70 MECHANICAL. [introd. 



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

 this method can be understood. 



It has been seen that variations may be either Meristic or 

 Substantive, and that in each group discontinuous and definite 

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

 We are now seeking the factors which determine this totality and 

 define the forms assumed in Variation. In this attempt we may, 

 by arbitrarily confining our first notice to very simple cases, recog- 

 nize at least two distinct factors which may possibly be concerned 

 in this determination. Of these the first relates to Meristic 

 Variation and the second to Substantive Variation. 



1. Possible nature of the Discontinuity of Meristic Variation. 



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

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

 a fair suggestion that the definiteness of these variations is deter- 

 mined mechanically, and that the patterns into which the tissues 

 of animals are~3Ivided represent positions in which the forces that 

 effect the division are in equilibrium. On this view, the lines or 

 planes of division would be regarded as lines or planes at right 

 angles to the directions of the dividing forces ; and in the lines of 

 Meristic Division we are perhaps actually presented with a map 

 of the lines of those forces of attraction and repulsion which 

 determine the number and positions of the repeated parts, and 

 from which Symmetry results. If the Symmetry of a living body 

 were thus recognized as of the same nature as that of any sym- 

 metrical system of mechanical forces, the definiteness of the sym- 

 metry in Meristic Variation would call for no special remark, and 

 the perfection of the symmetry of a Tulip with its parts divided 

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

 nized as in nowise more singular than the symmetry of the type. 

 Both alike would then be seen to owe their perfection to me- 

 chanical conditions and not to Selection or to any other gradual 

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

 Division should appear, the frequency with which in any given 

 form a particular pattern of Division or of Symmetry recurs, 

 would be found to be determined by and to be a measure of the 

 stability of the forces of Division when disposed in that particular 

 pattern. (It will of c ourse be understood that_jn_these remarks 

 no suggestion is offered as to the causes which determine whether 

 a tissue shall divide into four or into three, but merely as to the 

 conditions of perfection of the division in either case!^ It will also 

 Be" clear that l^ough~the symmetry of aTflower or" of any other 

 tissue depends also on symmetrical growth, it is primarily dependent 

 on the symmetry of its primary divisions, upon which symmetrical 

 growth and secondary symmetrical divisions follow. 



