46 COMMUNITY FUNCTIONS— DYNAMICS OF BIOTIC FORMATION 



cially numerous or carry some form of internal parasite; (d) internal 

 insect parasites that infect only other insects and their eggs and 

 cause the death of the host. 



The life histories of the external parasites of the mammals and 

 the insect parasites of other insects are very simple and not essentially 

 different from those of animals feeding in some other way, but they 

 are of much importance in any attempt to evaluate their coactive 

 effect. The internal parasites are chiefly worms of the several phyla, 

 protozoans and a few arthropods. The life history of the metazoan 

 parasites is often complicated, as it very commonly involves two hosts. 

 Transfers from host to host are divisible into three chief classes: (a) 

 through contact with feces; (5) through the devouring of herbivores 

 by carnivores and omnivores; (c) through contact with water in drink- 

 ing, bathing, or swimming. 



Motile Animals. The breeding activities are the center of the 

 environmental relations of such animals (Merriam, 1890; Shelford, 

 1911, a-e). This does not mean, however, that factors operating at 

 other periods and with reference to other activities may not be more 

 important in some cases (Kendeigh, 1934). 



Two types of life history resulting from method of reproduction 

 may be recognized, i.e., some animals are oviparous and others are 

 viviparous. From the standpoint of ecology, the life history of an 

 egg-laying animal may well begin with the adult, usually with the 

 laying female. She selects a place for the eggs that is suited not only 

 to them but also to the young at the time of hatching. The adult 

 appears to be in a physiological state similar in many cases to that 

 of the young at hatching. This is a noteworthy characteristic of the 

 tiger beetles investigated by Shelford (1915). JMany fishes construct 

 nests in which the young pass their early sensitive period. In this 

 case both sexes reach a physiological condition similar to that of the 

 young, recurrently each year. The same is true of birds which re- 

 turn each season to the same climatic and habitat complex to breed. 

 The anadromous salmon, however, breeds only once in waters suited 

 to the young, but the young migrate to the sea and back again to 

 the breeding places. In a lesser way, this is true of many animals, 

 notably insects, fishes, amphibians, and reptiles. 



In the egg-laying organisms with a metamorphosis, each species 

 presents several seasonal forms in its life history. In Amhystoma, the 

 adults, the eggs, the tadpoles, the gilled larvae, and finally the adult 

 form api^ear in turn between February and July. Since many ani- 

 mals without metamorphosis show similar phenomena, there is a gen- 

 eral sequence throughout the year, often made striking by migration 



