MERISTIC VARIATIONS 131 
composite plants—e.g., the daisy and chrysanthemum. 
It is suggested that meristic variations are connected 
with definite changes in the mechanical relations of 
dividing parts, and that it is in the mechanics of cell- 
division that the explanation of their discontinuous 
appearance is to be sought for. 
Thus when, for example, a tulip-flower appears 
having its parts perfectly developed in sets of four 
instead of in sets of three, it is suggested that the 
arrangement in fours, like the arrangement in threes, 
fulfils certain conditions of equilibrium among the 
forces which affect the cell-divisions in the rudiment 
of the flower, and that these conditions of stability 
would not be equally well provided for by any inter- 
mediate arrangement. 
Substantive variations are changes in the actual 
constitution or substance of the parts themselves. 
For example, a plant with coloured flowers may give 
rise to offspring the flowers of which are white. There 
seems to be no mechanical necessity for such varia- 
tions to be discontinuous rather than continuous; it 
is quite possible to imagine a gradual dilution of colour 
taking place throughout a long series of generations. 
Discontinuous substantive variations are, however, not 
infrequent, and in such cases it is suggested that they 
may be associated with definite changes in chemical 
composition. Thus, for example, definite alterations 
in the colour of offspring as compared with their 
parents are almost necessarily of a chemical nature. 
The further evidence contained in the book we are 
considering refers entirely to meristic variation. 
9Q—z 
