xvi PREFACE. 



uidniiig patience and unsurpassed accuracy, most of the microscopic illustrations wliicli 

 adorn my work. I owe it to IVIi-. Clark to say, that he has identified himself so thoroughly 

 with my studies since he took his degree in the Lawrence Scientific School, that it would 

 be didicult for me to say when I ceased to guide him in his work. But this I know 

 very well, — that he is now a most trustworthy observer, fully cajrable of tracing for 

 himself the minutest microscopic investigation, and the accuracy of his illustrations 

 challenges comparison. I esteem myself happy to have been able to secure the continued 

 assistance of my old friend, Mr. A. Sonrel, in drawing the zoological figures of my work. 

 More than twenty years ago, he began to make illustrations for my European works ; and 

 ever since he has been engaged, with short interruptions, in executing drawings for me. 

 The mastery he has attained in this department, and the elegance and accuracy of his 

 lithographic representations, are unsurpassed, if they are anywhere equalled. For all these 

 invaluable services, it is but justice that I should make this public acknowledgment. 



As questions of omission or oversight may come up hereafter respecting the different 

 topics discussed in these volumes, it is proper for me to state, that the printing of the 

 text of the first volume has been completed more than ten months ; indeed, the First Part 

 passed through the press fifteen months ago. My object in delaying its publication was 

 chiefly to await the time when I could lay before my readers a fair specimen of the 

 plates, no one of which relates exclusively to the first volume. The text of the second 

 volume was finished in June last. But here I met with another difficulty. The subject 

 of this volume did not require a sufficiently large number of plates to be fully equivalent 

 to that required for two volumes, when counting the plates as they now are, as sim]ile 

 plates, notwithstanding the large increase of figures crowded upon each, and it seemed inap- 

 propriate to bind together plates belonging to different volumes. I shall therefore have to 

 make up for this deficiency by a sufficient addition of plates to the third volume, the sub- 

 ject of which naturally requires very numerous illustrations. I hope no disappointment 

 will be felt, on this account, by my subscribers, for in the course pursued by the pub- 

 lishers and by myself, they will readily see that we have aimed to do every thing in our 

 power to respond to the liberality of the subscription ; and I ti-ust the following volumes 

 will afford additional evidence of this disposition. 



LOUIS AGASSIZ. 



Cambkidge, October 3, 1857. 



