BIOPSYCHOLOGICAL 617 



stimulus which evokes changes in the behaviour, such as the inter- 

 ruption of the close brooding by excursions for food. The bird's 

 prolonged sitting on addled eggs is not a mark of stupiditj^ it 

 merely means that the stimulus leading from circle 5 to circle 6 has 

 not been forthcoming. Many anomalies in the reproductive behaviour 

 become more intelligible in the light of the idea of chain-instincts. 

 One term in the series may be weakened, or even dropped out, if 

 the stimulus to the next term comes into operation prematurely, 

 or if the stimulus to the antecedent term be continued for an un- 

 usually long time. Thus the cuckoo, highly sexed and polyandrous, 

 has a prolonged sex-urge, and has suppressed nest-building and all 

 that follows. Or more generally, we may say with Herrick that the 

 normal cycle may be disturbed so that there is a lack of attune- 

 ment. Thus many an individual bird drops its eggs casually on the 

 ground, the nest-building instinctive activity remaining dormant 

 or awakening too late. 



Many a bird, especially a male, may build two or even more 

 nests, instead of being satisfied with one ; the third circle has usurped 

 more than its normal share of the time and energy available during 

 the short reproductive period. Occasionally the autumnal migrating 

 instinct is awakened prematurely, and the parent birds leave their 

 offspring to perish. As to the causes of the disturbance or lack of 

 attunement in the normal sequence, they may be looked for in 

 some nervous, endocrinal, gonadial, or other variation affecting the 

 outcrop of impulse, or in some unusual vicissitude in the environ- 

 mental conditions or in the pairing. 



Yucca Moth. — One of the best-known instances of chain-instinct 

 is the behaviour of the Yucca Moth {Pronuba yuccasella) when it 

 visits the beautiful Yucca flower [Yucca filamentosa) and effects 

 pollination. The facts were clearly described by Riley [Reports 

 Missouri Botanic Garden, 1892, pp. 99-158, figs.), and are in several 

 ways very instructive. The Yucca is one of the Liliaceae, and pro- 

 duces (sometimes at intervals of years) tall spikes of creamy flowers, 

 one or more of which opens every evening or so for several weeks in 

 succession. It is a night-flower and gives forth a pleasant fragrance, 

 which attracts the female Yucca Moth, without which there can be 

 no fertile seed. None is formed in the Yuccas that occasionally flower 

 in British gardens. 



The female Yucca Moth, emerging from her pupa-case into a 

 world of which she has had no experience, becomes fertilised, and 

 thereafter flies to the freshly opened Yucca flowers, and collects 

 pollen from the stamens. She uses her peculiarly shaped maxilLne, 

 and holds a little ball of pollen by means of two sickle-shaped 

 maxillary tentacles in front of her head. She then lays an egg in the 

 ovary, but usually in that of a different and older flower. Having 

 deposited the e^^, she carefully places the ball of pollen on the 



