380 ENVIKONMENT AMONG ANIMALS AND PLANTS 



the domestic boar as 18-5 to 1, a difference which is in part 

 probably due to diet. 1 The colour of birds' plumage is affected 

 by their diet. Hempseed causes bullfinches and other birds 

 to become black. Cayenne pepper causes yellow to change to 

 orange red. In the New York Zoological Gardens it has been 

 shown that some birds, such as the bobolink, may be so dieted 

 that they keep their breeding plumage throughout the year and 

 will sing their spring song in mid-winter. 2 Warren produced 

 marked changes in the common water-flea, Daphnia, by keeping 

 them in a confined space for many generations. ' Semper and 

 de Varigny found that when freshwater snails were reared in 

 vessels of a shape that allowed them abundant water but very 

 little surface in which to take exercise they developed into 

 dwarf forms. Every precaution was taken to procure abundant 

 food, perfect aeration and thorough removal of waste products. 

 De Varigny's experiments were particularly careful and point 

 convincingly to the conclusion that the condition of dwarfing was 

 the restricted area for exercise.' 3 Lastly it may be remembered 

 that, as is well known, parasites may induce remarkable changes 

 in their hosts. 



8. From what was said in the third chapter it follows that the 

 results of artificial subjection to abnormal stimuli just described 

 are to be interpreted as examples of the fact that different 

 responses are given to different stimuli by the same or approxi- 

 mately the same germinal constitution. But large and conspicuous 

 changes of this nature are not visible only as the result of artificial 

 circumstances. Many species are normally subject to more or 

 less definite and sudden changes in the surroundings, and some of 

 them exhibit more or less definite responses to such changes. 

 This is especially obvious in the case of sessile organisms the 

 outward form of which is clearly modified by the surroundings. 

 Thus ' the water ranunculus, when growing submerged beneath 

 the surface of a pond, produces leaves the blades of which are 

 cut up into a number of fine thread-like segments. As soon as 

 the top of the plant reaches the surface of the water, those leaf- 

 rudiments which are just commencing their existence, proceed 

 to develop in a totally different fashion. The leaves to which 

 they give rise possess a wide and unpointed blade, which floats 



1 Vernon, loc. cit., p. 294. 2 Thomson, Animal Life, p. 391. In the case of 



the Porto Santo rabbit examined by Darwin the change in colour was found to be 

 due to the environment. 8 Ibid., p. 383. 



