On the Migrations of Animals. 169 



rather special interest to the circumstance that the 

 two men met as has been described and afterwards 

 kept up a friendly correspondence. 



In February, 1867, Mr. Robertson writes to Mr. 

 Edward a letter which shows that the two men did 

 not merely interchange idle compliments. 



" MY DEAR SIR, 



"Many thanks for your two valuable and 

 interesting papers. I agree with you that much is 

 yet to be learned regarding habitat. Too much 

 weight has been attached to incidental observations, 

 or to those of only a particular season of the year, or 

 observations made (repeated) under similar circum- 

 stances, and from such vague finding permanent 

 habitat has been fixed. We might as well set down 

 a crow as the parasite of a tree, because we often find 

 them congregating on it. No doubt there are many 

 animals with fixed (special) habitats, away from which 

 they could not exist, and many parasites which can 

 only live on one particular species of plant or animal, 

 and, in some cases, only on particular parts of the 

 animal or plant ; but many, as you have clearly 

 proved, that have been considered to belong to par- 

 ticular habitats are decidedly migratory in their 

 habits, (their movements) depending, no doubt, in a 

 great measure, on the season of the year, the state of 

 the weather, and the age of the animals. 



" In the case of amphipods and such obscure and 

 diminutive creatures, they have to be sought for and 

 studied the whole year round before trustworthy 

 conclusions can be come to. 



