82 THE EVOLUTION THEORY 



slight changes in the form of the body, especially of the limbs, with 

 their teeth, bristles, spines, and so on. These new or transformed 

 parts are formed before the throwing-ofF of the old chitinous shell, 

 and under its protection, and they are brought about by an elaboration 

 or transformation of the living soft matrix of the skeleton, the hypo- 

 dermis, which consists of cells, and is the true skin. They must thus 

 have arisen in the ancestors of our modern Arthropods in the same 

 way, that is, not by a gradual modification during use, but by a slight 

 sudden transformation before use. The steps in the transformation 

 may have been very small, a bristle may have become a little longer 

 in the second stage of life than it was in the first, or instead of five 

 bristles a particular spot may bear six in the second or third stage of 

 life ; but the variations in the phyletic development must always be 

 caused by germ-variations Avhich eft'ect from within the variation in 

 the relevant stage of dev^elopment. But the part which has varied 

 can only function after it has become firm and immodifiable. 



If these circumstances be kept clearly in mind, they fur- 

 nish a quite overwhelming mass of proof against the views of the 

 Lamarckians. 



Furthermore, it is not even true that the thickest parts of the 

 external skeleton are those at which the muscles are inserted. The 

 wing-covers of beetles ofier the best proof to the contrar}^, for there 

 are no muscles at all in them, yet they are, in many species, the 

 hardest and thickest part of the whole chitinous coat of mail. The 

 reason is not far to seek ; they protect the wings and the soft 

 skin of the back, which lies concealed beneath them, and the muscles 

 are inserted in this ! — a relation which can be explained only by 

 its suitability to the end, and not as due to any direct effect. 



When we remember the origin — which we have just described — 

 of the external skeleton from the soft layer of cells underneath it, the 

 thickness of the chitinous skeleton, which is very difierent at different 

 places in the same animal, but always adapted to its end, furnishes 

 a case of co-adaptation in parts which have a purely passive function. 

 The thickened part cannot be due to the insertion of a muscle, but it is 

 always there in advance, from internal causes, so that the muscle finds 

 sufficient resistance. Close to it there may lie, perhaps, the edge of 

 a segment, and at this spot the chitinous skeleton becomes almost 

 suddenly thinned to a joint membrane capable of being bent or folded, 

 not because there was no pull from the muscles at this spot, but in 

 order that the two segments may be connected movably. Thus, 

 now^here in the whole body of the Arthropod can the adaptation of the 

 skeleton, in regard to thickness and power of resistance, be regulated 



