ASPECTS OF KINETIC EVOLUTION 369 



groups may show only single lines, higher groups only double 

 lines, except at the actual points of junction where conjugation 

 takes place. 1 



In alternation of generation and metamorphosis the organism 

 changes its external form without altering the figure of descent. 

 Alternation of generations, like the differentiation of separate 

 sexes, exists in simple-celled as well as in double-celled organ- 

 isms. The phenomena are of an entirely different and minor 

 order of significance compared with the diversities of the dif- 

 ferent types of cellular structure. Wonderful as the changes 

 are, they are still of a merely morphological and adaptive 

 character and do not indicate new evolutionary departures of the 

 scope of the double-celled structures. 



SPECIFIC CONSTITUTIONS MODIFIED BY SPECIALIZED HETERISM. 



There are two principal groups or kinds of specific constitu- 

 tions which can be studied or thought of as cross-sections of the 

 networks of descent. These two series of special types of species 

 arise through two forms of specialization of methods of descent. 

 Instead of remaining uniform or homogeneous throughout, the 

 network of descent becomes variously subdivided or separated 

 into subspecific strands. 



The first form of subspecific differentiation consists in special- 

 izations of heterism, that is, the establishment within the species 

 of definite forms of diversity of descent, so that individuals are 

 not merely different individually, but fall into two or more 

 groups regularly distinguishable by definite characters. These 

 groups are not formed by isolation, and their existence does not 

 interfere with interbreeding, but usually has the contrary effect 

 of encouraging or compelling interbreeding, since the members 

 of the same group may be unable to interbreed with each other, 

 but are specially adapted for interbreeding with the members of 

 the other group or groups of which the species is composed. 



SPECIES WITHOUT SPECIALIZATION OF HETERISM (ARROPIC). 



The diversity of normal symbasic descent remains miscel- 

 laneous and unspecialized. The individuals may be more or less 



1 Diagrams of networks of descent in the various types of double-celled struc- 

 tures have been given in another place. Bulletin Si, Bureau of Plant Industry, 

 U. S. Department of Agriculture. 



