196 DEVELOPMENT AND HEREDITY. 



Still produced in equal numbers. Further changes 

 in environment might entail some periodic change 

 in the food-supply, or some periodic struggle with 

 enemies. In the former case we might have the 

 original form gradually producing an alternate new 

 form to meet the periodic demand for the new 

 method of getting food. In the latter case the 

 tendency would be to produce an alternate new 

 form to carry on the regularly recurring wars. 

 Through later changes of environment these new 

 forms might also lose their fertility, the old original 

 form being the only form remaining fertile. At 

 the same time, instead of being alternate or peri- 

 odic generations, all four forms might become 

 simultaneous, the difference of development in each 

 form depending on the treatment received by the 

 embryo. Forms of this kind once produced would 

 persist unless some hostile change of environment 

 and natural selection should eventually obliterate 

 them. 



We must not forget to make allowance for the 

 effect of instinct in modifying the growth of these 

 social animals. While many are to be regarded as 

 imperfectly developed, some particular instinct seems 

 to be perfectly developed, or, perhaps, over-devel- 

 oped, thus influencing them to a particular line of 

 activity, which must have its effect upon their 



