16 THE EYE IN EVOLUTION 



the endogenous diurnal rhythm, is seen in the fiddler-crab, Uca 

 (Brown et al., 1952-54) and also in marine forms of the protozoon, 

 Euglena (Pohl, 1948). A lunar as well as a diurnal rhythm of activity 

 and oxygen consumption is seen in the earthworm, Lumbriciis (Ralph, 

 1957), and in crayfish such as Cambarus virilis (Guyselman, 1957). 

 The timing of these rhythms with respect to the solar day when the 

 animals are kept in darkness has been altered by lowering the tem- 

 perature (the fiddler-crab, Uca, Stephens, 1957). ^ 



THE SEXUAL CYCLE OF ANIMALS, as with flowering in plants, is also 

 frequently determined by the influence of light as expressed by the 

 gradual change in the length of day in the annual solar cycle ; in this 

 way the onset of the breeding season becomes rhythmic as though 

 there has been implanted on the central nervous and hormonal 

 systems a pattern of behaviour automatic and innate so that it can 

 only be altered experimentally by a prolonged disturbance of the 

 natural phases. In some species, it is true, particularly in Invertebrates 

 and the lower Vertebrates, other factors such as temperature and 

 humidity also enter into the question, but controlled experiments have 

 shown that these and other extraneous circumstances, such as physical 

 activity and feeding, are often secondary and in many cases can be 

 excluded and that the most important factor is the duration of the 

 period of light — not its intensity or wave-length. These phenomena 

 have been particularly studied in animals inhabiting the northern 

 hemisphere. Spring in these regions with its increasing days is the 

 appropriate season for reproduction if survival is to be maintained, 

 and in those species with a long gestation period, the shortening days 

 of autumn are most suitable for mating. In general, when species with 

 a breeding periodicity of this type are experimentally subjected to 

 artificially lengthening days in late autunrn or winter, they can be 

 brought from their sexually quiescent condition into the ripeness 

 typical of spring, while conversely, if the lengthening days of spring 

 are artificially curtailed, sexual regression occurs ; indeed, it is possible 

 by these means to bring some types (birds, for example) into breeding 

 condition several times in the year — a change which applies not only 

 to anatomical considerations such as the development of the gonads, 

 but also to those habits and modes of conduct which are 'essentially 

 sexual in origin such as (in birds) singing and migration. 



Such phenomena have been investigated in many species of Inverte- 

 brates, Fishes, Amphibians, Reptiles, Birds and Mammals. 



In the INVERTEBRATES, even among Protozoa, Ehret (1951) found that 

 the diurnal rhythm of the mating reaction of Paramceciuni bursaris persists for 

 several days in complete darkness and can be altered by varying the illumination 



^ Compare p. 22. 



