DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION IIQ 



to the home and family, which we regard as the real basis of 

 modern society ; and such social organization in turn has been a 

 most powerful factor in the progress of the human species. 

 Death and the length of life must also be considered as special 

 adaptations. This differs in different species very widely. 

 Life in general, where natural selection acts, will be the period 

 of youth, plus the period of fertility, plus the time necessary 

 to rear the latest offspring. For the species, the death of the 

 individual becomes an advantage at the completion of this 

 period, and this fact is sufficient to insure that death will nor- 

 mally occur at this time. 



152. Practical Exercises. Add instances of parental care which have fallen 

 under your own observation, and give a statement of the facts in the case. Com- 

 pare the mammals with which you are acquainted, in this regard. Compare the 

 condition of the young of the robin, the quail, the blue-jay, the pigeon as to 

 maturity at hatching. Do any animals of your acquaintance reproduce more than 

 once in a year? Why is one reproductive period per year a common adaptation. 

 Compile statistics concerning the longevity of various animals, and its relation to 

 size, to reproductive period, and to the time demanded to reach the adult stage. 



153. Colonies. In some of the lower groups of animals, 

 as the polyps and jelly-fishes, in which the reproduction by 

 fission or budding is prominent, the newly formed individuals 

 remain for a longer or shorter time in association with the parent 

 or with each other. These units which otherwise might be 

 separate individuals are originally connected and often come, 

 by the continuation of the process, to form immense masses, 

 as in the coral. Such organic associations are called colonies, 

 Colonies rarely occur in animals in which the organs are highly 

 specialized. Very often the individuals become specialized for 

 the performance of a special portion of the work, and thus we 

 get several quite differently constructed individuals within the 

 colony (polymorphism, Fig. 87). The whole colony may then 

 behave somewhat as an individual, the polyps taking the place 

 of organs (Siphonophora) . Colonial animals are almost always 

 attached to fixed or floating objects. These polymorphic indi- 

 viduals are closely adapted to each other in structure and divi- 

 sion of labor; and the colonial habit in general, even where there 

 is no division of labor, is a successful device whereby limited 

 areas are completely occupied by the members of a species (as 



