32 PROBLEMS OF GENETICS 



the process of division, or, more generally, repetition of parts, 

 plays in the constitution of the forms of living things. 



That there may be a real independence between the Meristic 

 and the Substantive phenomena is evident from the fact both 

 that Meristic changes may occur without Substantive Variation, 

 and that the substances composing an organism may change 

 without any perceptible alteration in its meristic structure. 

 When the distinction between these two classes of phenomena 

 is perceived it will be realised that the study of genetics has on 

 the one hand a physical, or perhaps more strictly a mechanical 

 aspect, which relates to the manner in which material is divided 

 and distributed; and also a chemical aspect, which relates to 

 the constitution of the materials themselves. Somewhat as 

 the philosophers of the seventeenth and eighteenth centuries 

 were awaiting both a chemical and a mechanical discovery which 

 should serve as a key to the problems of unorganised matter, 

 so have biologists been awaiting two several clues. In Mendelian 

 analysis we have now, it is true, something comparable with the 

 clue of chemistry, but there is still little prospect of penetrating 

 the obscurity which envelops the mechanical aspect of our phe- 

 nomena. To make clear the application of the terms chemical 

 and mechanical to the problem of Genetics the nature of that 

 problem must be more fully described. In its most concrete 

 form this problem is expressed in the question, how does a cell 

 divide? If the organism is unicellular, and the single cell is 

 the whole body, then the process of heredity is accomplished 

 in the single operation of cell-division. Similarly in animals and 

 plants whose bodies are made up of many cells, the whole process 

 of heredity is accomplished in the cell-divisions by which the 

 germ-cells are formed. When therefore we see a cell dividing, 

 we are witnessing the process by which the form and the proper- 

 ties of the daughter-cells are determined. 



Now this process has the two aspects which I have called 

 mechanical and chemical. The term " Entwicklungsmechanik" 

 has familiarised us with the application of the word mechanics 

 to these processes, but on reflexion it will be seen that this com- 

 prehensive term includes two sorts of events which are sometimes 



