The evolution theory . thing, it w^ould have beennot their broad surfaces, but their narrow edges,which would most easily cut through the earth,that would have been directed outwards. Thepeculiar curving, first concave, then convex, ofthe outer surface of the digging foot is exactly what is best adaptedfor cutting into the earth and for the pushing aside Avliich follows,but it is not what it would have become if the chitin-wall had yieldedto the pressure of the earth and adapted itself to it. But, as we areagain dealing with the chitinous skeleton, there can be no question ofthe direct effect
The evolution theory . thing, it w^ould have beennot their broad surfaces, but their narrow edges,which would most easily cut through the earth,that would have been directed outwards. Thepeculiar curving, first concave, then convex, ofthe outer surface of the digging foot is exactly what is best adaptedfor cutting into the earth and for the pushing aside Avliich follows,but it is not what it would have become if the chitin-wall had yieldedto the pressure of the earth and adapted itself to it. But, as we areagain dealing with the chitinous skeleton, there can be no question ofthe direct effect of use, and, it seems to me, it must be admitted thathere we have a case of co-adaptation of at least seven different 2:)arts,which have varied independently of each other, without anyassistance from the Lamarckian principle. But much more complicated cases than this might be cited, ifwe were in a position to estimate exactly the functional value of theindividual parts of the wing-venation in the different insects, for. Fig. the Mole - cricketiG-ryllotalpa). cox, coxaattaching the limb tothe thorax, fe, the shortbroad femur. iih, thetibia forming a broadspade with six largesharp teeth, tars, thetarsal joints, which areturned upwards andcannot be used in loco-motion. After Rosel. TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS 87 it is well known that this venation serves the systematist as a basisfor the definition of genera, especially in Lepidoptera and Hymenop-tera. That is to say, it varies from genus to genus in a characteristicmanner, obviously corresponding to the differences in the wing-form,and in the flight itself. But, unfortunately, we are still far frombeing able to make more than quite general hypotheses as to themeaning of the lengthening and strengthening, or conversely, thedegeneration or elimination, of this or that vein. From extreme cases,however, as for instance the rich venation in cfood fliers with lart^ewings, and the scanty venation in poor fliers w
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