BIRDS AND EVOLUTION 369 



that has gone on during fifty years of ostrich-farming has 

 affected the quality of feathers but not the quantity. Indeed, 

 Professor Duerden beUeves that the wing of the ostrich is 

 degenerating as regards the number of feathers, the 42- 

 plumed wing being interpreted as a hark-back to an ancestral 

 condition, just as a guinea-pig sometimes shows four toes 

 instead of the normal three. It is pointed out in this con- 

 nection that the third finger of the ostrich is almost buried in 

 flesh, and that it bears no claw as is often alleged. 



Terminology. — When we carefully examine a large 

 number of specimens of the same kind — sparrows let us say — 

 we find that they are very far from being all alike. They 

 differ from one another in details, just as we see among the 

 members of a human family. These individual peculiarities 

 can be measured and registered : they may be called 

 " observed differences " or " observed divergences." We 

 need some term that will commit us to no theory. 



A more intimate study of the observed differences shows 

 that some of them are due to peculiarities in " nurture," e.g. 

 in habits, nutrition, and surroundings. These peculiarities 

 directly due to peculiarities in exercise, diet, habitat and 

 so forth are technically called " acquired characters " or, 

 preferably, somatic modifications. They may be defined as 

 structural changes in the body, acquired during the individual 

 lifetime as the direct results of peculiarities or novelties in 

 function, nutrition, and environment, and so transcending 

 the limits of organic elasticity that they persist in the indi- 

 vidual after the inducing conditions have ceased to operate. 

 They are " exogenous " dints or imprints, hammered on 

 from without, or they are direct parries of nurtural thrusts. 

 The word " nurture " was used by Sir Francis Galton to 

 include all manner of extrinsic influences that play on the 

 body, in contrast to the intrinsic hereditary " nature " which 

 expresses itself in development. When the colour of the 

 canary's plumage changes in response to pecuUar food, when 

 the lining of the gull's stomach changes when it turns from 

 fish to grain, when the muscles of the breast degenerate in 

 captive birds, we speak of somatic modifications. They 



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