independent of external conditions. They are, so to say, means for bridging 

 a special interval of time. And sometimes they also span space, as in the case 

 of migratory spores or the seeds of many species. 



As we survey life forms from the smallest and simplest to the familiar 

 and complex animals most like ourselves, we see a progressive increase in the 

 amount of differentiation that takes place during the individual's develop- 

 ment. That is, there come to be more kinds of cells, more kinds of organs and 

 tissues. This differentiation includes the appearance of specialized repro- 

 ductive structures and processes. These culminate in sexual, as distinguished 

 from vegetative or asexual, reproduction. In this process, among plants as 

 among animals, two germ cells or units of protoplasm unite into one, which 

 becomes the beginning of a new individual. 



In the simplest forms of sexual reproduction, almost any cell may act as a 

 gamete. But there is a progressive differentiation of gametes into male and 

 female. The two gametes differ in the simplest forms chiefly in size. But in 

 later forms they show other distinctive characters, such as relative motility 

 and relative amount of accumulated food material. There appear highly 

 specialized gamete-bearing structures, with various adaptations to the distri- 

 bution of gametes and to the bringing together of sperm cells and tgg cells. 



Along with speciaHzation of gametes there is a progressive development 

 of secondary sexual characters. These involve, among the more complex 

 members of the various plant and animal phyla, modes of behavior that dis- 

 tinguish the male and the female of the species. And there is further develop- 

 ment of specialized structures and modes of behavior that have to do with the 

 protection of zygotes and their distribution. 



In the higher vertebrates, organs and processes related to perpetuating the 

 species develop side by side with organs and processes that increasingly free 

 the organisms from external conditions and dangers. And from a human point 

 of view, there is a tremendous increase of free activity that brings satisfac- 

 tions over and above merely keeping alive. There is in particular the excep- 

 tionally long period of childhood, in which relative freedom and security make 

 it possible to develop talents and interests of great personal and community 

 significance. 



434 



