Competitive System Amongst Lower Animals 753 



among themselves as a rule; may show defensive contrivances 

 by spines or surface secretions that are disliked by other ani- 

 mals; or they may burrow or form protective structures around 

 themselves. They are often heavily preyed on by other ani- 

 mals, but they reproduce much more abundantly than do 

 species of the preceding group, and so they hold their own well 

 in the "struggle for existence." Since they prey on small 

 forms or on decomposing animal bodies, they do not show the 

 same degree of antagonistic fighting action as do the last. 



So, in the wear and tear of life they are not exposed to so 

 great strains. 



The third group may be, according to opportunity, wholly 

 phytivorous or to slight degree may resemble the last. They 

 are often highly social, and may fashion community dwell- 

 ings, or may travel in social groups or herds, which by their 

 unity and compactness alike afford shelter to the young and 

 a degree of protection to each other. Not a few of them are 

 protected like the last group. They often have a well-devel- 

 oped nervous system, and their social habits enable them to 

 organize and evolve to a high degree. Included here are a 

 few Crustacea, a fair number of insects, some molluscs, many 

 fishes, some amphibians, a few reptiles, many birds, and many 

 mammals. They are often decidedly dominant species, owing 

 to the wide area from which they may draw food supplies, 

 the degree of protection and even strength that the social 

 habit gives to them and their young, also the extreme fertility 

 that characterizes them. 



The fourth group includes nearly all of the most highly evolved 

 and organized insects, a small number of fishes, a considerable 

 number of birds, and these amongst the most highly organized, 

 also the greater number of species of the mammalia. It is 

 amongst this biological group that we meet with the most 

 elaborately and diversely modified types, alike as to nervous, 

 alimentary, and reproductive organization. They include, on 

 the whole, the most highly colored, the most defensively pro- 

 tected by secretions, the most abundantly reproductive, the 

 most highly evolved in social organization, and the types that 

 are now richest in individuals. 



