216 DEVELOPMENT AND HEREDITY. 



changes from egg to maturity occurring with grad- 

 ually decreasing rapidity throughout the whole period 

 of development, and no sudden and striking meta- 

 morphosis takes place in the last stage. This 

 method we must suppose also to have been followed 

 at some time in the past by the ancestors of the 

 butterfly. What causes may have effected a change 

 in this simple manner of development and produced 

 the present method, we cannot definitely determine. 

 But still, from our knowledge of the nature of the 

 hereditary impulse on which this change has been 

 wrought, and from our knowledge of the present 

 manner of growth, we may gain some idea of the 

 nature of these causes. Imagine some ancestral 

 form of the butterfly in which the development was 

 simple and gradual from beginning to end, without 

 any sudden metamorphosis, in fact, such a method of 

 development as that of the grasshopper. Suppose 

 such a change in the environment of this organism 

 that on reaching the stage of development equivalent 

 to the caterpillar stage, the organism should meet 

 with conditions of existence much easier than hith- 

 erto. Let its food become very abundant, and let 

 its external conditions remain for a long time un- 

 changed. This of itself, we have seen in discussing 

 parasitism, would be enough to hinder its develop- 

 ment, through the withdrawal of its customary 



