INSTINCTS AND HABITS IN CHICKS 75 



if the word " trial " in the above expression is meant to refer to 

 the principle of random activity, and " error " to learning based 

 on the avoidance of harmful stimuli, still " trial and error " is 

 an inadequate descriptive term. One could as well say the 

 general law is, " Prove all things; hold fast that which is good." 

 If the term selection be construed broadly enough to include 

 both the positive and negative aspects, then trial and selection 

 appears on the whole to be a more satisfactory name for this 

 type of modification. 



If the " index of modifiability," to borrow Yerkes' term and 

 definition, be considered " that number of tests after which no 

 errors occur for at least thirty tests," the index of modifiability 

 of these Barred Plymouth Rock chicks, under the conditions 

 described, is 72.2. 



The rate of modification for the same nine chicks is set forth 



graphically in fig. 11. This is a success curve plotted from 



data in tables 13 and 20. The averages upon which the curve 



is based, including the preference test, are as follows: 4.7, 3.3, 



5.2, 6.1, 7.8, 7.9, 8.8, 8.9, 9.8, 10, 9.4, 10, 10, 10. It is quite 



possible that valuable accretions to our knowledge of habit 



and instinct may come through a careful study of the course 



of development of reactions of both types and a comparison 



of'the quantitative relations involved. I offer the curve of 



the development of the complete pecking co-ordination (fig. 7) 



and that of the development of the black-blue modification 



(fig. 11) as suggestions for the beginning of a more specialized 



investigation. 



SUMMARY AND CONCLUSIONS. 



The early post-embryonic life of the chicks continued the 

 scope of activities already begun in the egg. The alternations 

 of passivity and activity, the lifting movements of the head 

 combined with stretching movements of the legs, the occasional 

 reflex forward thrust of the bill followed usually by movements 

 of the mandibles, loud chirping along with other violent activity, 

 — all these were common aspects of the behavior of chicks just 

 before -as well as irrimediately after hatching. The pecking 

 reaction might have assisted in the process of exclusion, but 

 by far the most common reaction while the chick was struggling 

 in the egg was the lifting movement of the head and bill. This 

 reaction actually broke shell and tore confining membranes. 



