12 COMPARATIVE ANATOMY. 



Prop. 1st. That while plants inspire carbonic acid they 

 expire oxygen the process being reversed in animals. Obj. 

 Plants, in absence of sunlight, or in times of flowering, ex- 

 hale carbonic acid. 



Prop. 2d. That plants do not possess cilia animals do. 

 Obj. Many zoospores of plants arc ciliated. Entire groups of 

 animals, as higher articulates and osseous fishes, are without 

 them. 



Prop. 3d. That plants are without stomachs all animals 

 possess stomachs. Obj. The amoeboid stage of development 

 of (Ethalium has a stomach in the same sense as Amceba; 

 while Tcenia (tapeworm) is without a stomach. 



Prop. 4th. That plants have cellulose and starch as chemi- 

 cal constituents of tissue animals have not. Obj. The 

 mantle of the Tunicata contains cellulose ; the ventricles of 

 human brain contain starch granules. 



Prop. 5th. That plants are stationary while animals are 

 contractile and motile. Obj. Many plants have active con- 

 tractile zoospores, and Volvox globator and the Diatomaceae 

 are essentially motile. 



We cannot, therefore, well express the difference between 

 a plant and an animal.; but can only ascertain the position of 

 each disputed form by contrasting the sum of its life-phases 

 with that of acknowledged types, as determined by develop- 

 ment and sexual condition. 



Contrasts between animals arise from the variety of 

 methods by which the processes of life are performed. In 

 the Amoeba these processes are carried on in a common tissue 

 (sarcode), so that at different times the same structure is sub- 

 servient to different uses. But among the higher animals, 

 which have many tissues, each organ has a limited functional 

 power ; the degree of such limitation bearing a fixed relation 

 to the multiplicity of organs, so that in the highest animals 

 each organ has but a single function to perform. Indee'd, 

 every animal, it is thought, has its tissues so specialized that 

 the degree of its specialization is expressive of its position in 

 the class to which it belongs. 



