The biologist, as well as the philosopher, learns to recognise that the whole is not merely the sum of its parts. It is this, and much more than this. For it is not a bundle of parts but an organisation of parts, of parts in their mutual arrangement, fitting one with another, in what Aristotle calls ãa single and indivisible principle of unityã; and this is no merely metaphysical conception, but is in biology the fundamental truth which lies at the basis of Geoffroyãs (or Goetheãs) law of ãcompensation,ã or ãbalancement of growth.ã
Nevertheless Darwin found no difficulty in believing that ãnatural selection will tend in the long run to reduce any part of the organisation, as soon as, through changed habits, it becomes superfluous: without by any means causing some other part to be largely developed in a corôÙreôÙsponôÙding degree. And conversely, that natural selection may perfectly well succeed in largely developing an organ without requiring as a necessary compensation the reduction of some adjoining part28.ã This view has been developed into a doctrine of the ãindependence of single charactersã (not to be confused with the germinal ãunit charactersã of Mendelism), especially by the palaeontologists. Thus Osborn asserts a ãprinciple of hereditary correlation,ã combined with a ãprinciple of hereditary separability whereby the body is a colony, a mosaic, of single individual and separable characters29.ã I cannot think that there is more than a small element of truth in this doctrine. As Kant said, ãdie Ursache der Art der Existenz bei jedem Theile eines lebenden KûÑrpers ist im Ganzen enthalten.ã And, according to the trend or aspect of our thought, we may look upon the co-ordinated parts, now as related and fitted to the end or function of the whole, and now as related to or resulting from the physical causes inherent in the entire system of forces to which the whole has been exposed, and under whose influence it has come into being30.
It would seem to me that the mechanical principles and phenomena which we have dealt with in this chapter are of no small importance to the morphologist, all the more when he is inclined to direct his study of the skeleton exclusively to the problem of phylogeny; and especially when, according to the methods of modern comparative morphology, he is apt to take the skeleton to pieces, and to draw from the comparison of a series of scapulae, humeri, or individual vertebrae, conclusions as to the descent and relationship of the animals to which they belong.
It would, I dare say, be a gross exaggeration to see in every bone nothing more than a resultant of immediate and direct physical or mechanical conditions; for to do so would be to deny the existence, in this connection, of a principle of heredity. And though I have tried throughout this book to lay emphasis on the direct action of causes other than heredity, in short to circumscribe the employment of the latter as a working hypothesis in morphology, there can still be no question whatsoever but that heredity is a vastly important as well as a mysterious thing; it is one of the great factors in biology, however we may attempt to figure to ourselves, or howsoever we may fail even to imagine, its underlying physical explanation. But I maintain that it is no less an exaggeration if we tend to neglect these direct physical and mechanical modes of causation altogether, and to see in the characters of a bone merely the results of variation and of heredity, and to trust, in consequence, to those characters as a sure and certain and unquestioned guide to affinity and phylogeny. Comparative anatomy has its physiological side, which filled menãs minds in John Hunterãs day, and in Owenãs day; it has its {716} clasôÙsiôÙfiôÙcaôÙtory and phylogenetic aspect, which has all but filled menãs minds during the last couple of generations; and we can lose sight of neither aspect without risk of error and misconception.