594 APPENDIX C. 



fact to be interpreted is, that in the same individual plant homol 

 ogous parts, which, according to the type of the plant, should be 

 equally woody, become much thicker one than another if subject 

 to greater mechanical stress. And of this too an interpretation is 

 similarly afforded. 



Now the sufficiency of the assigned actions to account for so many 

 phenomena not otherwise explained, would be strong evidence that 

 the rationale is the true one, even were it of a purely hypothetical 

 kind. How strong, then, becomes the reason for believing it the 

 true one when we remember that the actions alleged demonstrably 

 go on in the way asserted. They are ever operating before our 

 eyes ; and that they produce the effects in question is a conclusion 

 deducible from mechanical principles, a conclusion established by 

 induction, and a conclusion verified by experiment. These three 

 orders of proof may be briefly summed up as follows. 



That plants which have to raise themselves above the earth's sur- 

 face, and to withstand the actions of the wind, must have a power of 

 developing supporting structure, is an a priori conclusion which may 

 be safely drawn. It is an equally safe a priori conclusion, that if 

 the supporting structure, either as a whole or in any of its parts, has 

 to adapt itself to the particular strains which the individual plant 

 is subject to by its particular circumstances, there must be at work 

 some process by which the strength of the supporting structure is 

 everywhere brought into equilibrium with the forces it has to bear. 

 Though the typical distribution of supporting structure in each kind 

 of plant may be explained teleologically by those whom teleological 

 explanations satisfy ; and though otherwise this typical distribution 

 may be ascribed to natural selection acting apart from any directly 

 adaptive process ; yet it is manifest that those departures from the 

 typical distribution which fit the parts of each plant to their special 

 conditions are explicable neither teleologically nor by natural selec- 

 tion. We are, therefore, compelled to admit that, if in each plant 

 there goes on a balancing of the particular strains by the particular 

 strengths, there must be a physical or physico-chemical process by 

 which the adjustments of the two are effected. Meanwhile we are 

 equally compelled to admit, a priori, that the mechanical actions to 

 be resisted, themselves affect the internal tissues in such ways as to 

 further the increase of that dense substance by which they are re- 

 sisted. It is demonstrable that bending the petioles, shoots, and 

 stems must compress the vessels beneath their surfaces, and increase 

 the exudation of nutritive matters from them, and must do this act- 

 ively in proportion as the bends are great and frequent ; so that 

 while, on the one hand, it is a necessary deduction that, if the parts of 

 each plant are to be severally strengthened according to the several 

 strains, there must be some direct connexion between strains and 

 strengths, it is, on the other hand, a necessary deduction from 



