320 ZOO LOOT. 



perhaps only from 50,000 to 100,000 species, but of flying 

 forms tlie insects uloiie number by estimate over 1,000,000 

 species. 



Causes of Variation. — Variation and adaptation in species, 

 genera, and classes are primarily due to the same kind of 

 causes as liave dii'ected the development and form of the 

 individual, tlie causes being both external a7id internal. 

 The chief external causes are the action of atmospheric 

 pressui'e, gravity, electricity, light, change of temperature 

 (heat and cold), and tlio mechanical and chemical state of 

 tlie medium in which animals live; also tlie kind of food. 

 Thus whole classes of animals, as, for example, fishes, are 

 adapted to their aquatic life, and the classes of insects and 

 birds are fitted for an aerial life. The shape of the bodies 

 of these animals is more or less spindle-likc, enabling them 

 to most readil' cleave the water or air. The action of the 

 external world on organisms has been recognized by the 

 greatest and most thoughtful minds in biology. Herbert 

 Spencer says: "The direct action of the medium was the 

 primordial factor of organic evolution." Darwin says that 

 "organic beings, wlien subjected during several genera- 

 tions to any change whatever in their conditions, tend to 

 vary." Again: "Changes of any kind in the conditions of 

 life, even extremely slight changes, often suffice to cause 

 variability.'' The botanist Sachs asserts: "A far greater 

 portion of the jihenomena of life are called forth by exter- 

 nal influences tlian one formerly ventured to assume. 

 Every phenomenon of life arises from two factors: on the 

 one hand from the structure transmitted from tlie mother 

 organism, and on the other ivom. external forces working 

 on this structure."' Semper claimed that " of all the prop- 

 erties of the animal organism variability is that which may 

 first and most easily be traced by exact investigation to its 

 efficient causes." 



It is apparently the variations in the unceasing action of 

 tliese pliysical agents, which are called the primary causes 

 of organic evolution, operating ou the most primitive one- 



