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I highly recommend the the preface, introduction and the first three chapters of this book as a guide to how to organize one's own thoughts on the subject of evolution's mechanisms and how to assess the ideas of others. (The remainder of the book is about Sheldrake's own hypothesis of morphic resonance.)
The writing is delightfully simple, the clarity of thought impressive. Opening paragraph:
Most biologists take it for granted that living organisms are nothing but complex machines, governed only by the known laws of physics and chemistry. I myself used to share this point of view. But over a period of several years I came to see that such an assumption is difficult to justify. For when so little is actually understood, there is an open possibility that at least some of the phenomena of life depend on laws or factors as yet unrecognized by the physical sciences. The more I thought about about the unresolved problems of biology, the more convinced I became that the conventional approach is unnecessarily restrictive. I started trying to imagine the possible outlines of a broader science of life.
Sheldrake begins by distinguishing between two kinds of theory; vitalism and holism. "Any new theory capable of extending or going beyond the mechanistic theory will have to do more than assert that life involves qualities or factors at present unrecognized by the physical sciences: it will have to say what sorts of things these qualities or factors are, how they work, and what relationship they have to known physico-chemical processes." Theories merely asserting there are such qualities or factors he labels "vitalism." Several such theories have been proposed during the 20th century, he says, "but none has succeeded in making predictions that can be tested, or in suggesting new kinds of experiments." I find this too restrictive. New theories could provide us with more powerful forms of discourse or help us think more fruitfully about life, even merely pinpoint where straight reductionism fails. His (for me) over-concern with prediction and experiment impairs my appreciation of the potential of his own theory of morphic resonance.
Organismic or holistic philosophy "recognizes the existence of hierarchically organized systems which, at each level of complexity, possess properties that cannot be fully understood in terms of the properties exhibited by their parts in isolation from each other; at each level the whole is greater than the sum of its parts." He quotes A. N. Whitehead: "Biology is the study of the larger organisms, whereas physics is the study of the smaller organisms." As an example he gives us "morphogenetic fields," supposed "to help account for, or describe, the coming-into-being of the characteristic forms of embryos and other developing systems." While he evidently favors such theories over simple vitalism none have proved capable of being tested through experiment, he claims, except his own theory of morphic resonance.
What issues must any new science of life take up? In his first chapter Sheldrake takes us through sections titled morphogenesis, behavior, evolution, origin of life, limitations of physical explanation, psychology, and parapsychology. Of evolution he remarks that the neo-Darwinian theory can never be more than speculative" and he issues this very acute observation: "evolution will always have to be interpreted in terms of ideas which have already been formed on other grounds." He warns us not to expect these grounds to emerge from the study of life's origin, that he anticipates will tell us little about its subsequent evolution.
More fruitful are likely to be challenges in accounting for behavior. He illustrates these under the headings instinct, for example a spider's ability to spin a functional web; regulation--that is, the ability to substitute one mode of performance for another, as a dog can if necessary learn to walk on three legs; and learning and intelligence. "An enormous gulf of ignorance lies between all these phenomena and the established facts of molecular biology, biochemistry, genetics and neurophysiology. How, for example, could the migratory behavior of young cuckoos ultimately be explained in terms of DNA and protein synthesis?"
Sheldrake then drives two darts into the heart of physical explanation. First, the possibility of dualism: If "the mind were non-physical and yet causally efficacious, capable of interacting with the body, then human behavior could not be fully explained in physical terms. The possibility that mind and body interact is by no means ruled out by the available evidence.... it is possible that human behavior, at least, might not be explicable entirely in physical terms, even in principle." Then, the dependence of science itself on mind: attempting "to account for mental activity in terms of physical science involves a seemingly inevitable circularity, because science itself depends of mental activity." Under the heading "Psychology" he says: "at present the idea that all the phenomena of psychology are in principle explicable in terms of physics is itself no more than speculative."
In accounting for morphogenesis he first defines it, then identifies three remarkable problems: "Biological morphogenesis can be defined as the 'coming-into-being of characteristic and specific form in living organisms.' The first problem is precisely that form comes into being. Biological systems are epigenetic: new structures appear which cannot be explained in terms of the unfolding or growth of structures which are already present in the egg at the beginning of development. The second problem is that many developing systems are able to regulate; in other words if a part of a developing system is removed (or an additional part is added), the system continues to develop in such a way that a more or less normal structure is produced" and he goes on to give examples. "Results of this type show that the developing systems proceed towards a morphological goal, and that they have some property which specifies this goal and enables them to reach it even if parts of the system are removed and the normal course of development is disturbed. The third problem is that of regulation, whereby organisms are able to replace or restore damaged structures.... for example, if the lens is surgically removed from a newt's eye, a new lens regenerates from the edge of the iris; in normal embryonic development the lens if formed in a very different way, from the skin."
"The only way in which these phenomena can be understood is in terms of causal entities which are somehow more than the sum of the parts of the developing systems, and which determine the goals of the processes of development. Vitalists ascribe these properties to vital factors, organicists to morphogenetic fields, and mechanists to genetic programmes."He goes on to analyze these positions.
Sheldrake then devotes to morphogenesis an entire chapter. I'll give a single quote from this chapter: "Within the same organism, different patterns of development take place while the DNA remains the same. Consider, for example, the arm and leg of a man: both contain identical cell types (muscle cells, connective tissues etc) with identical proteins and identical DNA. So the differences between the arm and the leg cannot be ascribed to DNA per se; they must be ascribed to pattern-determining factors which act differently in the developing arm and leg."
The final chapter I refer you to is titled "The Causes of Form." It starts: "It is not immediately obvious that form presents any problem at all. The world around us is full of forms; we recognize them in every act of perception. But we easily forget that there is a vast gulf between this aspect of our experience, which we simply take for granted, and the quantitative factors with which physics concerns itself: mass, momentum, energy, temperature,pressure, electric charge, etc." Sheldrake goes on to give a nice analysis of the springs of mechanistic thinking, one of which is "a mathematical mysticism of the Pythagorean type: the universe is seen as dependent upon a fundamental mathematical order which somehow gives rise to all empirical phenomena; this transcendent order is revealed and becomes comprehensible only through the methods of mathematics. Although this attitude is rarely advocated explicitly, it has a strong influence within modern science, and can often be found, more or less thinly disguised, among mathematicians and physicists." He then explores for us the influence on contemporary biological thinking of Plato's theory of forms.
In the next chapter Sheldrake tackles morphogentic fields, transitioning into his own theory of morphic resonance.
I recommend the first three chapters of Sheldrake's book as a measure of the problems to be dealt with in coming up with new theories of evolution.