184 THE BIOLOGY OF THE SEASONS 



the making of the brain and spinal cord, the making of the 

 transitory skeletal axis (the notochord) and the foundation- 

 laying of the future backbone, the formation of the food- 

 canal, the establishment of the musculature, the body-cavity, 

 and so on. 



Within a few days, it may be a week or more, the embryo 

 is recognisably a bird-embryo, within a few more days it is 

 recognisably a particular kind of bird with cuckoo's feet 

 or kiwi's breast-bone, or the hoatzin's claws, or a pattern 

 of feather-tracts (before there are any feathers), which 

 is distinctive in most birds. Soon, moreover, it is 

 visibly, as it has been throughout invisibly, a particular 

 species of bird. Even audibly, for one may hear from 

 within the shell the characteristic call-note of the future 

 bird, e.g. of peewit, or gull, or coot, speaking before it is 

 born. 



Not infrequently, however, some of the individual char- 

 acteristics of the bird are, as it were, held back from ex- 

 pression the hereditary cheque not cashed until some time 

 after birth. The facts go to show that this retardation of 

 expression is especially marked in regard to characters which 

 have been fixed relatively late in the racial history of the 

 species. 



To put it concretely, it is a noteworthy fact that the 

 characteristic bills of spoon-bill, humming-bird, and scissor- 

 bill are not expressed until long after birth, just as part of our 

 own inheritance is often held latent for many years until 

 the appropriate, or it may be, to us, inappropriate time for 

 its expression comes when its liberating stimulus 

 A uslosungsreiz arrives. 



Meanwhile the patient task of incubation proceeds. 

 Usually the hen sits ; sometimes the two parents co-operate ; 

 rarely the patience is wholly masculine, as in the bustard- 

 quails. Sometimes, moreover, the incubation is left to the 



