THE PEDIGREE OF BIRDS 361 



cards that are shuffled at the beginning of each individual 

 Hfe, so that changes occur in the " hand " which each 

 individual holds. These changes begin in the germ-cells ; 

 they find expression in development ; and they are tested in 

 the individual experience. They begin in the germ-cells 

 as the outcome of the opportunities for permutations and 

 combinations of factors that occur in maturation and fer- 

 tilisation ; they may also be incited by deeply-saturating 

 variational stimuli which are traceable to peculiarities in the 

 nutrition, environment, and functioning of the parent ; or 

 they may be due to subtle changes, e.g. growth-changes and 

 active re-organisations, in the living germ-cells. This 

 remains highly speculative. 



It is probable that in each type, say a stork type, there is 

 a considerable bloc of hereditary characters, perhaps en- 

 registered in the cytoplasm of the egg-cell, in which there is 

 little change from generation to generation. It is also 

 probable that the germinal variations that occur are within 

 a limited range, that they have to be congruent with what 

 has been already securely established. It is certain that no 

 variation can take root unless it is consistent with stability 

 of architecture and constitution. This is prevented by the 

 conditions of development and also by the winnowing action 

 of Natural Selection. 



Within certain limits, then, of morphogenic stability and 

 bionomic success new departures are of unceasing occurrence, 

 and these have formed the basis of new species. The 

 individual gets a new hand of hereditary cards, and if it 

 plays them well it may find some niche of opportunity which 

 ensures success either for itself or for the progeny. 



What is particularly interesting in a general survey of 

 bird evolution is the recurrence of similar adaptations along 

 lines which have only a distant genetic affinity. In other 

 words, what is called convergence, the occurrence of similar 

 adaptations in unrelated types, is abundantly illustrated in 

 the evolution of birds. 



Success in the struggle for existence depends on many 

 factors, such as vigour and parental care, catholicity of 



