510 THE EYE IN EVOLUTION 



a necessity at an early stage if an organism were to combine large size with 

 mobility and reactivity — attributes necessary for its survival. Even in 

 unicellular organisms, however, a foretaste of two fundamentally different 

 methods of response to light or other stimuli is evident — ^the first and most 

 primitive, a simple quantitative kinetic resjwnse the mechanism of which 

 is essentially chemical, and the other, a more qualitative shock-reaction the 

 basis of which is electrical. 



In the kinetic response the amount of light absorbed by a photo- 

 sensitive substance determines a proportional increase or decrease of activity, 

 a change which may be transmitted beyond the confines of the cell by the 

 spread of the chemical products of the reaction. In the shock-response, the 

 rate of change in the amount of light absorbed by the photosensitive tissue 

 is of importance ; the precarious electro -chemical balance of protoplasm in 

 cellular form is maintained until the strength of the stimulus is sufficient 

 to fire it into sudden activity, like an explosive, by a trigger-action. The 

 first type of reaction is seen in plants and is typically evident as a regulator 

 of basic activities in animals ; the second is characteristic of the animal 

 world and is seen in the lower organisms in their orientation to light and in 

 the higher is typical of the economy -oi nervous activity. 



The classical distinction between plants and animals as given, for example, by 

 Haldane and Huxley in their standard work on biology as the only valid differentiation, 

 concerned the type of foodstuffs they utilized, in the first case derived innocently 

 from the air and the soil, in the second, from the syntheses accomplished by other 

 living things. With some exceptions, such as insectivorous orchids, this is true, 

 although difficulties arise among unicellular organisms on the border-line between 

 plants and animals ; thus some Flagellates have green chromatophores, others are 

 colourless and live saprophytically and may be regarded as the starting point on the 

 one hand of unicellular Thallophytes, on the other of Protozoa. A more fundamental 

 differentiation, however, lies in the mechanism of their response to stimuli, a differentia- 

 tion which determines the relative simplicity and serenity of vegetable existence with 

 its close affinity to the sun's energy and the earth's chemistry, in contrast with the 

 - complexity and hurry-scurry of animal life with its mobility and independence. 



In both plants (Blaauw, 1914-15) and animals (Northrop and Loeb, 1923) the 

 stimulatory mechanism is purely photochemical, but there is a fundamental difference 

 in the manner of conduction and in the effector mechanism. It is probable that all 

 living cells are able to conduct waves of excitation, the common mechanism both in 

 plant and animal cells being a wave of depolarization which passes along the plasma 

 membrane and momentarily increases its permeability. But in plants the stimulus 

 must be received directly by each cell, and propagation of the stimulus to a point at 

 a distance is effected, not by the direct transmission of an imjxilse from cell to cell, 

 but by the transfer of the products of the primary change, photochemical or otherwise, 

 by a process somewhat more rapid than simple diffusion. ^ Such stimulatory substances 

 have an obvious analogy to the prodvicts of the endocrine glands of animals but none 

 to the propagation of nervous impulses. We have seen that substances of this type are 

 responsible for the flowering of plants ^ and that a group of hormones, particularly the 

 auxins,^ are responsible for the growth and movement of plants — substances which 



^ 10 mm. per hour in the case of auxin, p. 39. 

 2 p. 10. » p. 39. 



