W. B. DWIGST. 135-(105 



of having even a rudiment of a tail. Under the same 

 law, centipedes are inferior to true insects ; salamanders, 

 or tailed batrachians, to frogs, or tailless batrachians ; 

 snakes to lizards ; and ganoids with vertebrated tails, to 

 those fishes whose tails are not vertebrated. 



It will be noticed that in some of these cases, the same 

 conclusions were reached by the other principle ; this 

 mutual corroboration is important, as showing their cor- 

 rectness. 



I may now mention other criteria of rank, of more or 

 less importance, yet none of them having so wide an ap- 

 plication as either of the two grand laws already given. 

 In many cases, however, they are really outgrowths of 

 one of these laws 



One of these, is the principle that water species of an'y 

 type, are inferior to land species of the same type. Thus 

 whales, the mammals of the sea, in spite of their vast 

 size, are inferior to the mammals of the land ; water- 

 snails to land-snails, crustaceans or water-articulates, 

 such as crabs, to insects, which are land-articulates. 



Again, we have the law of the specialization of organs. 

 All true progress in the development of animal life is a 

 process of specialization. The lowest forms have only 

 one or a few organs with which to perform all necessary 

 functions. Thus the amoeboid and infusorian animal- 

 cules have only a few cavities, of the simplest character, 

 and a series of hair-like cilia, to perform for them all 

 the acts of moving, seizing, selecting, swallowing, digest- 

 ing and distributing food, throwing off excreta, the cir- 

 culation of aerated fluids and reproduction. Each high- 

 er grade has some organs fitted or specialized to do 

 separately some portion of this work, until as we reach 

 the high-grade, well-proportioned frame of man, we find 

 a great many complex organs doing each a special thing, 

 and all together bringing about the perfection of high 

 physiological service. We can understand how under 

 the working of this law, by the simple examination of 

 structure .under the microscope, even the exceedingly 

 minute animalcules have been at last distinguished into 

 widely separated grades. For instance, of two which 

 are sporting side by side among hundreds in a drop of 

 water, one, a ciliated infusorian. is pronounced very 

 low. while his neighbor, though considerably similar in 



