Chap. I. FUNDAMENTAL RELATIONS OF ANIMALS. 5 



accept thoni as natural divisions much smaller ; few of tlieni having expressed a 

 belief that genera have as distinct an existence in nature as species. And as to 

 families, orders, classes, or any kind of higher divisions, they seem to be univer.sally 

 considered as convenient devices, framed with the view of facilitating the study of 

 innumerable objects, and of grouping them in the most suitable manner. The indif- 

 ference with which this part of our science is generally treated becomes unjustifiable, 

 considering the progress which Zoology in general has made of late. It is a matter 

 of consequence, whether genera are circumscribed in our systematic works within 

 these or those limits ; whether fiimilies inclose a wider or more contracted range of 

 genera ; whether such or such orders are admitted in a class, and what are the natu- 

 I'al boundaries of classes; as well as how the classes themselves are related to one 

 another, and whether all these groups are considered as resting upon the same foun- 

 dation in nature or not. 



Without venturing here upon an analysis of the various systems of Zoology, — the 

 prominent features of which are sufficiently exemplified for my purpose by the sys- 

 tems of Linnajus and Cuvier,^ which must be familiar to every student of Natural 

 History, — it is certainly a seasonable c^uestion to ask, whether the animal kingdom 

 exhibits only those few subdivisions into orders and genera which the Linntvan 

 system indicates, or whether the classes differ among themselves to the extent which 

 the system of Cuvier w^ould lead us to suppose. Or is, after all, this complicated 

 structure of Classification merely an ingenious human invention, -which everj' one may 

 shape, as he pleases, to suit himself? When we remember that all the works on Nat- 

 ural History admit some system or other of this kind, it is certainly an aim wor- 

 thy of a true naturalist, to ascertain what is the real meaning of all these divisions. 



Embryology, moreover, forces the inquiry upon us at every step, as it is impos- 

 sible to establish precise comparisons between the different stages of growth of young 

 animals of any higher group and the permanent characters of full-grown individuals 

 of other types, without first ascertaining what is the value of the divisions with 

 which we may have to compare embryos. This is my reason for introducing here, 

 in a work chiefly devoted to Embryology, a subject to Avhicli I lune paid the most 

 careful attention for many years past, and for the solution of which I have made 

 special investigations. 



Before I proceed any further, however, I would submit one case to the consider- 

 ation of my reader. Suppose that the innumerable articulated animals, which ai'e 

 counted by tens of thousands, nay, perhaps by hundreds of thousands, had never 

 made their appearance upon the surface of our globe, with one single exception : 

 that, for instance, our Lobster (Ilomarus americanus) were the only representative of 



' Compare Chap. III. 



