663 



fdct to be inteq)reted is, that in the same individual plant homologoaa 

 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 mterpretation is 

 similarly afforded 



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

 phenomena not otherwise explained, would be strong eyidence that 

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

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

 true one when we remember that the actions alleged demonstrably go 

 on in the way asserted. They are ever operatmg before our eyes-, 

 and that they produce the effects in question is a conclusion dedu- 

 cible from mechanical principles, a conclusion established by induction, 

 and a conclusion verified by experiment. These thi-ee 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 ol 

 developing supporting structure, is an a priori conclusion which may 

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

 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 i? 

 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 thougli otherwise this typical distribution 

 may be ascribed to natural selection acting apart from any directly 

 adaptive process ; yet it is manifest that those departm-es 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 m 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 fur- 

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

 It is demonstrable that bending the petioles, shoots, and stems must 

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

 lion 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 sirengtheiied according to the several strains, 

 tliere must be some direct connexion between strains and strengths, 

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



