POSTSCRIPT. 521 



It is diflficult to keep uj) a continuous uniformity under 

 repeated interruptions and considerable lapse of time. 

 To write a work that is novel in its plan and purpose, by 

 successive instalments, and to print it in like manner, 

 could scarcely fail to give to it something of the piece- 

 meal character. The earlier portions could not be made 

 to lead to the after portions in a sufficiently direct and 

 connected manner ; because the form in which it would 

 be found most convenient to draw out those after portions 

 for printing, especially the tabular lists, was only decided 

 as they were successively prepared for the press. Some 

 repetitions, and frequent back references, have hence 

 become necessary in this last volume, which might have 

 been less required under other circumstances. 



Wide differences of date in the volumes have also 

 tended to augment bulk. For instance, upwards of two 

 hundred pages of the third volume were devoted to 

 " additional species and notes," in order to bring up the 

 contents of the two preceding volumes to the same date. 

 But if those three volumes could have been printed and 

 published almost simultaneously, most of the requisite 

 alterations might have been made by a few figures and 

 statements in the manuscripts, without augmentation of 

 bulk. Such alterations have been made in re-stating the 

 distribution of several species in tliis fourth volume ; so 

 that the ai'eas and census, the altitudes and temperatures, 

 as indicated in the lists, wiU not always correspond with 

 those stated for the same species separately in the former 

 volumes. 



Doubtless another dozen or score of years will again 

 accumulate another store of facts, to yield another supply 

 of "additional species and notes," such as may again lead 

 to various alterations in minor details. But the distribu- 

 tion of the several species, as now set' forth in the lists, 



VOL. IV. 3 X 



