544 



Dr. J. C. Willis. On the Lack of 



are practically no characters in the orders that can possibly be regarded as 

 adaptational, and most certainly not the characters — of thallus formations, 

 dorsiventrality of flower, etc. — which really characterise the orders, and 

 enable them to be separated in the natural system. But are even these 

 characters which we have enumerated really adaptations ? The development 

 of secondary shoots upon roots is by no means uncommon, though not usually 

 carried to such a pitch of regularity and perfection as here. The absence of 

 large intercellular spaces may be directly ancestral ; if the orders came, as is 

 quite possible, directly from plants growing upon the banks of the water, 

 their members may never have had large intercellular spaces at all, these 

 being quite useless in their mode of life. Haptera, perhaps, are really 

 adaptations, as they are all but unique, and appear to be modified root 

 branches, but they form the only feature in the two orders that one can point 

 to that is at all probably an adaptation. 



Geographical Distribution. — Another argument in favour of our main con- 

 tention may be derived from the geographical distribution of the Tristichacese 

 and Podostemaceae. The genera which are widely distributed are Tristicha* 

 in the one family and Podostemonj- in the other, both of them genera which 

 are comparatively little modified from the earlier types of the orders. Of the 

 other genera the distribution is, roughly, the following : — 



Lawia W. India and Ceylon. 



Weddellina Guiana and N. Brazil. 



GEnone Guiana and N". Brazil. 



Marathrum Mexico to Brazil. 



Ehyncholacis Guiana. 



Apinagia Guiana and Brazil. 



Lophogyne Province of Kio de Janeiro. 



Mourera Guiana, Brazil. 



Lonchostephus Eiver Amazon. 



Lacis Biver Amazon. 



Dicraea Madagascar, India, Ceylon. 



Hydrobryum India, Ceylon. 



Ceratolacis \ Brazil. 



Mniopsis Brazil. 



Oserya Brazil to Mexico. 



Castelnavia Brazil. 



* Mexico, S. America, Africa, Madagascar, India, 

 t Ohio to Argentina, Ceylon, India, probably Africa. 



