306 Willis.—The Origin of the Tristichaceae and Podostemaceae. 
must have been a fairly large one—either the plants can live as these plants 
do, or they cannot. But if so, we must allow that mutations may be ‘ large , , 1 
and we must admit that Natural Selection, which works by small variations, 
could not effect the change, both of these being points for which I have con¬ 
tended in other papers. No amount of ‘small’ changes will transfer these 
plants from another mode of life to their present remarkable one. As 
I find that very few people in Europe realize the conditions under which 
these plants really grow, it may not be amiss to mention that the edge 
of such a waterfall as Stonebyres, where the water is actually pouring over, 
would be covered with them were the fall in Brazil. Or the shallower parts 
of the Strid, near Ilkley, Yorks., where there was light enough, would 
similarly be covered with them in Ceylon. 
In a later paper, I propose to deal with the evolution of these families, 
proposing a theory which appears applicable to evolution in general. 
1 There may have been a struggle for existence among the ancestors, but only a ‘ large ’ mutation 
would set a plant free from that, and we have no evidence to show that desirable changes may occur 
in response to any need for them, and evidence as for example with the seeds of these families, to 
show that they do not. 
