214 DEVELOPMENT AND HEREDITY. 



the strongest where the separate acts of practice, 

 i.e. the successive repetitions, are exactly the same ; 

 any slight changes in the successive repetitions tend 

 to lessen their power to form nervous associations. 



This larval period, when the imperfect animal is 

 shifting for itself, living a free life in a variable 

 environment, might be a period of considerable 

 diversity of individual conditions, and be followed 

 later by a period in which the conditions would be 

 the same for all individuals. Take, for example, the 

 development of the butterfly. First, comes the 

 embryonic period, where the conditions are the same 

 for all individuals of the species ; the hereditary im- 

 pulse is accordingly strong, and the development 

 rapid and alike in all individuals. Then comes the 

 larval period, when the organism must seek its own 

 food, and the individuals must meet with diverse 

 experiences of life. In this period the development 

 is slower, and individuals of the same brood some- 

 times develop slight differences, e.g. in coloration. 

 Third, comes the chrysalis stage, when the condi- 

 tions are again uniform for all individuals. In this 

 stage the hereditary impulse is again very strong, 

 and the greatest changes of development take place, 

 and follow with great certainty and uniformity for 

 all individuals and generations. The rapidity of de- 

 velopment, however, at this stage, is modified by and 



