24 RETROGRESSIVE DEVELOPMENT IN NATURE. [IX. 



these examples. Almost all wild animals, mammals as well as 

 birds, possess the instinct to escape : they are not only 

 extremely attentive to the slightest sound and smell, and to 

 every movement taking place within their field of vision, but all 

 of them, the predaceous animals not excepted, are continually 

 mindful of their safety, and though not always consciously on 

 the watch, are so to a great extent instinctively. A wild bird 

 flies away at the least sound ; a hedgehog which has been 

 surprised, and has rolled itself up, only unrolls itself to run away 

 after the lapse of a considerable time, while the slightest suspi- 

 cious sound will make it roll up even more tightly. These acts 

 are not the result of reflection, but are purely instinctive, the 

 act of rolling-up being always associated with the perception 

 of sound, so that the former follows instantaneously upon the 

 latter, before the animal has had time to reflect on its meaning, 

 just as we shut our eyes the instant that anj^thing touches 

 them. In the higher animals these movements are certainly 

 under conscious control, i. e. they are capable of suppression, 

 and hence it is that animals in a state of captivity lose the 

 instinct of being startled and of escaping. This instinct is 

 nevertheless deeply implanted in them, and many generations 

 must be passed in domestication before the natural timidity is 

 lost. I believe that the loss is brought about by cessation of 

 the action of natural selection, and a consequent gradual degene- 

 ration of the instinct. Of course it is difficult to judge of the 

 amount of influence exercised by custom upon the life of the 

 individual, but it may at least be considered as certain that the 

 young of our domestic fowls, geese, and ducks, have lost much 

 of the instinct to escape possessed by their wild ancestors, and 

 that they would never become quite wild again even if placed 

 under the care of a wild mother from the first. 



The length of time which may be necessary before domes- 

 tication can get the better of this passive kind of wildness, as 

 the instinct to escape may be called, is seen in the case of the 

 guinea-pig. These animals have been domesticated ever since 

 the discovery of South America about 400 years ago, — a period 

 of time which has not sufficed to overcome their natural timidity. 

 Any loud noise will make them start violently and seek to 

 escape, although they may never in their lives have had any 

 experience of real danger : even shortly after birth the same 



