BIB J. LUBBOCK ON THE 011IGIN OF INSECTS. 



423 



however slightly, it immediately becomes subject to the action 

 of distinct forces : natural selection affects it in two different 

 and, it may be, very distinct manners, gradually leading to dif- 

 ferences which may become so great as to involve an intermediate 

 period of change and quiescence. 



There are, however, peculiar difficulties in those cases in which, 

 as among the Lepidoptera, the same species is mandibulate as a 

 larva and suctorial as an imago. From this point of view, how- 

 ever, Campodea and the Collembola (Podura, &c.) are peculiarly 

 interesting. There are among insects three principal types of 

 mouth : — first, the mandibulate ; secondly, the suctorial ; and, 

 thirdly, that of Campodea and the Collembola generally, in which 

 the mandibles and maxillae are retracted, but, though far from 

 strong, have some freedom of motion, and can be used for biting 

 and chewing soft substances. This type is intermediate between 

 the other two. Assuming that certain representatives of such a 

 type found themselves in circumstances which made a suctorial 

 mouth advantageous, those individuals would be favoured by na- 

 tural selection in which the mandibles and maxillae were best 

 calculated to pierce or prick, and their power of lateral motion 

 would tend to fall into abeyance ; while, on the other hand, 

 if powerful 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 mastication 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 will not be denied that the 

 ten thousand variations in the mouth-parts of insects have special 

 reference to the 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 sug- 

 gested, the last of which seems to explain the possible origin 

 of species which are mandibulate in one period of life and not in 

 another. The change from the one condition to the other would 

 no doubt take place contemporaneously with a change of skin. 

 At such times we know that, even when there is no change of 

 form, the temporary softness of the organs often precludes the 



