966 ON FORM 'AND MECHANICAL EFFICIENCY [cii. 



other forces of restraint. We have seen how the egg is automatically 

 stream-lined, after a simple fashion, by the muscular pressure which 

 drives it on its way. The contours of a snowdrift, of a wind- 

 swept sand-dune, even of the flame of a lamp, shew endless 

 illustrations of stream-lines or eddy-curves which the stream itself 

 imposes, and which are oftentimes of great elegance and complexity. 

 Always the stream tends to mould the bodies it streams over, 

 facilitating its own flow; and the same principle must somehow 

 come into play, at least as a contributory factor, in the making of 

 a fish or of a bird. But it is obvious in both of these that even 

 though the stream-lining be perfected in the individual it is also 

 an inheritance of the race ; and the twofold problem of accumulated 

 inheritance, and of perfect structural adaptation, confronts us once 

 again and passes all our understanding*. 



When, after attempting to comprehend the exquisite adaptation 

 of the swallow or the albatross to the navigation of the air, we try 

 to pass beyond the empirical study and contemplation of such per- 

 fection of mechanical fitness, and to ask how such fitness came to 

 be, then indeed we may be excused if we stand wrapt in wonderment, 

 and if our minds be occupied and even satisfied with the conception 

 of a final cause. And yet all the while, with no loss of wonderment 

 nor lack of reverence, do we find ourselves constrained to believe 

 that somehow or other, in dynamical principles and natural law,' 

 there lie hidden the steps and stages of physical causation by which 

 the material structure was so shapen to its endsf. 



The problems associated with these phenomena are difficult at 

 every stage, even long before we approach to the unsolved secrets 

 of causation ; and for my part I confess I lack the requisite know- 

 ledge for even an elementary discussion of the form of a fish, or of 

 an insect, or of a bird. But in the form of a bone we have a problem 



* Mechanical perfection has often little to do with immunity from accident or 

 with capacity to survive. Legs and wings of locust or mayfly are indescribably 

 perfect for their brief spell of life and narrow sphere of toil; but they may be torn 

 asunder in a moment, and whole populations perish in an hour. Careful of the 

 type, but careless of the single life, Nature seems ruthless and indiscriminate 

 in the sacrifice of these little lives. 



t Cf. Professor Flint, in his Preface to Affleck's translation of Janet's Catises 

 finales: "We are, no doubt, still a long way from a mechanical theory of organic 

 growth, but it may be said to be the quaesitum of modern science, and no one 

 can say that it is a chimaera." 



