THE INSURGENCE OF LIFE 181 



success of one of the most daring of life's experiments. 

 Let us glance very briefly at the various suggestions that 

 have been made in regard to the way-finding. (1) It has 

 been suggested that success in way-finding may be due to 

 inherited experience, slowly cumulative from generation 

 to generation, enriched and specialized by individually 

 minute contributions. There is probably very little sound- 

 ness in this suggestion, for we have no secure evidence of 

 the direct entailment of the results of experience, and we 

 find it difficult to state what content the experience could 

 have in the case of birds flying by night, and often at great 

 heights, and across the sea, as so many do. 



(2) An attractive theory is that of social tradition, and 

 in this there may be some truth. The idea is that those 

 lead well one year who followed well for several years 

 before. Ornithologists are not quite omniscient ; there 

 may be some old experienced hands amongst that rushing 

 troup of youngsters. But the difficulties are great. 

 How could the old hand become experienced in the matter 

 of a night journey across the Mediterranean ? In the case 

 of the cuckoo there does not seem to be a single adult left 

 in Britain when the youngsters begin to migrate. But 

 there is no evidence that cuckoos are less successful migrants 

 than other birds. It has been said that they may migrate 

 with their foster-parents, but this, if true, cannot be the 

 whole truth, since a number of the species who act as 

 foster-parents are non-migratory birds. 



(3) A third theory, that has a great deal to be said for it, 

 lays all the emphasis on sensory acuteness. Birds have 

 very keen senses of sight and hearing ; the migrants 

 sometimes follow coast-lines, river-valleys, lines of islands, 

 and so on. But it is quite plain that this cannot be the 



