236 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



system. The experiments were conducted in two 

 ways. In the first, the larvae were induced to suspend 

 themselves from sheets of clear glass, by placing them 

 in wide shallow boxes, so that the ascent to the glass 

 roof was easily accomplished. As soon as suspension 

 (stage iii.) had taken place, each larva was covered 

 with a cardboard tube, divided into two chambers by 

 a horizontal partition, which was fixed rather below 

 the middle. There was a central hole in the partition 

 just large enough to admit the body of the larva. The 

 tube was fixed to the glass sheet with glue ; the upper 

 chamber was lined with one color, e. g. gilt, and the 

 lower chamber with the opposite color, e. g. black, 

 with which the outside of the cylinder was also cov- 

 ered, in case the larva should stretch its head beyond 

 the lower edge. The partition was fixed at such a 

 height that the larval head and rather less than half of 

 the total surface of skin were contained in the lower 

 chamber, while rather more than half of the skin sur- 

 face was contained in the upper chamber. 



"The second method of conducting the conflicting 

 color experiments was superior in the more equal illu- 

 mination of the upper and lower colors. The bottom 

 of a shallow wooden box was covered with alternate 

 areas of black and gilt papers, and partitions were 

 fixed along the lines where the two colors c'ame in 

 contact. Each partition was gilt toward the gilt sur- 

 face, and black toward the black surface, and was per- 

 forated close to the bottom of the box with holes that 

 would just admit the body of a larva. The box was 

 th,en placed in a vertical position towards a strong 

 light, so that the partitions became strong shelves, 

 while the black and gilt surfaces were uppermost alter- 

 nately. As soon as a larva was suspended to a glass 



