563 



fact to be interf)reted 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 sub- 

 ject to greater mechanical stress. And of this too an interpre- 

 tation is similarly afforded. 



Now the sufBciency 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 

 bind. How strong, then, becomes the reason for beUe™g 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 dedn- 

 cible from mechanical principles, a conclusion estabhshed 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 d, pi-iori conclusion which may 

 be safely drawn. It is an equally safe d, priori conclusion, that il 

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

 to adapt itself to the particular strains w,hich 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 ie 

 OYcrywhere brought into equiUbrium 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 teleologica] 

 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, cbpriori, that the mechanical actions to be 

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

 ther the increase^ of that dense substance by which they are resisted. 

 It i=i demonstrable that bending the petioles, shoots, and stems musi 

 compress the vessels beneath their surfaces, and increase the exuda- 

 tion of nutritive matters from them, and must do this actively in pro- 

 portion 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, 

 tUere must be some direct connexion between strams and strengths, 

 It is, on the other hand, a necessary deduction from mechanical prin- 



