Evolution.

Society for Experimental Biology (Great Britain)

Date:: 1953

Catalogue details

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

Credit: Evolution. Source: Wellcome Collection.

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392 DETERMINING FACTORS IN THE EVOLUTION off rods against cones for the same purpose; still others may have developed other carotenoid photopigments. In this connexion it is interesting to note that in many strongly diurnal animals, where perhaps a colour sense is most likely to develop (though there is no theoretical reason why wave-length differences should not also be detectable at lower illuminations if more than one type of receptor remained active under such conditions), there are often found the so-called double cones. Sometimes both members of these pairs morphologically resemble cones, but often they give the appearance of consisting of a rod and a cone. One wonders if the purpose of these closely associated pairs is not perhaps to produce elements in the retina with a sensitivity intermediate between that of two others, e.g. between rods and cones or between two types of cones. Secondly, such double receptors would cover a wider band of the spectrum than either element would by itself. Thirdly, if a cone cell, when stimulated, produced a certain quantity of chemical transmitter with which it activated its bipolar cell, then a rod cell incorporated into the synapse might well, by producing its opposing transmitter substance, reduce the response in proportion to the difference between the degree to which rod and cone were stimulated by the light ; such a path would then give a response which would be charac¬ teristic of the wave-length, while if the cone were also connected alone to another bipolar cell this path would give a measure of the light intensity. Visual purple, itself a carotenoid derivative closely related to vitamin A, together with similar carotenoids, associated with cells belonging to two main families and displaying properties not unlike those of other cells of neural origin and almost certainly mutually antagonistic, seem to form the basic materials for vision in the vertebrates. The process of evolution has developed this or that aspect or property of these materials according to the needs of each species. Increased primary photosensitivity, coupled with the development of summating neural pathways, favours nocturnal activity. Parallel development of both paths, perhaps assisted by inhibition of the more sensitive by the less sensitive, could lead to the 24-hour-day eye. Specialization of individual pathways from receptors to brain cells, together with lateral inhibitory mechanisms favours high visual acuity; again this may be combined with a highly sensitive nocturnal apparatus by spacial separation of the groups of cells involved and probably also by direct inhibition of the more sensitive. Development of two or more independent pathways of approximately equal sensitivity, but each having a different spectral distribution of sensitivity, might lead to a colour- discriminating mechanism, provided that the necessary central machinery developed at the same time for co-ordinating and correlating the informa¬ tion thus transmitted by the separate paths. Two such pathways could thus