238 COOK 



species have even a general similarity of structure and external 

 appearance. 



There have been extensive and not altogether profitable dis- 

 cussions of the relation of growth-characters to those of the adult 

 and to the evolutionary history of the species. The older em- 

 bryologists worked out a doctrine of recapitulation to explain 

 larval and juvenile characters, but it is evident in some groups, 

 such as the insects, that preliminary stages may be quite as adap- 

 tive as the adult form of the species, and sometimes distinctly 

 more so. The differences of growth-stages are themselves of 

 very different types in the various natural groups, as a result of 

 the great diversity of methods by which evolution has been 

 accomplished. 



THREE TYPES OF CELLULAR STRUCTURES. 



The most fundamental diversity of form and structure which 

 exists among the members of the same species is that which 

 arises from the existence of different types of cell-organization. 

 In many of the lower groups of plants the vegetative organism, 

 like a filamentous alga or a moss-plant, is composed of simple 

 cells which have not conjugated and which have in many cases 

 no power of conjugation. In the higher types of plants and 

 animals the body of the organism, in its highest and most com- 

 plete form, is built up of cells in a double or conjugating condi- 

 tion. The higher fungi differ from the ferns, flowering plants, 

 and higher animals in that the cells associate themselves while 

 in the first stage of conjugation, before the nuclei have fused, 

 while the cells of the other groups represent the second stage of 

 conjugation. The nuclei have fused, but the chromatin gran- 

 ules still remain distinct. 1 



The great diversity of the cells which compose the bodies of 

 the higher plants and animals may be viewed as a phenomenon 

 of social organization. The lower the organism the more alike 

 are the cells until in the lowest all cells are similar and equal. 

 Where socialization, the habit of joining together or living in 

 groups, has not progressed too far, the cells of compound indi- 



1 Cook, O. F., and Swingle, W. T., 1905. Evolution of Cellular Structures. 

 Bulletin 81, Bureau of Plant Industry, U. S. Department of Agriculture. 



