ANIMALS AND THEIR ENVIRONMENT 291 



limited to, a single species of plant: for example, an aphid that lives 

 almost solely on the chrysanthemum. Carnivorous animals are less com- 

 monly or less rigidly limited; lady beetles nearly always feed on aphids, 

 but accept a number of species, and can eat other small insects, such as 

 thrips. They also devour insect eggs. 



A highly specialized interspecific relation is parasitism, which has 

 already been mentioned as one means of securing nutrition. It is 

 referred to here again as an example of interspecific relations, because 

 of the great lengths to which life cycles of parasites have sometimes gone 

 in affecting other species. 



In simple cases a parasite has only one host. The trematode Gyro- 

 dactylus is parasitic on the skin and gills of the goldfish. When it 

 reproduces, the offspring become attached to the same or another gold- 

 fish. The liver fluke, however, employs two hosts. Its egg-producing 

 stage is spent in the liver of the sheep, or certain other large mammals, 

 but the offspring developed from these eggs must find a snail — any one 

 of a number of genera. In the snail it undergoes a series of developmental 

 changes, after which in a larval form it emerges from the snail and either 

 floats in the water or becomes attached to grass. Here it is drunk or 

 eaten by a sheep (or cow, or man) and the cycle is repeated. 



A parasite in the human lung passes through three hosts in its cycle. 

 Escaping in the sputum into water, it enters a snail. Then at a certain 

 stage of its development it emerges into the water again, and penetrates 

 the body of a crustacean. If the crustacean is eaten raw, as is the cray- 

 fish by people in Japan or sometimes shrimps in America, the human 

 host is reentered and the cycle is concluded. And finally, the trematode 

 Alaria passes through four hosts. From a carnivorous mammal, often 

 a dog or a member of the mink family, it goes through a snail, then a frog, 

 next a mouse or some other small mammal, and thence to a dog or 

 other mammal which eats the mouse. 



Ecological Succession. — No community of organisms is in a stable 

 condition. It is to be expected that the component species will vary 

 in relative abundance seasonally and from year to year. Occasionally 

 a species seems to disappear, perhaps to return later, and other species 

 may be added from some outside source. While these frequent changes 

 are of interest, they are far surpassed in importance by the alterations 

 known as succession. Ecological succession is an orderly sequence of 

 substitutions of species in a community. Certain species increase in 

 numbers, become perhaps dominant members of the group, then decline or 

 even disappear. Other species rise in succession, enjoy dominance for a 

 time, and then recede. Were this succession a purely random change, it 

 would have little more meaning than do the irregular seasonal and 

 sporadic fluctuations mentioned above. But in ecological succession, the 



