THE ELEMENTS OF ORGANIC EVOLUTION. 8r 
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 
