72 ON THE ORIGIN AND [CHAP. 



abeyance ; while, on the other hand, if masticatory 

 jaws were an advantage, the opposite process would 

 take place. 



There is yet a third possibility namely, that 

 during the first portion of life, the power of mastica- 

 tion should be an advantage, and during the second 

 that of suction, or vice versa. A certain kind of food 

 might abound at one season and fail at another ; 

 might be suitable for the animal at one age and 

 not at another. Now in such cases we should have 

 two forces acting successively on each individual, and 

 tending to modify the organization of the mouth in 

 different directions. It cannot be denied that the 

 innumerable variations in the mouth-parts of insects 

 have special reference to their mode of life, and are of 

 some advantage to the species in which they occur. 

 Hence, no believer in natural selection can doubt 

 the possibility of the three cases above suggested, 

 the last of which seems to throw some light on the 

 possible origin of species which are mandibulate in 

 one period of life and not in another. Granting then 

 the transition from the one condition to the other, this 

 would no doubt take place contemporaneously with a 

 change of skin. At such times we know that, even 

 when there is no change in form, the softness of the 

 organs temporarily precludes the insect from feeding 

 for a time, as, for instance, in the case of cater- 

 pillars. If, however, any considerable change were 

 involved, this period of fasting must be prolonged, 

 and would lead to the existence of a third condition, 

 that of the pupa, intermediate between the other two. 

 Since the acquisition of wings is a more conspicuous 



