CRUSTACEA AND MOLLUSCS 237 



■ — only appeared about the Tertiary period, and so come 

 after many groups of vertebrates, such as the reptiles, 

 had reached their greatest height. In fact, the whole 

 intricate mechanism of the plants — the leaves, flowers, 

 etc. — only came as an adaptation to life on land ; marine 

 plants have remained at the primitive stage of seaweeds 

 and algae. 



The plant-eaters are, of course, older than the flesh- 

 eaters, since the first animals can only have had organic 

 food in the shape of plants. But we must not generalise 

 too much on the strength of this fact. Once the world 

 was filled with the tiniest animals, the higher animals 

 could be developed from them, and grow and advance 

 at their expense. It has been rightly pointed out that 

 animal food comes more naturally to the animal than 

 plant-food. Every animal has been accustomed to an 

 animal diet in its earlier period, whether this consisted 

 of the yolk of an egg or the milk of the mother. The 

 break from this early diet to plant-food with its hard 

 substance, the cellulose, is very considerable, and we 

 can understand why the higher plants have been 

 avoided and the earlier vegetal diet retained. 



The chief difficulty in following the food-series occurs 

 in connection with the land-animals. In the sea there 

 are lower algae that are eaten by the smallest animals, 

 and these in turn are devoured by the larger. Here 

 nearly all are carnivores ; vegetarians are very rare 

 amongst the larger salt-water animals. We can thus 

 see that these tiny, lowly algae provide the organic 

 matter for the whole animal life of the sea, since this 

 is prepared from inorganic substances by the algae 



