EXCURSIONS ON THE COAST. 393 



should keep in mind this fact of the striking contrasts 

 presented under different latitudes by the same families. 

 Of course the equatorial regions teem with plants and 

 trees belonging to families either entirely unknown or 

 but poorly represented in more temperate latitudes ; and 

 these distinct groups naturally arrest the attention of the 

 botanist, and perhaps awaken his interest more than those 

 with which he is already familiar under other forms. 

 But, while these different families are recognized as dis- 

 tinct, and no doubt deserve to be considered by them- 

 selves as natural groups, I believe that much might be 

 learned of the deeper relations of plants by studying, 

 not only the representatives of the same families in dif- 

 ferent latitudes, such as the Mimosas and the Milk-weeds, 

 but also what I may call botanical equivalents, groups 

 which balance each other in the different climatic zones. 

 This idea is suggested to me by my zoological studies in 

 the Amazons, which have led me to perceive new relations 

 between the animals of the temperate and the tropical zone : 

 it seems probable that corresponding relations should ex- 

 ist in the vegetable world also. Struck, for instance, by 

 the total absence of sturgeons, perches, pickerels, trouts, 

 carps and other white fishes, cusks, sculpins, <rc., I have 

 asked myself, while studying the fishes of the Amazons, 

 what analogy could exist between those of our Western 

 rivers and those of the tropics, as well as between the 

 latter and those of the intermediate latitudes. Looking 

 at them with this view, I have been surprised to find 

 how closely related the Goniodonts are to the Sturgeons ; 

 so much so, that the Loricaria? may be considered as gen- 

 uine Sturgeons, with more extensive shields upon the body. 

 I am satisfied also that the Cychla is a porch to all intents 



