidence that we can push 

 onward. The solution we are looking for does not lie 



