Evolution.

Society for Experimental Biology

Date:: 1953

Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolution. Source: Wellcome Collection.

38/484 (page 14)

H THE ORIGIN OF LIFE minerals which are known to adsorb many organic substances on to their surfaces. This is undoubtedly a powerful idea in view of the known catalytic activity of clays and the fact that quartz is the one common inorganic substance showing optical activity. But, as we have seen, the absence of aluminium from the list of biologically important elements presents a difficulty, and there is, indeed, no need to invoke the aid of any mineral element to explain the process of concentration, provided that the substances involved exist in a dynamic equilibrium of the type described. VL THE APPEARANCE OF HETEROGENEITY Turing (1952) has demonstrated by mathematical methods that certain types of dynamic system which are initially homogeneous undergo a pro¬ gressive change which leads to the appearance of spatial heterogeneity. The essential minimum for this behaviour to occur appears to be the presence of two substances, which he calls 'morphogens', the concentration of one of which is dynamically maintained by a balance between a generation process whose rate is controlled by its concentration (autocatalysis in the wide sense) and a destruction process; the second being formed at a rate dependent mainly on the concentration of the first and being destroyed at a rate proportional to its own concentration ; and that there be movement from one point to another according to the laws of diffusion. Turing has so far published an account of the behaviour of only one set of equations showing this effect, namely, the linear differential equation for morphogens He has, however (personal communication), also considered the more general problem involving non-linear terms. These equations are related to but not identical with those proposed by Volterra (1931), but the new feature of Turing's work arises from the simultaneous consideration of diffusion as a factor influencing the concentrations in a region of space. By variation of the constants and the form of the equations Turing finds cases in which stationary wave patterns of concentration are generated, and it is clear that where dynamic equilibria of the right type exist for more than one substance with interaction between the two chemical systems, a local con¬ centration can occur without the intervention of adsorption phenomena on to pre-existing particles. This discovery opens up a new field of speculation and experiment, since it may be possible to devise conditions in which the XandY f is) J