Lack of Adaptation in the Tristichacece and Podostemacece. 533 



different conditions of life. The climate, the soil, the competition with other 

 living beings, many conditions, vary in the most complicated way, and it is 

 very difficult, if not impossible, to disentangle the effect of the different 

 factors that may have a hand in producing the result. But in the case of 

 these two closely allied families we are able to study the problem of evolution 

 with many of the complication of factors removed or simplified. There is no 

 competition with other families, for these two have their habitats to themselves. 

 There is no difference in soils, for all grow on naked waterworn rock. There 

 is no difference in climate, for all grow in water in the tropical or sub-tropical 

 zone, and are represented only by seeds in the colder weather. There is no 

 difference in circumambient medium, for all grow in running water. The 

 illumination is the same for all. The conditions of their life are the most 

 absolutely uniform that it is possible to couceive ; even in a laboratory it 

 would be very difficult to produce conditions more uniform, and they must 

 have been the same since the original founders of the orders first took to life 

 in running water. 



We cannot for one moment suppose that these plants took to the water at 

 different stages of evolution into a family upon land ; they must obviously 

 have gone through the whole of their evolution in water from an extremely 

 early stage, during which the ancestors probably retained a certain power of 

 surviving upon land. Now the most essential point of this argument is that 

 here we have two entire families, containing about 30 genera and over 100 

 species of the most varied morphological structure possible, entirely evolved 

 under perfectly uniform conditions which cannot have varied, except for all 

 alike,* since the evolution of the families began. The evolution consequently 

 cannot have been in any sort of response to a necessity of adaptation to 

 different conditions, for there are and have been no different conditions to 

 which to be adapted, since the first members of these families began to live in 

 running water. 



We shall see, first of all, that the conditions of life are absolutely uniform 

 (2) that the families contain the most astonishing variety of morphological 

 structure, (3) that all the different stages, so to speak, in the attainment of the 

 most bizarre of these forms, live together, and that the least bizarre are the 

 most widespread and common, and in consequence (4) that as there are 

 no changed conditions to which to be adapted, there can be no adaptation 

 to conditions other than to the general conditions which are common to 

 all, and have been common to them since the families began, and (5) that 

 therefore there can, in all probability, be no selection of infinitesimal variations, 

 but progress must have been by something of the nature of mutations or fixed 

 * As, for instance, if the climate became warmer. 



