260 CHAPTERS ON EVOLUTION. 



phosis," in the sense in which that term is used as regards the 

 butterfly, does not occur at all. The male insect, it is true, develops 

 wings at a late stage of development, but there is no chrysalis-stage 

 and no quiescence, as in the butterfly or beetle. 



How, then, it may be asked, are the differences between the 

 metamorphoses of insects to be accounted for ? On the theory that the 

 development of the individual recapitulates the evolution of the race, 

 it may be asked, do not the facts and differences of metamorphosis in 

 insects seem to present very grave difficulties? There is not that general 

 likeness seen, for instance, in the young of different Crustaceans, nor 

 the similarity in development witnessed in the Echinoderms or starfish 

 group. These apparent difficulties, however, become greatly lessened, 

 or may disappear entirely, if we bear in mind the fact, already insisted 

 on, that as adult animals vary and alter, and so evolve new species, 

 so the young and developing forms are even more subject to modifi- 

 cation whilst in the process of growth. In other words, let us clearly 

 understand that the changes an animal or plant may undergo are two 

 in number. Firstly, there are developmental changes, or those we have 

 been tracing in previous papers, which belong to the animal as part 

 of its inheritance, and which cause it to travel along the line of its 

 ancestry towards the likeness of its parent. Then there are, secondly, 

 changes which must be named adaptive; which arise from the opera- 

 tion of surrounding circumstances heat, cold, food, &c. and from 

 the action upon the living being of external forces. These latter are 

 changes " adapting " it to, it may be, new ways and walks of life, 

 differing from those of its parents and ancestors, and remodelling its 

 frame in a novel guise. The young insect, in truth, may be described 

 as living between two sets of forces or conditions. One set may be 

 named " centripetal," or centre-seeking, for want of a more descrip- 

 tive term. These are developmental changes which tie it to its 

 type and which cause it to travel along the beaten track of its race. 

 Then there are the " centrifugal " or adaptive forces, which tend to 

 make its development erratic, which may cause it to fly off at a 

 tangent, so to speak, from its normal way of growth, and which 

 necessarily cause it to differ materially from its type and race. Says 

 Darwin, in speaking of development at large : " Many insects, and 

 especially certain crustaceans, show us what wonderful changes of 

 structure can be effected during development." A little later he 

 proceeds to remark that, whilst similar organs in the young of different 

 animals " often have no direct relation to their conditions of exist- 

 ence " (e.g. the gill arches of a quadruped, a bird, a frog, and a fish), 

 the case is " different when an animal, during any part of its embry- 

 onic career, is active and has to provide for itself. The period of 

 activity," says Darwin, " may come on earlier or later in life ; but 

 whenever it comes on, the adaptation of the larva to its conditions 



