506 GENERAL CONSIDERATIONS 



time but has excelled it in the development of methods and in the definite- 

 ness and certainty of its conclusions. 



532. Physiological Life Histories.— The life histories of animals have 

 been referred to many times but always with particular reference to the 

 structural changes which the animal passes through in its development. 

 The life history of an animal may be viewed from another aspect, and 

 that is as controlled by its physiological reactions. This aspect of an 

 animal's life history belongs to ecology. Shelford has enumerated five 

 types of physiological life histories. One is that in which the annual 

 cycle and the life-history cycle agree and in which the life history occupies 

 but one year. A second is that in which the development of the animal 

 occupies two or more years and the adults are produced at such intervals. 

 Usually broods of insects appear every year, but in some cases many 

 years may elapse between broods in any particular locality. In the 

 seventeen-year cicada, seventeen years intervene between the appearance 

 of the adults of one generation and those of the next. In a third type 

 the adult lives over a number of years and reproduces a number of times. 

 This is generally true of higher forms. A fourth type includes those 

 animals in which there are a number of generations in each year. The 

 fifth and last type includes those which reproduce continuously and either 

 at a uniform rate because of uniform conditions or at different rates under 

 varying conditions. Included in this group are certain plankton organ- 

 isms which hve where conditions are nearly uniform throughout the 



year. 



533. Habitat. — The particular locahty in which any one species of 

 animal is found is known as its habitat. Some animals are capable of 

 occupying several habitats, affording, perhaps, a variety of conditions; 

 others are very narrowly restricted in their choice. Perhaps the most 

 narrowly restricted of all animals are parasites that can live only in a 

 certain part of the body of a particular species of animal. There are, 

 however, many free-living forms which can live only under a very precise 

 set of conditions which are rarely found. Generally speaking, aquatic 

 animals cannot adjust themselves to life outside the water, although 

 there are those which make brief excursions outside their aquatic habitat 

 or which can remain living for some time when deprived of water. Exam- 

 ples of such types are the animals living above low tide or the many 

 animals of ponds and rivers which have to endure periods when the body 

 of water in which they live becomes dry. 



The inabiUty of a plant to move, generally speaking, forces it either to 

 live or to perish in the habitat in which the seed germinates. It is thus 

 possible for botanists to study the reactions between plants in any given 

 habitat and to define very exactly the composition of plant communities. 

 However, the mobility of animals and the different reactions they display 

 at different times in their physiological life histories present difficulties 



