DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 121 



parental protection accompanies the development of more com- 

 plex and highly organized instincts, and intelligence. The 

 lengthened period of dependence, while a burden to the parent 

 in one sense, is an advantage to it in the saving in number of 

 offspring, and serves to benefit the species, not merely by keep- 

 ing the offspring alive until they may reproduce, but in the 

 greater development of such parental instincts as gentleness, 

 self-sacrifice, and the like. In the human race it has given rise 

 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 life of the 

 individual beyond this period becomes a disadvantage. This 

 disadvantage to the species, in the long run, fixes the normal 

 length of life in the species. 



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 rohin, 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 organically 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 



