HIPPOLYTE VARIANS. 629 



So far^ we have considered the nocturnal condition as 

 induced by darkness^ or facilitated by the use of muslin 

 covers on the jars containing the animals. This phase may, 

 however, be effected in bright light. If specimens of Hip- 

 polyte which have stood for some hours in a white dish, 

 or which have just been caught, be exposed towards evening 

 in a porcelain dish to the full glare of the incandescent light, 

 they (in most cases) gradually assume the characteristic 

 nocturnal colour. These " light-induced " nocturnes differ in 

 several points from normal nocturnes, in which the phase 

 has been induced by waning light. They are no longer 

 keenly sensitive to moderate changes of light intensity ; 

 v\rhen placed in the dark or in dim light, if any colour-change 

 at all occurs in them, it is of the nature of an emphasis of 

 the nocturnal colour. In fact, the only means by which a fairly 

 rapid recovery of these " light-induced nocturnes^' can be 

 effected is the very drastic method of illumination by the 

 light of a sub-stage condenser. For these reasons " light- 

 induced nocturnes " have a peculiar value in experimental 

 work, and we have used them extensively for determining 

 the effects of stimulation on colour-changes. It may be 

 urged that the expression '^light-induced nocturnes" is a 

 misnomer, or at least far-fetched. If the criticism is delayed 

 till after the section on periodicity has been perused we think 

 that it will prove unfounded. We describe the phenomenon 

 here in order to collect together in one section all the facts 

 we have discovered concerning the conditions under which 

 nocturnal colour is assumed ; we offer the explanation later, 

 since it is only in the light of other phenomena that these 

 can, we think, be interpreted. 



Recovery from the Nocturnal Condition. — The 

 facts bearing on the recovery of natural and artificially in- 

 duced nocturnes require further discussion. Do nocturnes 

 always recover to the diurnal tint of the previous day ? 

 Have they the power, at the moment of recovery, to assimi- 

 late their colour with that of new weed if, meantime, they 

 have left weed of one colour and attached themselves to 



