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Throughout the natural life‐cycle of an individual of any species there are progressive changes in most of the homeostatic set‐points, e.g. body measures change, pulse‐rate drops, hormone levels change, habitat area increases, representational models become more differentiated and detailed. These developmental changes are indices of growth and maturation and usually follow a fairly predictable course. There is thus stability in the pathway of change. The tendency for organisms to maintain relatively steady developmental pathways, despite variation of environment and despite limited deviation along the route, is termed by Waddington (1957)⁹ homeorhesis. The term he gives to each stable pathway is ‘creode’.

For each measure and category of homeostasis there is (probably) a corresponding measure and category of homeorhesis. This is certainly so in the case of most morphological, physiological and ecological measures, all of which show a fairly high degree of homeorhesis in all species. In the case of the other two categories of measure (personal‐environmental and representational) the position is less clear, because understudied. I suspect, however, that in these cases too there is a fairly high degree of homeorhesis.

When a species is endowed genetically with a high degree of developmental homeorhesis, the course of development of individuals is rendered relatively independent of even large fluctuations of environment. This can make for adults of a fairly high average degree of adaptedness to the usual environment even should their development have occurred in atypical environments. If this genetic strategy is carried too far, however, the species loses its adaptability. In the long run the species´ environment might change so much that the single creode characteristic of its development might prove maladaptive, and, with no adaptability left, the species would become extinct.

An alternative genetic strategy is to provide a range of alternative creodes each suitable for one of a range of probable or potential environments and each tending to be the one followed when development happens to take place in that particular environment. An example is the capacity of a mammal’s immunological system to develop persisting responses appropriate to particular features met with in the environment. Whilst an increased epigenetic adaptability of this kind has obvious advantages there are limits to the variety of environments to which any one repertoire of creodes is adapted. Furthermore, every increase in species adaptability, by increasing developmental instability, is probably bought at the cost of greater risk that some individuals will develop along maladaptive lines. Examples from the field of immunology are liability to anaphylactic responses or those arising from rhesus incompatibility.

In ordinary language an individual (or species) endowed with a high degree of homeorhesis is regarded as ‘tough’. No matter what the environment (within limits) he seems to come through untouched. Conversely, an individual (or species) with low homeorhesis and high adaptability is regarded as ‘sensitive’. How he develops depends on the particular environment in which he grows up. The result may be high adaptedness to that environment but it may also lead to deviant development and low adaptedness.

In some species, development in an atypical environment, i.e. one markedly different from the species environment of evolutionary adaptedness, can result in an organism becoming adapted to that atypical environment. Such adaptedness can be achieved in a number of ways. Some entail a shift in one or other homeostatic parameter, others a shift in one or another homeorhetic pathway. Waddington gives as an example the several possible ways in which development at high altitude may result in an organism becoming adapted to living at low pressures of atmospheric oxygen:

by means of a shift in homeorhetic pathway, the area of lung alveoli is increased; this method permits both blood oxygen pressure and pulse rate to be maintained within usual homeostatic limits;

1 by means of a shift in one of these homeostatic measures, namely increasing heart‐rate, the other homeostatic measure, blood oxygen pressure, and also homeorhesis of lung development are permitted to remain unchanged;

2 by means of a shift in the other measure of homeostasis, namely reduction in blood oxygen pressure, other measures can remain unchanged, though energy output is reduced.

Because adaptedness to any particular environment is never absolute and may be purchased at some cost to homeostatic steady states or homeorhetic stable pathways, it is often not possible to make unitary judgments about what is healthy and what not. A main long‐term criterion, however, can always be applied, namely the degree to which any one solution contributes more or less successfully to species survival.