488 LOUIS AGASSIZ. 



To this period .belongs also the following 

 fragment of a letter to Humboldt. 



TO ALEXANDER VON HUMBOLDT. 



[Probably 1852, date not given.] 



. . . What a time has passed since my last 

 letter ! Had you not been constantly in my 

 thoughts, and your counsels always before me 

 as my guide, I should reproach myself for 

 my silence. I hope my two papers on the me- 

 dusae, forwarded this year, have reached you, 

 and also one upon the classification of insects, 

 as based upon their development. I have 

 devoted myself especially to the organization 

 of the invertebrate animals, and to the facts 

 bearing upon the perfecting of their classifi- 

 cation. I have succeeded in tracing the same 

 identity of structure between the three classes 

 of radiates, and also between those of mol- 

 lusks, as has already been recognized in the 

 vertebrates, and partially in the articulates. 

 It is truly a pleasure for me now to be able 

 to demonstrate in my lectures the insensible 

 gradations existing between polyps, medusae, 

 and echinoderms, and to designate by the 

 same name organs seemingly so different. 

 Especially has the minute examination of the 

 thickness of the test in echinoderms revealed 



