582 APPENDIX C. 



It is not the aim of the foregoing reasoning to show that mechani- 

 cal actions are the sole causes of the formation of dense tissue in 

 plants. Dense tissue is in many cases formed where no such causes 

 have come into play as, for example, in thorns and in the shells of 

 nuts. Here the natural selection of variations can alone have ope- 

 rated. It is manifest, too, that even those supporting structures the 

 building up of which is above ascribed to intermittent strains, may, 

 in the individual plant of a species that ordinarily has them, be de- 

 veloped to a great extent when intermittent strains are prevented. 

 We see this in trees that are artificially supported by nailing to 

 walls ; and we also see a kindred fact in natural climbers. Though 

 in these cases the formation of wood is obviously less than it would 

 be were the stem and branches habitually moved about by the wind, it 

 nevertheless goes on. Clearly the tendency of the plant to repeat the 

 structure of its type (in the one case the structure of its species,- and 

 in the other case that of the order from which it has diverged in be- 

 coming a climber) is here almost the sole cause of wood-formation. 

 But though in plants so circumstanced intermittent mechanical 

 strains have little or no direct share, it may still be true, and I believe 

 is true, that intermittent mechanical strains are the original cause ; 

 for, as before hinted, the typical structure which the individual thus 

 repeats irrespective of its own conditions, is interpretable as a typical 

 structure that is itself the product of these actions and reactions be- 

 tween the plant and its environment. Grant the inheritance of func- 

 tionally-produced modifications ; grant that natural selection will 

 always co-operate in such way as to favour those individuals and 

 families in which functionally-produced modifications have pro- 

 gressed most advantageously ; and it will follow that this mechani- 

 cally-caused formation of dense substance, accumulating from gen- 

 eration to generation by the survival of the fittest, will result in an 

 organic habit of forming dense tissue at the required places. The 

 deposit arising from exudation at the places of greatest strain, re- 

 curring from generation to generation at the same places, will 

 come to be reproduced in anticipation of strain, and will continue 

 to be reproduced for a long time after a changed habit of the 

 species prevents the strain eventually, however, decreasing, both 

 through functional inactivity and natural selection, to the point 

 at which it is in equilibrium with the requirement. 



individual cell these structures are determined by these mechanical actions. 

 The facts clearly negative any such conclusion, showing us, as they in many 

 cases do, that these structures are assumed in advance of these mechanical 

 actions. The implication is, that such mechanical actions initiated modifi- 

 cations that have, with the aid of natural selection, been accumulated from 

 generation to generation; until, in conformity with ordinary embryological 

 laws, the cells of the parts exposed to such actions assume these special 

 structures irrespective of the actions the actions, however, still serving to 

 aid and complete the assumption of the inherited type. 



