THE PSYCHIC DEVELOPMENT OF YOUNG ANIMALS 161 
I have noticed in these and other puppies a quick 
recognition of human association through what I have 
termed the lip-call, not to be identified with any other 
sound. Is this the result of heredity to any extent, 
this sound having been used more than any other in 
attracting the attention of dogs? But so readily are 
psychic associations formed that one must not be sure 
of this. The dog, above all our domestic animals, is a 
plastic creature, and his life is made up largely of 
associative reflexes and kindred neuroses with corres- 
ponding psychoses. This principle I regard as a key 
that unlocks more of the secret places of canine 
nature than perhaps any other, unless it be heredity 
itself. 
Humovur.—tThe records of the 26th and 33rd days 
seem to show that even such young puppies appreciate 
fun or humour, much as a child does, and this can be 
almost daily observed in mature dogs. 
ATTENTION AND FATIGUE.—My observations on these 
subjects, some of which I have attempted to record, 
show, in the plainest way, how very readily a puppy is 
fatigued, but also indicate a gradual improvement in 
this respect. This readiness in experiencing fatigue 
explains why, moreover, one observer may be led to 
question the observations of another on very young 
animals. Again and again have I failed in my attempt 
to get the same result on repetition. In fact, the rule 
up to about the 20th day was, that success on repetition 
of certain stimuli was very doubtful owing to fatigue. 
This is well illustrated in the case of the growling 
reflex, etc. of the 17th day, but it applies to all the 
senses and the whole life of the animal, somatic and 
psychic. 
For this reason sleep follows at once on the exertion 
of play, with its physical movements and its sensory 
stimuli tending to exhaust. 
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