ZOOGENESIS 



about ten feet in length. But relatively few, either 

 in the sea or on the land, exceed two inches. 



So we find that the vertebrates and the arthropods 

 in the great majority of their included species are 

 really non-competitive. Each of these two major 

 groups for the most part occupies its special economic 

 niche which is determined by its structure and the 

 correlated limitations in efficient size. 



The structure of a mollusk with its unjointed body 

 does not permit the development of jointed supports 

 or supports articulated with the body, like the legs 

 and wings and the paired fins of vertebrates and the 

 legs and wings of insects. So the mollusks cannot 

 compete with vertebrates or with insects in running 

 or flying on the land. On the other hand, since their 

 motions are very slow they require much less food and 

 air and so are able to exist in situations impossible 

 for any members of these other groups. 



So also in the sea and in fresh water the mollusks 

 are capable of existing in many different ways and 

 under many different sets of conditions not possible 

 for any vertebrate or arthropod. 



On the land the mollusks compete more or less 

 directly with some insects and a few vertebrates, such 

 as the small land tortoises, in consuming fungi and 

 soft vegetable material. In the sea some, as mussels, 

 oysters and others, live attached to firm supports and 

 feed on the same materials as the barnacles (fig. 30, 

 p. 47), which are curious distorted arthropods. 

 Others live suspended in the water of the open ocean 

 (fig. 48, p. 97) and feed on the minute suspended 



