62 BIRDS' NESTS 



the young of each species are brought to maturity. 

 This fact must never be neglected by the student of 

 birds' nests, for it is one of the fundamental principles 

 upon which the science of caliology is based. It is 

 true, if we analyse the construction of certain nests, 

 we are confronted by facts that seem to suggest a 

 gradual development from a crude and simple to an 

 elaborate model ; but after a prolonged study of the 

 subject, extending over many years, I am convinced 

 that such is not really the case. For instance, we 

 might endeavour to build up a theory of the gradual 

 evolution of a Nest in this manner. In the first place, 

 take for the sake of our argument the nest of a Bull- 

 finch. In this structure we find a recapitulation of 

 various cruder nest types before the elaborate and 

 beautiful home is completed. We have the few 

 twigs as a foundation, which represent the full and 

 complete nest of such a simple architect as a Pigeon, 

 for instance ; and then we may gradually trace the 

 more and more elaborate nest through each succes- 

 sive stage of its construction from the roots and bits 

 of dry grass until we reach the final lining of hair, 

 wool and feathers, the whole combined representing 

 the perfect model of a nest of this particular type. 

 So again we might illustrate the theory of nest evolu- 

 tion by a comparative study of the nests of various 

 species of Crows. First, as the crudest type, we have 

 the nest of such a species as the Chough and the 

 Jackdaw built in holes and fissures, the materials 



