7l) MKCIIANICAL. [iXTROD. 



that lie ahead in the Study of Variation, that the great value of 

 tin- in. -th. ..| can be understood. 



h ha- 1..-OI s.-en that variations may be either Meristic or 



Sub-taut i\. . and that in each group discontinuous and definite 



one may occur by steps which may be integral or total. 



\\ are imw seeking the factors which determine this totality and 



.1. tin.- th.- forms assumed in Variation. In this attempt we may, 



l.v jirl.it tariK confining our first notice to very simple cases, recog- 



n"i/e at 1. a-t two distinct factors which may possibly be concerned 



ii, th;- : initiation. Of these the first relates to Meristic 



at ion and tin- second to Substantive Variation. 



I. Possible nature of the Discontinuity of Meristic Variation. 



Looking at simple cases of Meristic Variation, such as that of 

 tin- Tulip or of Aurelia, or of the Cockroach tarsus, there is, I think, 

 a fair sugg( -ii"! that the definiteness of these variations is deter- 

 mini-'l mechanically , and th&t the patterns into which the tissues 

 of animal- an- divided represent positions in which the forces that 

 . tf.-.-t tin di\i.-ion are in equilibrium. On this view, the lines or 

 plan-- of di\ision would be regarded as lines or planes at right 

 angl. > t.. tin- directions of the dividing forces; and in the lines of 

 M. ii-tii- ]>i\i-ion we are perhaps actually presented with a map 

 of the lines of tho-e forces of attraction and repulsion which 

 di-tcrmim- th>' number and positions of the repeated parts, and 

 ti-.in ulii.-h Symmetry re.-ults. Jf the Symmetry of a living bod}' 

 \\t-n- tlni- i. '"_; ni/. d as of the same nature as that of any sym- 

 ni. nical -\-teiii 'f mechanical forces, the definiteness of the sym- 



4 V 



in. n\ in Meri-tic Variation would call for no special remark, and 



the \ Tlei-tion of th>- -ymmetry of a Tulip \\ith its parts divided 

 int.. four, though occurring suddenly as a " sport," would be recog- 

 ni/. d a> in nowis.- more sin--nlar than the symmetry of the type. 

 I loth alike would tln-n In seen to owe their perfection to me- 

 dianical condition- and not to Selection or to any other gradual 

 process. It reason for adopting such a view of the physics of 

 l>i\i-ion >hould a|i)iear. the frei|iiency with which in any given 

 l-'im a particular pattern of Division or of Symmetry recurs, 

 \\oiild lie toiind to l.e determined by and to be a measure of the 

 stability of the forces of l)i\ivj,,n \\hen disposed in that particular 

 patt.rn. It will ..f course lie understood that in these" remarks 

 H" ! !"ii i- offered as t" the causes \\hich determine whether 



a tis-.ii.- shall di\id.- into four or into three, but merely as to the 

 Conditions "f pert'. -t ion ..f the di\ ision iii eit her case. It will also 

 1" clear that though the symmetry of a flower or of any other 



tissue depend- a I- -\ minelrical gr.. \\tli, it is primarily dependent 



on tin- -\miiietn of its primary di\isions. ujion which symmetrical 

 ;ji..uili and -econdary sy nniiet rical di\i-ions follow. 



