6i8 LIFE : OUTLINES OF GENERAL BIOLOGY 



stigma, and the down-growing pollen tubes effect the fertilisation 

 of the ovules — usually by cross-fertilisation. 



The Yucca Moth proceeds to repeat the sequence: visiting a 

 flower, collecting pollen, making a ball and holding it, visiting 

 another riper flower, laying an egg in its seed- box, depositing 

 the pollen-ball on the stigma, and then flying off to begin 

 anew. It is evident that there are some six links in this instinct- 

 chain. 



The egg of the moth develops in the seed-box, and the larva 

 feeds on some of the seeds which would not have grown if the 

 ovules had not been fertilised. As there are lar more seeds than 

 the larva requires, the linkage works well for the plant as well as 

 for the insect. What we are concerned with here is the chain of 

 activities, but we may digress to notice two interesting points, 

 illustrative of the "wheels within wheels" — the subtlety of these 

 inter-relations. The Yucca, as we have mentioned, does not flower 

 every j^ear; and it is remarkable that there is in the Yucca Moth 

 family (Prodoxid^e) a variability in the length of time that is passed 

 in the pupa-state. The quiescent phase sometimes lasts a year or 

 two longer than usual! We do not, of course, say that moth and 

 plant keep time with one another in their respective emergence 

 and flowering; but it is striking that both should be variable. Fine 

 weather conditions probably favour the simultaneous appearance 

 of the moths and the flowers in the same year; but in their native 

 haunts in south-western North America there is probably some 

 flowering and some emergence of moths every season. None the 

 less it is very remarkable and unusual that one particular species 

 of plant and one particular species of insect should be absolutely 

 bound up with one another in the continuance of their respective 

 generations. 



The other linkage concerns another genus of moths (Prodoxus) 

 which also lays its eggs in the ovary of the Yucca flower. With this 

 great difference, however, that the species of Prodoxus have not the 

 mouth-parts characteristic of the species of Pronuba and are there- 

 fore unable to pollinate the Yucca. But without pollination the 

 ovules do not become seeds, and the ovary does not grow larger. 

 Thus it follows that this "bogus Yucca Moth", as it is called, is 

 dependent indirectly on its relative the "true Yucca Moth"; Pro- 

 doxus is dependent on Pronuba. But let us return to further illustra- 

 tion of the linking of instincts. 



L.AMB-TiiEFT. — A suggestive illustration, worked out by Mr. A. H. L. 

 Frascr [Jonrn. of Psychol., 1926), concerns the lambing instincts of 

 Cheviot sheep. Two saving clauses may be emphasised at the outset: 

 (i) That here, as with birds, there is a frequent mingling of intel- 

 ligent control with instinctive routine; and (2) that the behaviour 



