182 MENTAL EVOLUTION IN ANIMALS. 



although every dog adheres to his particular attitude through 

 life. Nearly all domestic animals exhibit slight but indi- 

 vidually constant differences of movement when caressed, 

 when they are threatened, when at play, &c. But perhaps a 

 more striking view of this subject may be obtained by con- 

 sidering the sum of the neuro-muscular conditions, leading to 

 individual peculiarities of movement, which we comprise 

 under the term " disposition," or, if more prominent, " idiosyn- 

 cracy." Thus many dogs develop the meaningless habit — 

 which has all the strength of an incipient instinct, and in 

 the case of the collie breed, as we shall subsequently see, 

 inherited or innate — of barking round a carriage. Some cats 

 take to " mousing " with avidity, while others can never be 

 taught to care about the sport. All who keep pet birds — 

 and indeed domestic animals of any kind — must have 

 noticed the diversity of their dispositions in respect of play, 

 boldness, amiability, &c. ; and Mr. W. Kidd, who had a very 

 large experience, is sure that the diversity of disposition in 

 larks and canaries is displayed by nestlings reared from the 

 nest.* 



Almost innumerable instances might be given of indi- 

 vidual variations in the instincts of nest-building, f Even as 



* See Gardener's Chronicle, 1851, p. 181, which is referred to in this 

 connection in Mr. Darwin's MSS. 



f For example, the Nut-hatch usually builds in the hollow branch of a tree, 

 plastering up the opening with clay ; but Mr. Hewetson found a pair which 

 for many years occupied a hole in a wall (YarreVs Birds), and Mr. Bond 

 describes another nest placed in the side of a hay-stack, built up with a mass 

 of clay weighing no less than eleven pounds, and the nest measuring thirteen 

 inches in height (Zoologist, 2nd ser., p. 2850). The golden-crested Wren, also, 

 frequently exhibits variations in the structure and situation of its nest (Hist. 

 Brit. Birds, 4th ed., vol. i, p. 450). The Golden Eagle builds in precipitous 

 crags of rock ; but Mr. D. E. Knox {Autumns on the Spey, 1872, pp. 141-3), 

 describes a nest which he himself examined on a fir-tree, not above twenty 

 feet from the ground. Couch says that " more than one pair of birds will 

 sometimes unite in occupying one nest, and either rear their broods in com- 

 mon, or one of them will surrender the future care of them to the other (Illus- 

 trations of Instinct, p. 233). Mr. S. Stone, writing of the Missel-thrush says, 

 " From what has been written, it appears plain that some individuals use clay or 

 plaster in the construction of the nest, while others contrive to do without it, 

 which agrees with my own observation, for although I have found nests 

 which did not contain plaster, the greater part of those which have fallen in 

 my way — and they have been not a few — certainly have had a plastering of 

 some kind between tlie twigs and lichens outside and the fine grasses which 

 invariably constitute the lining ; this has been more especially the case when 

 the bird has selected as a site the horizontal branches of a tree (Field, Jan. 8, 

 1861. This is a clipping which I find among Mr. Darwin's MS notes). As 



