72 THE QUANTITATIVE METHOD IN BIOLOGY 



fundamental unit of weight, but they have wisely adapted the 

 new system to the needs of practical appUcation and therefore 

 they have chosen the gramme. Nowadays we use the series 

 milligramme, centigramme, gramme (C.G.S. system), kilo- 

 gramme, metric ton, the gramme being the standard. 



In physics and chemistry the molecules and atoms are in a 

 certain sense individuals or units of a certain order. For many 

 years the atom has been the ultimate term, in fact the 

 standard. In the new chemistry the atom is divided in its 

 turn into smaller units ; the classic system of units has been 

 completed, but neither altered nor disturbed by this innovation. 



In a similar way we may express in biology the notion of unit 

 or individual by means of one (or several) series of terms, the 

 cell being the standard and (provisionally) the simplest term. 

 The first series of units (see a second series in § 70) is based upon 

 the principle of segmentation and includes the terms :*:, x + i, 

 x + 2 . . . x + n. If necessary we may ascribe to a; a definite 

 value according to the case under consideration. If we want, 

 for one reason or another, to proceed from the unicellular units 

 to the more complex individuals, we may call the ceU an indi- 

 vidual {x + n), the next (pluricellular) term being {x + n)-i, 

 etc.i 



I do not attach much importance to the adopted notation of 

 the successive xinits. More important is the principle of the 

 adaptation of a series of biological units of successive order, a unit 

 of a given order being a component of a unit of the preceding 

 order and including several units of the next order. 



I hope that the proposed system is elastic enough to render 

 possible various apphcations. If we were compelled to divide 

 the cell itself into smaller units, the system would be com- 

 pleted but not altered, x + n remaining x + n. 



§55.— FIRST EXAMPLE: SPIROGYRA (continued).— 

 An individual x (adult specimen) of Spirogyra consists of a 

 number of individuals x + i (cells). This number is a primor- 

 dium of the individual x. We may look upon all the cells as 

 being cylindric, except both terminal cells, which are cylindric 

 and rounded at their distal (external) extremity.^ We find 

 here a very simple example of differentiation : the individuals 

 x + i (cells) are indeed of two kinds, terminal and intermediate. 



1 In Spirogyra n—i. In a leaf of an elm, in the salivary gland of a horse, 

 the value of n is practically unknown. In these and similar complicated cases 

 a given cell is called (x + n). This means that it is the last term of a series, 

 the number of terms « + i and the value of the most complicated term x being 

 unknown. The value (x+n) is therefore undetermined, but the value oi 

 (x + n)-i is determined with regard to the cell. 



^ We must content ourselves with the term rounded because of our ignorance 

 of the exact form of the extremity. 



