Summary
Some fifty species of animals have provided the material for the investigations reviewed. The majority have been birds, but mammals, reptiles, amphibia, fish and invertebrates have also been used. A list is provided.
Most of the animals investigated have been species with short breeding seasons restricted to the spring in the northern hemisphere. In their main aspects the results have shown considerable uniformity, exceptions coming primarily from species that normally have no breeding periodicity, such as domestic rabbits, or animals of tropical origin. Species hibernating during the winter, or aestivating in summer, and in their natural state removed from exposure to daylight for several months of the year, have also provided exceptions.
The underlying principle is to be found in the fact that when such periodically breeding species are subjected to artificially lengthened days in the winter or late autumn they can be brought from the sexually quiescent condition characteristic of winter into the breeding condition typical of spring. The basic factor appears to be the length of day to which they are exposed.
The analogous phenomenon in plants has been termed photoperiodism by botanists. A brief review of this phase is given.
Until 1924 the reproductive rhythm in birds was attributed to the rising temperatures of spring, yet in the practice of yogai in Japan, many centuries old, and the induction of the muit in Holland, also going back a long way in history, light had been applied practically to induce winter singing. The underlying principle was unrecognized. Increased egg production in hens through the use of artificial light, first practised a century ago in Spain, is shown to be essentially different from the principle here dealt with.
It may be stated in general that animals (primarily birds) of the northern hemisphere, with short reproductive periods, show analogous responses in the following respects:
Days increased to spring duration by means of artificial illumination in mid‐winter induce development of the sex organs.
Spring days artificially curtailed to winter length induce regression.
Various birds may be brought into breeding condition two or even three times in a year, but a specific period of rest is required by the organs between one developmental peak and another.
It has been shown by numerous investigators that the reproductive rhythm depends on pituitary activity and that upon removal of this organ atrophy of the sex organs follows. The effect of extended lengths of day thus appears to be secondary as far as the gonads are concerned, ceasing after hypophysectomy.
Methods of administering artificial light seem to be important. Small graduated increases result in highly developed organs completely normal in size and histology. Longer increases induce irregularities and premature maturation of the spermatic elements in the testes (of birds) and have, in an extreme case, resulted in a significant rate of mortality in the animals employed (starlings).
Wave‐length, beyond certain limits, is...
