134 K. S. LASHLEY AND JOHN B. WATSON 



age, a monkey is more helpful and intelligent, and in fact all its instincts are strongly- 

 developed at a comparatively early age. In about a month the young one begins 

 to pick up grain and other food, and then the struggle for life soon begins, and 

 the mother and the young one commence to fight over their food, although their 

 natural instincts bind them to each other at other times." 



SUMMARY OF DEVELOPMENT 



Physical 



At birth the little monkey was far advanced in physical 

 development. His body, although less than 20 cm. in length, 

 differed from the proportions of the adults to a much, less extent 

 than does that of most young animals. His head was dispro- 

 portionately large, but not so much so as that of the human 

 infant ; his hind-quarters were considerably smaller proportion- 

 ately than are those of the adult monkey. The body was cov- 

 ered with hair, distributed as in the adult with the exception 

 of a peculiar bare line following the sagittal suture of the skull, 

 where in the adult there is a heavy growth of hair. (This is 

 shown distinctly in Plate II.) 



The eyes were open at birth. They were at first light blue 

 and did not reach the dark brown color of the adult until the 

 ninth week. 



As far as the crude methods employed could determine, the 

 sensory development of the monkey seems to be complete at 

 birth or shortly afterward; reactions to light appeared on the 

 second day; painful and tactile stimuli brought responses on 

 the first. The time of the beginning of the auditory function 

 is not known with certainty but there is some evidence that 

 sounds were heard on the second day. It was impossible to 

 stimulate the other sense organs in order to determine their 

 condition until much later, at which time they were completely 

 functional. 



The early, post-natal development was largely that of the 

 motor apparatus, the range of activities keeping step with 

 growth and increase in muscular strength. The rapidity of 

 growth has been less than might have been expected to accom- 

 pany the rapid increase in motor agility of the little monkey; 

 the most marked increase in size being in the hind limbs. At 

 the age of four months the baby has gained the proportions of 

 the younger adults although his body length is less than 

 one -half that of the adult. 



