Chap. II. GRADATION AMONG ACALEPHS. 113 



the same famil}^, or those of one and the same genus, should be considered as 

 lineal descendants of a common stock ; for orders, flimilies, and genera are based 

 upon different categories of characters, and not iipon more or less extensive char- 

 acters of the same kind, as I have shown years ago (vol. 1, p. 150-163), and 

 numbers of different kinds of representatives of these various groups make their 

 appearance simultaneously in all the successive geological periods. There appear 

 together Corals and Echinoderms of different fiimilies and of different genera in 

 the earliest geological formation, and this is equally true of Bryozoa, Brachio- 

 pods, and Lamellibranchiates, of Trilobites and the other Crustacea, in fact of the 

 representatives of all the classes of the animal kingdom, making due allowance 

 for the period of the first appearance of each ; and at all times and in all classes, 

 the rej)resentatives of these different kinds of groups are found to present the 

 same definiteness in their characteristics and limitation. Were the transmutation 

 theory true, the geological record should exhibit an uninterrupted succession of 

 types, blending gradually into one another. The fact is, that throughout all 

 geological times, each period is characterized by definite, specific types, belonging 

 to definite genera, and these to definite families, refei'able to definite orders, con- 

 stituting definite classes, and definite branches built upon definite plans. Until the 

 facts of nature are shown to have been mistaken by those who have made them 

 known, and that they have a different meaning from that now generally assigned 

 to them, I shall, therefore, consider the transmutation theory as a scientific mistake, 

 untrue in its fiicts, unscientific in its method, and mischievous in its tendency. 



SECTIONVII. 



GRADATION AMONG ACALEPHS. 



Confident that I have correctly ascertained the limits of the class of Acalejjhs, 

 and that the method I have followed in that investigation is the only one that 

 may furnish the means of avoiding arbitrary decisions with reference to the natural 

 affinities of animals, I now proceed to an inquiry into the gradation or relative 

 standing of the different members of this class. Keeping in view the principles 

 laid down in the first volume of this work (p. 150), this inquiry should lead us 

 to a knowledge of the Orders among Acalephs, if orders, as natural divisions, are 

 based upon the different degrees of complication of the structure of the members 

 of one and the same class ; and that this is the true view to take of orders, 

 I have at present not the least doubt. It is certainly so m all the classes, of 



VOL. III. 15 



