360 



CHAPTERS ON EVOLUTION. 



leaf. Through the combined operation of this green colour either 

 singly or aided by the leaf protoplasm and the action of light, plants 

 decompose the carbonic acid of the air, as every schoolboy knows. 

 They then retain the carbon to aid in the formation of starch, and 

 set free the oxygen, which thus returns to the atmosphere, and is 

 welcomed by the animal hosts. The hydra, or common fresh-water 

 polype (Fig. 258), many animalcules, and certain worms of a low type 

 possess this chlorophyll. Like dishonest manufacturers, they seem to 

 have infringed the patent rights of the plant to elaborate this green 

 colour. And it is no longer matter of theory, but ascertained fact, 

 that these green animals are capable, like the plants, of absorbing 

 carbonic acid usually a fatal gas to the animal constitution and 



FIG. 258. HYDR. 

 (In both figures young hydrae are represented budding from the side of the parent.) 



of elaborating starch therefrom like their plant neighbours. Thus a 

 simpler mode of feeding, obviating the necessities of animal existence 

 in the way of digestive apparatus, has apparently led to the simplifica- 

 tion of structure. Degeneration has followed in the worms just 

 mentioned, as the result of their imitation and acquirement of 

 vegetative powers of feeding; and it is probable that other altera- 

 tions in the way of dietary, of less sweeping character than that 

 just mentioned, will affect, in like retrogressive fashion, the animal 

 constitution. 



Some of the most curious cases of degeneration known to us 

 illustrate the total disappearance of digestive apparatus even in 

 some beings, in which, as in the stylops already mentioned, one 

 sex becomes retrogressive whilst the other sex remains structurally 

 fully developed. Such a case is illustrated by the males of 

 those remarkable organisms, the Rotifera, or " wheel animalcules " 

 (Fig. 259). These minute creatures, inhabiting our fresh waters, may 

 be desiccated and dried, and revived, on the application of moisture, 

 many times in succession. But in their ordinary existence, and in 



