86 



THE EVOLUTION THEORY 



Cox 



mole-cricket [Gryllotal'pa) have varied greatly, yet quite differently, 

 and the whole together forms a most effective digging-tool. With 

 it the animal digs out the earth before it to right and to left, and to 

 do this it makes with both legs simultaneous outward movements, 

 which ai-e otherwise quite unusual among insects, and does so with 

 such strength that Rosel von Rosenhof saw two bodies each weighing 

 three pounds pushed away in this manner. In this case four chief 

 parts of the leg (Fig. 104), the coxa {cox), the femur (/e), the tibia itih), 

 and the tarsi {tars) are so adapted to each other in form, joints, thickness 

 of skeleton, and size, that they cannot have varied otherwise than in 

 relation to each other, but each piece has done so in an individual 

 manner. Most remarkable of all is the short 

 broad tibia, equipped with four large, hard teeth, 

 which has to perform the digging in the ground 

 after the manner of a spade,' while the dispro- 

 portionately thin and weak tarsal joints, the last 

 of which bears two perfectly straight spines 

 instead of claws, are directed upwards, and do 

 not touch the ground, being no longer used for 

 walking. Rbsel supposed, probably correctly, 

 that they are used for cleaning the spade when 

 it becomes clogged up with earth, since the 

 animal cannot clean it with its mouth. These 

 quite unusually formed parts of the limb cannot 

 have become what they are as the direct results 

 of use, because, for one thing, it would have been 

 not their broad surfaces, but their narrow edges, 

 which would most easily cut through the earth, 

 that would have been directed outwards. The 

 peculiar curving, first concave, then convex, of 

 the outer surface of the digging foot is exactly what is best adapted 

 for cutting into the earth and for the pushing aside which follows, 

 but it is not what it would have become if the chitin-wall had yielded 

 to the pressure of the earth and adapted itself to it. But, as we are 

 again dealing with the chitinous skeleton, there can be no question of 

 the direct effect of use, and, it seems to me, it must be admitted that 

 here we have a case of co-adaptation of at least seven different parts, 

 which have varied independently of each other, without any 

 assistance from the Lamarckian principle. 



But much more complicated cases than this might be cited, if 

 we were in a position to estimate exactly the functional value of the 

 individual parts of the wing-venation in the different insects, for 



Fig. 104. Digging leg 

 of the Mole - cricket 

 {Gryllotalpa). cox, coxa 

 attaching the limb to 

 the thorax, fe, the short 

 broad femur. iib, the 

 tibia forming a broad 

 spade with six large 

 sharp teeth, tars, the 

 tarsal joints, which are 

 turned upwards and 

 cannot be used in loco- 

 motion. After EOsel. 



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