ASPECTS OF KINETIC EVOLUTION 367 



lengthened to cover nearly all the activities of the species in 

 higher types of life. 



Among the lower groups the overlapping of the generations 

 appears to be a mere coincidence and serves no important evo- 

 lutionary purpose, but among the higher types it is a condition 

 of the utmost significance, since it has permitted the develop- 

 ment of parental instincts and of the numberless devices and 

 habits by which the eggs or seeds or the young individuals are 

 protected and nourished through periods of helplessness. The 

 lengthening of the embryonic and juvenile periods has been 

 necessary to permit the development of large and highly special- 

 ized organisms. The overlapping of the generations is also a 

 prerequisite for the development of social habits and instincts, 

 and especially in the transmission of the postnatal inheritance 

 on which the development of human culture and civilization 

 depends. Civilization has been developed and has persisted 

 only among those races in which the family unit of social organ- 

 ization was maintained, so that the children secured the advan- 

 tage of long and intimate contact with their parents and were 

 thus able to acquire, transmit and accumulate in the race the 

 collective experience and progress of the component individuals 

 and families. Thus the aborigines of tropical America who 

 live mostly in separate and isolated families have built up 

 numerous primitive civilizations, while the natives of tropical 

 Africa who live only in villages have never developed civiliza- 

 tions. Indian children are the constant associates and helpers 

 of their parents while the children of an African village are 

 herded among themselves in little troops or squads like the street 

 waifs of our slums. Even our highly developed systems of 

 formal education have this serious defect and danger, that they 

 tend to disconnect the generations, and to throw the young into 

 premature and reactionary forms of social organization instead 

 of permitting them to grow gradually into their normal places 

 in the general fabric of the community. 



DIFFERENT TYPES OF CELLULAR ORGANIZATION. 



The complexity of the constitution of species can not be fully 

 appreciated unless it be kept in mind that each individual of all 



