4 ZOOLOGY. 



do a large proportion of the higher animals. The two 

 kingdoms supplement each other, are mutually dependent, 

 and probably appeared simultaneously in the beginning of 

 things. It should be observed, however, that the animal 

 kingdom overtops the vegetable kingdom, culminating in 

 man. 



In speaking as we have of low animals and high animals, 

 we are comparing very unequal quantities; the distance be- 

 tween monad and man is well-nigh infinite. But there is a 

 series or chain, sometimes broken and often with lost links, 

 connecting the extremes ; and as there are wide differences 

 in form, so there are great extremes in the organs and de- 

 gree of complication of function of the simple as compared 

 with the more complex forms. The improvised stomach of 

 an Amoeba is not comparable with the stomach of an hydra, 

 nor is the stomach of the latter creature with that of a 

 horse ; there is a gradual perfection and elaboration or spe- 

 cialization of the stomach as we ascend in the animal series. 

 So it is with organs of locomotion ; the pseudopods and cilia 

 of the Protozoans are replaced in the star-fishes and worms 

 by hollow tentacles or various fleshy soft appendages ; in 

 crabs and insects by stiff, jointed limbs, with different lev- 

 erage systems ; and these are replaced in vertebrates by 

 genuine limbs supported by bones. A comparative view of 

 the origin and structure of organs succeeds in this book the 

 systematic account of the animals themselves. 



We thus sec that the organs of the higher animals are 

 merely modifications of organs often having the same 



v O 



general functions as in the lower animals ; the lower or 

 simpler have preceded in geological history the higher or 

 more specialized forms, and thus we are, in ascending the 

 animal series, going from the simple to the complex. For 

 this reason the plan of this work has been to lead the stu- 

 dent from the simpler forms of animal life to the more 

 complex ; and though the vertebrate animals, such as fishes 

 and dogs, are more familiar and interesting to us, the seri- 

 ous student of zoology will feel that it is more logical and 

 better in the end to study the animal world in the order in 

 which the different forms have appeared as we believe, 



