3 i2 PLANT-BREEDING 



less large number of units, whilst divergence must evidently 

 be the result of the occurrence of different units. The 

 larger the number of common units, and the smaller that of 

 varying constituents, the greater will be the affinity. 



Wide as are the prospects of solving the most difficult 

 problems of systematic science and comparative investiga- 

 tions, and tempting as it is to indulge in a discussion of the 

 possibility of the discovery of their ruling laws on the ground 

 of the principle of units, we must, of course, now limit our- 

 selves to questions more directly concerned with our inquiry. 



Among these the chief point is, what is to be considered 

 as a unit ? This question is intimately connected with our 

 main subject, and sufficient facts are at hand to draw at 

 least a preliminary sketch. At once it brings us upon the 

 ground of the internal causes of the phenomena of correlation 

 and so completes the scheme made by our distinction be- 

 tween these and the external factors. 



Our units may be considered from two different points 

 of view. We may be content with analyzing the visible 

 characters and with reducing them to independent groups, 

 or we may ask for some invisible, although material cause, 

 which constitutes the real source of each unit. This latter 

 inquiry, however, is as yet wholly of a hypothetical nature, 

 and so it may suffice to have suggested it, and to return to 

 the visible features for our further discussion. 



There is no reason for assuming that a unit should be 

 limited to one organ, to one tissue, or to one cell. Quite on 

 the contrary, it seems probable that a unit may show its 

 activity in different organs, sometimes even in almost all parts 

 of a plant. This conception affords a broad principle for 

 the explanation of a large group of correlations, the corre- 

 lated external marks being simply assumed to be the expres- 

 sions of the same internal character. The faculty of pro- 

 ducing a red or blue color may be taken as an instance. If 



