752 Causes and Course of Organic Evolution 



or more of the Amphibia, many reptiles, a relatively small 

 number of birds, and possibly one-third of living mammals. 

 Nearly all of these have the prehensile parts — the limbs and 

 claws, the beak, or the teeth — modified in varying degree for 

 seizing animal prey; the eyes and nose are usually keen for 

 sight and smell-perception; their alimentary system is modi- 

 fied for digestion of rich readily decomposable animal material; 

 their general nervous organization is seldom equal to that of 

 semi-phytivorous or phytivorous types. Biologically they show 

 a merciless and relentless relation to other animal species, 

 and even in many cases to individuals of their own species. 

 Their movements accordingly become stealthy, suspicious, 

 sly, sudden, and aggressive; while other animals — even indi- 

 viduals of their own species — show a constant dread of or 

 hostile relation to them. As a whole they reproduce rather, 

 or even very, slowly or sparingly, and the young are exposed 

 to considerable risks, at times even are preyed on by their 

 parents or by each other. 



To this biological assemblage belong many of the protected, 

 often encased, or even heavily mailed animals like the larger 

 crabs and lobsters, the scorpions and spiders, some groups 

 of insects, the odontophorous molluscs, the heavily scaled 

 fishes, amphibians, and reptiles. Though they have evolved 

 into a rather varied assemblage of species, palseontological 

 as well as recent evidence show that they constantly become 

 scarcer in species and individuals, and at length suffer extinc- 

 tion. 



The second group includes species that often are social or 

 semi-social. Some of the smaller crabs and lobsters, a con- 

 siderable number of insects, many molluscs and fishes, a fair 

 number of amphibians, reptiles, birds, and mammals are 

 here included. These feed usually on forms much smaller 

 than themselves, or on decaying animals. They vary from 

 active to rather sedentary, may secure their prey either by 

 pursuit, by rapid picking, by protrusible viscid tongue, or 

 by passing currents of water through some channels. They 

 are at times graceful and active in movement; live peacefully 



