244 REVIEWS — NATURAL HISTORY. 



style. The accompanying plates are beautifully executed and very 

 valuable, doing the highest credit to all engaged upon them, and the 

 contents of the volumes, consisting of a general introduction and ex- 

 position of the author's views on classification, with an admirable mon- 

 ograph on the North American Testudinata, cannot fail to be accounted 

 an important contribution to science. There are occasionally points on 

 which we cannot agree with the learned professor, but we fully feel the 

 importance of his labours, and cordially thank him for the additions he 

 has made to our knowledge in a highly interesting department, as well 

 as for a clear and elegant exposition of his views respecting the princi- 

 ples of classification, and their practical application. We ' quote the 

 following passage from Section VII. of the Essay on Classification, as a 

 concise statement of the principles maintained : 



" Thus far I have considered only those kinds of divisions which are introduced 

 in aIn;ost all our naodern classifications, and attempted to show that these 

 groups are founded in nature, and ought not to be considered as artificial devices 

 invented by man to facilitate his studies. Upon the closest scrutiny of the subject, 

 I find that these divisions cover all the categories of relationship which exist 

 among animals, as far as their structure is concerned. 



"Branches or types are characterised by the plan of their structure: 

 " Classes, by the manner in which that plan is executed, as far as ways and 

 means are concerned : 



" Orders, by the degrees of complication of that structure : 

 " Families, by their form, as far as determined by structure : 

 " Genera, by the details of the execution in special parts, and 

 " Species, by the relations of individuals to one another, and to the world in 

 which they live, as well as by the proportions of their parts, their ornamentation, 

 &c." 



The author goes on to speak of such divisions as sub-classes, sub- 

 orders, sub-families, sub-genera, varieties, respecting which he thus 

 expresses himself : 



" These distinctions have long ago been introduced into our systems, and every 

 practical naturalist who has made a special study of any class of the animal king- 

 dom must have been impressed with the propriety of acknowledging a large num- 

 ber of sub-divisions, to express all the various degrees of affinity of the difi'erent 

 members of any higher natural group. ISTow, while I maintain that the branches, 

 the classes, the ordei"s, the families, the genera, and the species are groups, estab- 

 lished in nature respectively upon diiferent categories, and while I feel prepared to 

 trace the natural limits of these groups, by the characteristic features upon which 

 they are founded, I must confess at the same time that I have not yet been able 

 to discover the principle which obtains in the limitation of their respective sub- 

 divisions. All I can say is, that all the different categories considered above, 

 upon which branches, classes, orders, families, genera and species are founded, 

 have their degrees, and upon these degrees, sub-classes, sub-orders, sub-families. 



