Phylctic Parallelism in Mctamorpkic Species. 403 



equal degree in all the three stages of larva, pupa, 

 and imago, but that sometimes the caterpillar is 

 very variable and the pupa and imago quite 

 constant. On the other hand, all three stages may 

 be equally variable or equally constant, although 

 this seldom occurs. 



If variability is to be understood as indicating 

 the period of re-modelling of a living form, whether 

 in its totality or only in single characters or groups 

 of characters, from the simple fact of the hetero- 

 chronic variability of the ontogenetic stages, it fol- 

 lows that the latter can be modified individually, 

 and that the re-modelling of one stage by no 

 means necessarily entails that of the others. It 

 cannot however be doubted that variability, from 

 whatever cause it may have arisen, is in all cases 

 competent to produce a new form. From the 

 continued crossing of variable individuals alone, 

 an equalization of differences must at length take 

 place, and with this a new, although not always a 

 widely deviating, constant form must arise. 



That the different stages of development of a 

 species may actually be partly variable and partly 

 constant, and that the variable or constant cha- 

 racter of one stage has no influence on the other 

 stages, is shown by the following cases, which are, 

 at the same time, well adapted to throw light on 

 the causes of variability, and are thus calculated to 

 contribute towards the solution of the main pro- 

 blem with which this investigation is concerned. 



D d 2 



