551 



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 

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

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

 I uilding up of which is above ascribed to intermittent strains, ma/, 

 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 

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

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

 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 becom- 

 ing 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 irre- 

 spective of its own conditions, is interpretable as a typical structure 

 that is itself the product of these actions and reactions between the 

 plant and its environment. Grant the inheritance of functionally- 

 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 progressed most ad- 

 vantageously ; and it will follow that this mechanically-caused forma- 

 tion of dense substance, accumulating from generation to generation 

 by the survival of the fittest, will result hi an organic habit of form- 

 ing dense tissue at the required places. The deposit arising from 

 exudation at the places of greatest strain, recurring from generation 

 to generation at the same places, will come to be reproduced in an- 

 ticipation of strain, and will continue to be reproduced for a long 

 time after a changed habit of the species prevents the strain even- 

 tually, however, decreasing, both through functional inactivity and 

 natural selection, to the point at which it is in equilibrium with tho 

 requirement. 



in Hvidudl cell these structures are determined bv 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. 



