CASSELL'S NATURAL HISTORY 



CHAPTER I 



THE PROTOZOA THE DAWN OF LIFE 



THE simplest members of the animal kingdom are called the 

 Protozoa, 1 and, with few exceptions, are too minute to be clearly 

 visible without the aid of a microscope, those which may be 

 detected by the unaided eye appearing as tiny, inert specks or 

 moving particles. They are universal in their geographical dis- 

 tribution, from the Equator to the Arctic and Antarctic, and 

 are to be found living under the most varied conditions, in the 

 soil, in stagnant pools, in rivers and lakes, and in the sea ; while 

 many live in the bodies of the higher animals and are responsible 

 for some of the most serious and fatal diseases from which man 

 and beast can suffer. Malaria, sleeping sickness and blackwater 

 fever are inflicted by them on mankind, and rinderpest, nagana 

 and surra on cattle and horses, the organisms being conveyed from 

 one host to another through the agency of various biting insects. 

 Primitive as these organisms are, they are not sufficiently so to 

 be considered as primordial in the sense of being the first created 

 animals ; they must have been preceded by a still simpler organism, 

 of which we have no definite knowledge, but to which the Amoeba, 

 and some of the Mycetozoa, 2 at certain stages, probably most 

 nearly approach. The riddle of the origin of life still remains 

 unsolved, though its ultimate solution is within the bounds of 

 possibility. If it is ever accomplished, there is little doubt that 

 the key will be dredged up from out the sea. 



In the light of the most recent scientific investigation, it is 



1 Greek, prttos, first i zoa, animal. Greek, myees, a fungus ; xoa, animal. 

 B 



