ASPECTS OF KINETIC EVOLUTION 341 



they do not tend to stay uniform. Organisms are not naturally 

 pure-bred, and their tendencies are ever to be mixed more and 

 more. This is the overwhelming testimony of the facts of 

 nature, which the inventors of character-unit mechanisms would 

 do well to canvass before entering upon their labors. 



Chromosomes and granules as parts of cells are morpho- 

 logical entities, in the sense that they exist and can be made 

 visible by microscopical technique. It does not follow, how- 

 ever, that they are biological or evolutionary entities, or that 

 they can properly be thought of as having any general evolu- 

 tionary significance, except as parts or organs of cells or of or- 

 ganisms, which are the units of life. Moreover, as already in- 

 dicated from other considerations, not even organisms can be 

 considered units of evolution, which requires the coherent net- 

 work of descent of a normally diverse, interbreeding species. 



CONTACTS BETWEEN LINES OF DESCENT. 



The fact that the lines of descent are joined only in repro- 

 ductive cells should not be taken to mean that there is merely 

 a single or casual contact between them, nor prevent our 

 recognizing the possibility that the functions of the chromatin 

 granules may be physiological rather than morphological. It is 

 through them, evidently, that the reorganization of the proto- 

 plasm of the cells is accomplished. They represent the citadels 

 of life, the most vital points of the cell substance. The final 

 stage and apparent purpose of the process of conjugation is to 

 bring them into contact with other granules from other lines of 

 descent. The nature of this contact, whether the granules 

 exchange particles, or renew their vital energy by molecular or 

 other adjustments, is still unknown. 



The most recent results of cytological investigation are in 

 accord with the supposition that the ability of the higher plants 

 and animals to lessen the number of conjugations and prolong 

 the intervals of vegetative growth, has been attained by the 

 development of more and more efficient methods of conjuga- 

 tion. A few years ago the opinion was held that the proc- 

 ess of synapsis involved only a fusion and reduction of 

 the number of the chromosomes ; it now appears that the 



