ANALOGIES WITH PARTICULAR HUMAN MENTAL STATES Sleep and Dreams of Animals.—A condition corresponding to our sleep exists in vertebrates, particularly in mammals and birds. All sensory activities are then reduced. The capacity of executing voluntary muscular movements is interrupted, while the automatic movements and activities continue as usual. The eyes are generally protected against optical stimuli by the lids or nictitating membranes. At the same time many animals assume a characteristic sleep-position. A distinction between diurnal and nocturnal animals can be made according to the time of day at which the animals rest in this way. But not infrequently animals go through several phases of rest and activity during the 24 hours.
In the mouse the number of these periods is particularly great. Sleep is more especially a necessity for animals high up in the scale as regards their mental capacities. Thus it is comprehensible that guinea-pigs spend only i 1% of the whole 24 hours in resting and their longest periods of rest are only 10 minutes' duration. They feed continuously so long as food is offered to them.
Many reptiles and amphibians likewise assume typical sleeping postures. Snakes, lizards and urodele amphibians frequently curl themselves up, while tortoises withdraw their head, tail and extremities beneath their protective shield. But whether or not this outward rest is really accompanied by an internal recupera tion as in our own sleep must remain questionable. The opinion has also been advanced that sleep appeared first in the animal series with the development of the forebrain. Against this is the fact that numerous fishes exhibit a state of rest with reduced sensory activity and cessation of movement. Both in nature and in aquaria fishes may not infrequently be observed to rest motion less on the bottom or against a stone with scarcely a movement of the gill-covers or fins. Others float right up against the surface of the water as if lifeless. This resting condition is known in the big shark of the North Atlantic, in the moon-fish Orthagoriscus, and in bream, all of which are met with in calm water drifting motionless at the surface. Their sensory activity is so much reduced that they may be approached unperceived. Finally, in
various invertebrates temporary motionless states are known. Particularly is this to be observed in insects. According to the time of day at which they are active insects can also be divided into day and night animals. The males of many species of bees "sleep" in bell-shaped flowers which protect them from rain and dew. The position assumed by resting insects is generally that of the normal motionless state. Certain South African Hymenop tera of the genus Ammophila rest at night in a very peculiar fashion. They hold on to twigs with their jaws, and, letting go with the legs, allow the body to hang down freely. Among the higher Crustacea, too, and also in the flat-worms (planarians) resting states are known, with diminished sensitiveness to stimuli. In general, it may be stated that a condition of sleep comparable with our own exists in the higher vertebrates, but that the lower down we descend in the animal series the more we encounter resting states which differ from sleep. Among the lower inverte brates sleep is certainly absent.
During the sleep of higher mammals, particularly apes and carnivores, sounds and spontaneous movements may be observed which suggest a comparison with what can be seen to occur when our fellow men dream. During sleep a dog may bark, growl, wag its tail and jerk its limbs, while a cat will spit. Ungulates also frequently make movements in their sleep. Presumably phenomena take place in the central nervous system of these animals similar to those occurring in ourselves when we expe rience a dream. But we must be clear that the content of the animal's dream can never overstep the picture of the world with which the waking animal is endowed. A number of investigators prefer to deny the existence of animal dreams altogether and to see in the sleep-movements nothing but the effects of internal nerve stimuli. But if we concede any conscious life to the animals in question there is no reason for denying dreams to them. By analogy with our own dreams we are then obliged to suppose that in animals likewise the dream content is more confused and poorer than the waking consciousness.