THE EYE IN EVOLUTION 



plant as a whole, he transformed biology to a more factvial plane based on 

 observation and experiment, and was the first to show that in the higher plants 

 receptor tissues existed separately from motor tissues, and that the orientation 

 of plants to light was due to the transference over some distance of stimuli 

 appreciated by the former to be made effective by the latter. These observations 

 which appeared in the last of the classical books derived from his pen ^ form a 

 typical example of the revolutionary nature of Darwin's philosophy — the result 

 of a unique combination of experimental genius with penetrative powers of 

 interpretation which have rarely been equalled — and from these observations 

 have directly followed our understanding of the development of the sensory 

 organs and their effect on the evolution of the higher species in the animal scale. 

 The son of a doctor in the English country town of Shrewsbury, he went to 

 the University of Edinburgh to study medicine ; this, however, he forsook and 

 went to Cambridge with the intention of entering the Church ; but here Sedgwick 

 and Henslow, the professors of geology and botany, inspired him again with a 

 love of natural history which eventually was to become a passion. Darwin's 

 assessment of the qualities responsible for his own success is worth remembering : 

 " the love of science, unbounded patience in long reflecting over any subject, 

 industry in observing and collecting facts and a fair share of invention as well 

 as of common sense ". And again : "I have steadily endeavoured to keep my 

 mind free so as to give vip any hypothesis, however much beloved (and I cannot 

 resist foi-ming one on every subject), as soon as facts are shown to be opposed 

 to it ".- 



THE RESPONSES OF ORGANISMS TO LIGHT 



LIGHT — the visible radiant energy derived from the sun — is respon- 

 sible for the whole existence of living things on the earth, and without 

 question photosynthesis in plants — the reaction whereby the carbon 

 dioxide and water which permeate the atmosphere and the earth's 

 crust are converted into the organic substances which constitute the 

 basis of all living things — is the most fundamental and important 

 chemical process on our planet. Not only was photosynthesis respon- 

 sible for the origin of 'life but it maintains the perpetual cycle of the 

 activities of living things. By oxidation, living structures are con- 

 tinuously broken down to their initial constituents (carbon dioxide 

 and water), the process being accompanied by the liberation of the 

 energy required by organisms to perform their varied activities ; by 

 photosynthesis the carbon dioxide and water produced by the oxidation 

 of living matter are perpetually reunited by an opposite process of 

 reduction with the return of oxygen to the atmosphere, the high energy 

 requirements necessary being supplied by the capacity of the chloro- 

 phyll group of pigments in green plants to absorb sunlight. This 

 reaction whereby the chlorophyll system stores and then liberates 

 light-energy is thus not only the source of the activities of all living 

 things but supplies much of the energy at the disposal of the civilized 

 world in the stores of coal and petroleum formed throughout the ages. 



^ Pouer of Movements in Plants, London, 1880. 



^ Life and Letters of Darwin, by Francis Darwin, 1887. 



