Architecture in the Animal Kingdom. 97 



variety of performances is found in the jaws (mandi- 

 bles) of the ants. Of course, in digging burrows in 

 the earth and in constructing earth-works they are also 

 assisted by their fore-legs, which help partly to scrape 

 up the sand and partly to hold down and fasten pellets 

 of earth. In closely allied species the shape of these 

 instruments, and especially that of the all-important 

 toothed inner edge (cutting edge) of the mandible, 

 is as a rule so similar 1 that the specific differences in 

 architectural style can be accounted for only by the 



Fig. 2. Fig. 3. 



Left mandible of Formica rufa. Right mandible of F. sanguined. 



(Worker.) (Worker.) 



instinctive preferment of a particular style on the part 

 of different ant species. In the case of ants, therefore, 

 it will never do to resort to the mechanical automatism 

 of animal activities, .and to explain the differences of 

 instincts merely by differences of bodily organs. The 

 decisive factor is the psychic variety of instinctive dis- 

 positions. By them the bodily organs, in themselves 

 indifferent, are directed in their various modes of 

 operation. 



It is true, to a certain extent, that the nature of 



*) See the subjoined cuts. Both are drawn with the Zeiss* 

 microscope, syst. A., and Abbe's Camera lucida. 



7 



