57 



1899, writes as follows : "In a species of Dendrobium the 

 sepals and petals are white, ti[)i)ed with })Lirple, and the li^) 

 is bright orange with two crimson s[)()ts. This is probably 

 due to the marked tendency of the si)ecies to variation." 



C. Darwin's reference to iionoy guides will I)e found 

 farther on. Of these se\'en writers of repute, oidy one 

 hazards the (jpinion that they "have l)een determined by 

 insects," but not a hint is vouchsafed of the manner in 

 which they were determincnl, nor by what kind of insects. 

 Neither have I ever seen or heard of any explanation re- 

 garding their origin ; I shall therefore offer the following 

 as an original elucidation of the subject. 



For our first example in illustration, let us take a connnou 

 yellow garden Tropicolum, otherwise Nasturtium.* 



The plant is either climbing or dwarf, with round peltate 

 leaves, and the long-spurred flower has many varieties. We 



'The pen aud ink sketches iire l)y Joseph E. Lander? 



