262 CHAPTERS ON EVOLUTION. 



at first motionless and afterwards moving about. It is not difficult, 

 moreover, to show how very perfectly adapted to varied ways of life 

 different larvae have become. The worm-like form of those larvae 

 which live parasitically in the interior of other animals, or in plants, 

 may attract our notice as an adaptive feature. Such forms are well 

 represented in the young of the botflies, which pass their exist- 

 ence either within the digestive system of the horse or within 

 the tumours they form on the backs of cattle. Witness, on the 

 other hand, the strong-jawed, weak-legged larvae which burrow in 

 wood ; or the well-developed legs of those which feed on leaves or 

 which forage for animal matter. Compare with these features, 

 again, the degradation and modification causing those larvae which 

 are fed by parents or " nurses " (e.%. young ants and bees) in the 

 cells of their hives, to become fleshy, footless, inactive grubs ; whilst, 

 as a feature of highest interest, it may be noted that the bee grubs do 

 possess at one period of development rudiments of legs, which, how- 

 ever, soon disappear. The fact, as Sir J. Lubbock remarks, " seems 

 to show, not, indeed, that the larvae of bees were ever hexapod (or six- 

 legged), but that bees are descended from ancestors which had 

 hexapod larvae, and that the present apod (or footless) condition of 

 these larvae is not original, but results from their mode of life." The 

 changes which have converted bee larvae into footless grubs are, in 

 other words, not developmental, but adaptive. 



To follow out in detail the full history of insect metamorphosis 

 would be a task lying far beyond the scope or limits of the present 

 chapter, in which other details of varied developments have yet to be 

 noted. The key-note of metamorphosis, and its explanation, is 

 struck when the great fact of modification of the young, as well as of 

 the adult, becomes patent to us. Anything which may be said 

 further regarding metamorphosis, is*in reality an enlargement and 

 illustration of this thought. But we may, nevertheless, glance briefly 

 at one or two points in connection with the history of insects by way 

 of rendering clear the probable lines along which the production and 

 evolution of metamorphosis has taken place. We have seen that the 

 changes which an insect undergoes have reference, not so much to its 

 future form or adult state, as to its more immediate wants and to 

 the exigencies of its life when undergoing development. We have 

 noted likewise that the insect, like every other animal, is developed 

 and exists between two sets of conditions namely, those which 

 tend to keep it in its ancestral grooves, and those which tend to alter 

 its constitution through the influence of new surroundings. Insects 

 are known, further, to pass through every gradation of development ; 

 from slight change, limited to the moulting of the skin, to that which 

 is illustrated by the dissolution of the larval body and the rebuilding 

 of its frame to form the adult. Is there any information at hand, 



