322 ZOOLOGY. 



enemies or to seek food, and of keeping on in this kind of 

 life in order to njaintaiii existence, thus resulting in the 

 arboreal segregation of certain mammals, as the monkeys, 

 apes, slotli, etc., and of countless species of insects — all 

 this differentiation and specialization of forms are the 

 result of such agencies. 



The ease is paralleled by that of man. At first a hunter 

 and nomad, he became an agriculturist, and as soon as he 

 began to live in more crowded communities and cities the 

 professions and arts became gradually differentiated. If all 

 the inhabitants of a town or city were farmers, or bakers, or 

 carpenters, or lawyers, or physicians, through want of the 

 means of living all would either have to go back to a gener- 

 alized nomad existence or the species would cease to exist. 



There is thus a prucess of adaptation to tliis or that 

 mode of life, or of change of form from tlie general to the 

 special, and a narrowing or segregation to this or that 

 habitat or means of living, or kind of food and way of 

 getting it, which has been going on in the world ever since 

 the first Protozoan began to exist, and the operation of the 

 factors of organic evolation was with little doubt much 

 moi'e rapid then than now, i7i accordance with the more 

 rapid, widespread, and profound geological changes which 

 took place in Precambrian times. 



It was the varying conditions of life, tlie difference in 

 temperature, climate, soil, food, and in the medium, which 

 at once began to operate and to cause divergences in some 

 and degeneration in other organs; also fixed stationary and 

 parasitic modes of life in some cases as contrasted with 

 free unrestricted roving habits in others, resulted in the 

 origin of new features. These characters, being fixed by 

 heredity, became permanent. Meanwhile as soon as the 

 world began to get crowded and overstocked, a selective 

 principle began to operate and tend to eliminate or to 

 preserve this or that type. It is now apparent that the 

 amorplious or plant-like forms of sponges are primarily 

 due to their fixed mode of life. In polyps the vase-shaped 



