296 Animal Life and Intelligence. 



the question in a more satisfactory form — Are the limits 

 of sensibility to light-vibrations the same in them as in us? 

 M. Paul Bert concluded that they are. But Sir John 

 Lubbock has, I think, conclusively shown that they are 

 not. For the full evidence the reader is referred to his 

 *' Senses of Animals." * His experiments on ants, with 

 which those of M. Forel are in complete accordance, 

 satisfied him that these little animals are sensitive to the 

 ultra-violet rays which lie beyond the range of our vision. 

 Other experiments with fresh-water fleas (Daphnia) showed 

 that they have colour-preferences, green and yellow being 

 the favourite colours. 



The daphnias were placed in a shallow wooden trough, 

 divided by movable partitions of glass into divisions. 

 Over this was thrown a spectrum of rainbow colours. The 

 partitions were removed, and the daphnias allowed to 

 collect in the differently illuminated parts of the trough. 

 The partitions were then inserted, and the number of 

 crustaceans in each division counted. The following 

 numbers resulted from five such experiments : — 



Dark. Violet. Blue. Green. Yellow. Eed. 



3 18 170 36 23 



Special experiments seem to show that their limits of 

 vision at the red end of the spectrum coincide approxi- 

 mately with ours ; but at the violet end their spectrum is 

 longer than ours. Sir John covered up the visible spectrum, 

 so as to render it dark, and gave the daphnias the option 

 of collecting in this dark space or in the ultra-violet. To 

 human eyes both were alike dark. But not so to the 

 daplmian eye ; for while only 14 collected in the covered 

 part, 286 were found in the ultra-violet. The width of the 

 violet visible to man was two inches. Sir John divided 

 the ultra-violet into three spaces of two inches each. Of 

 the 286 daphnias, 261 were in the space nearest the violet, 

 25 in the next space, and none in the furthest of the three 

 spaces. From which it would seem that, though these 

 little creatures are sensitive to light of higher vibration- 

 * Chap. X. p. 202. 



