ANIMALS 47 



to different situations for dormant or unfavorable periods. ]\Iany of 

 the smaller forms, notably insects, produce a number of generations 

 in one season (Glenn, 1915). The different generations are often 

 divergent as to response system and other physiological characters. 

 Other animals, though present as adults each year, require several 

 years to reach maturity, and at a given time the various stages are 

 telescoped together in a somewhat confusing fashion. In fishes, mea- 

 surements of length, scales, and scale rings have served to separate 

 the age classes and illuminate the life history (Walford, 1932). In 

 a few species, such as some cicadas and a few wood beetles, unusu- 

 ally long life histories occur, extending over as much as seven, thir- 

 teen, or seventeen years. Again, the production of eggs is occasion- 

 ally associated with definite astronomical cycles, the palolo worms 

 laying eggs at a definite period of the moon. A correlation with con- 

 ditions is shown by various other marine worms (Mayer, 1908). 



In any community, a considerable series of important stages in 

 the life histories of different species occur together. This is due to 

 a certain similarity of response or of life histories. Some insects, not 

 conspicuous at other periods, occur in middle latitudes at the time 

 migratory birds are pausing on their w^ay to their northern breeding 

 grounds. At the same time, certain plants are in bloom, giving a sea- 

 sonal aspect to the biotic community, as the result of life history 

 demands. 



A few motile aquatic animals deposit eggs that float at the sur- 

 face and act essentially like disseminules of the sessile animals. The 

 fishes that produce this type of egg belong to diversified taxonomic 

 groups. For example, the cod moves into shallow water near shore 

 to breed, just as do many other fishes. After the pelagic eggs are 

 hatched, the fishes migrate into the protecting tangles of Zostera, 

 where they live during infancy (Blegvad, 1916). 



Viviparous species are distributed throughout the more highly 

 organized animal groups. In some cases viviparity does not differ 

 materially from egg laying; e.g., the blowflies and tsetse flies deposit 

 very young larvae instead of eggs. All degrees of adaptation may 

 occur, the condition in the marsupials and placental mammals being 

 more specialized than in the other forms. In these, there is a rutting 

 period often of much importance in that the strongest males fre- 

 quently become the fathers of the next generation, owing to an intense 

 competition among them for mates, as among the seals and certain 

 ungulates. Again the female sheep does not enter into estrus with 

 any certainty, except when stimulated by sharp changes of tempera- 

 ture from day to night (Johnson, 1924). Rutting time bears definite 



