THE ELEMENTS OF ORGANIC EVOLUTION. gl 



cells, male and female, to form a new fertilized cell, has 

 as its essential function the promotion of variation. 

 The processes of karyokinesis, the subdivision of the 

 nuclear material in the formation of a new cell, tend in 

 the same direction. By the result of the subdivisions 

 incident in forming the sperm cell or the ovum, no one 

 of these is left exactly like any other. From this point 

 of view we say that variation is, as Professor Osborn has 

 pointed out, "in reality a phase of heredity." The 

 same structures that provide for the continuance of the 

 species prevent the actual repetition of the individual. 



Besides these sources of germinal variation there are 

 the forces or laws which produce acceleration or retar- 

 dation in growth. Much of the advance in power or 

 specialization among organisms comes from the saving 

 of time in the process of development. As growth goes 

 on, the forms we call lower pass slowly through the 

 .various stages of life. Their development is finished 

 before any high degree of specialization is reached. 

 The embryo of the higher form passes through the same 

 course, but with a rapidity in some degree proportioned 

 to its future possibility. Less time is spent on non- 

 essentials, and we may say that by the saving of time 

 and force it is enabled to push on to higher devel- 

 opment. 



The gill structures of the fish by which its blood is 

 purified by contact with air dissolved in water last its 

 whole lifetime. The fish never outgrows this structure 

 and never acquires the function of breathing atmospheric 

 air. The frog is fish-like for a period in its life, but the 

 development is accelerated, organs for breathing atmos- 

 pheric air are produced, and the gills become atrophied 

 and disappear from view. Their traces remain, for by 

 the law of heredity no creature can ever wholly let go 

 of its past. That its ancestors once breathed in water 



