PROFESSOR LOUIS AGASSIZ. 617 



radiating circulatory tubes to the digestive cavity. The circulatory 

 fluid is chyme, and not chyle, as it is in the Articulata and Mollusca. 

 The author describes four species, and distinguishes a new genus — 

 JVemopsis. 



Ever since his arrival in America, Agassiz had been collecting ma- 

 terial for a series of " Contributions to the Natural History of the 

 United States." In 1857 appeared two volumes of these contributions ; 

 the first containing an " Essay on Classification " and the history of the 

 North American Testudinata ; the second on " the Embryology of the 

 Turtle." 



In the essay on Classification, Agassiz affirms that Nature is but 

 the expression of the thought of the Creator, and that a true classi- 

 fication will be found to be but an unfolding of the plan of creation, as 

 expressed in living realities ; that these realities do not exist in con- 

 sequence of the continued agency of physical causes, but appear suc- 

 cessively by the immediate intervention of the Creator. We find in 

 Nature a progressive series, from lower to higher forms ; but it is not 

 a uniform progress for the animal kingdom as a whole ; neither is it a 

 linear progress for the branches or classes, but a progress in which 

 each type has usually been introduced by the creation of species be- 

 longing to one of its higher groups, for the earliest representatives of 

 a class do not always seem to be the lowest. Yet, notwithstanding 

 these downward steps, the progress has continually tended toward the 

 production of higher and higher types, culminating at last in Man. 



We find a parallelism between the geological succession of animals 

 and the embryonic growth of their living representatives, as well as 

 a parallelism between the geological succession of animals and their 

 relative rank. The earlier types of animals were synthetic or pro- 

 phetic, foreshadowing a future group. Types likewise culminated and 

 disappeared in past ages — a feature parallel with the fact that in em- 

 bryological development parts fulfill their end and then disappear. 



Regarding Nature as the embodiment of a certain divine plan, Ag- 

 assiz sought for a classification that should be the expression of this 

 plan. Accordingly he based his classification on the following divisions, 

 which he deemed covered all the categories of relationship, as far as 

 their structure is concerned : Branches, or types characterized by 

 their plan of structure ; of these, he admitted four, Vertebrates, Ar- 

 ticulates, Molluscs, an/1 Radiates. Classes, which are characterized by 

 the mode of execution of the plan. Orders, by the degree of complica- 

 tion of structure. Families, by the form as determined by structure. 

 Genera, by the details of execution ; and Species, by the relation of the 

 individuals to one another, and to the world in which they live. 



In Part II., Agassiz believes that the Testudinata constitute an 

 order among the class of reptiles ; that their essential character lies 

 not so much in the shield as in the special development of different 

 regions of the body, thus giving them the highest rank in the class. 



