246 ME^'TAL EVOLUTION IN ANIMALS. 



the Nightingale a tendency to sing in the middle of the 

 night or in the day runs in families and is strictly in- 

 herited."* 



Professor Newton informs me that the Eing-plover on 

 the extensive sand-dunes of Norfolk and Suffolk habitually 

 displays a very curious and instructive case. These birds 

 naturally build on the sea-shore, depositing their eggs in a 

 hollow which they scoop out in the shingle. The sea has 

 retreated for miles from the extensive sand-dunes in question, 

 which have become covered with grass. Apparently the 

 Ring-plovers have gone on breeding for numberless genera- 

 tions on the site which was at one time the sea-coast, the 

 distance between them and the sea having therefore gradually 

 increased more and more.f Hence the birds are now living 

 on wide grassy surfaces instead of on shingle, but their 

 instinct of laying their eggs on stones remains ; so that after 

 having scooped out a hollow in the ground, they collect small 

 stones from all quarters and deposit them in the hollow. This 

 has the effect of rendering their nests very conspicuous, and 

 the fact shows in a striking way how a fixed ancestral instinct 

 may, while in the main persisting under changed conditions 

 of life, nevertheless so vary in reference to these changed 

 conditions as to constitute the beginning of a new instinct. 



For further instances of local variation in tlie instincts of 

 nest-building, I may in this connection again refer to the 

 highly instructive cases previously mentioned in illus- 

 tration of the plasticity of instinct under the moulding 

 influence of intelligence. J I allude to the fact that on the 

 American Continent various species of birds — notably a kind 

 of Owl, a Blue-bird, the Pewit Flycatcher, several species 

 of Wren, and nearly all the species of Swallow — have 

 adapted the structure of their nests to the artificial nesting- 

 places provided by man, in just the same way (though more 

 gradually and on a much larger scale) as did the colony of 

 Palm-swifts in Jamaica. But with still more special refer- 



* Stuben-Vogel, 1840, s. 323; see on different powers of singing in dif- 

 ferent places, s. 205 and 265. 



t That this is the explanation is not merely probable a priori, but 

 receives additional corroboration from the fact that these same sand-dunes 

 are now the habitat of a species of lepidopterous insect which elsewhere is 

 found upon the coast. 



X See above, p. 210. Compare also many of the cases given in the 

 Appendix. 



