X. THE SECONDARY ORGANISMS 203 



trast to the fact that Rickettsiae as well as viruses are all parasitic. 

 Is it really conceivable that man and bacteria share common ances- 

 try and that the latter have waited and waited throughout a dread- 

 ful long span of time in the most primitive unicellular form for their 

 brethren to be evolved into man, on which they were bound to be 

 parasitic ? Is there any doubt in assuming the parasitism as being 

 the most primitive mode of life of organisms ? 



3. The Limit of the Secondary Organisms 



The writer holds the opinion that free-living organisms can never 

 enter parasitic life unless they have a phylogenetical experience of 

 parasitism. Parasitic life involves too specialized and too limited en- 

 vironment to be newly adapted. 



Certain animals especially some insects are known to become 

 parasitic at their ^adult stage although free-living at the larval. As 

 will be fully discussed later, this must be a kind of atavism, never 

 the new acquisition of parasitism. This occurs generally much more 

 frequently in females than in males; it can be supposed that parasi- 

 tism is more profitable for females because of some physiological 

 function particular to females such as egg production or egg laying, 

 and hence females that could accomplish the reversible parasitism or 

 atavism might have been selected as the fittest. 



Parasitic botflies are known to show a complicated life cycle. 

 Botfly eggs can hatch only under the combined influence of the 

 horse's tongue and saliva with which they are carried into the mouth, 

 whence they burrow into the mucosa of the tongue or the palate and 

 tunnel their way through until they reach the pharyx. The second 

 larval stage is now reached. The maggots emerge from the mucosa 

 and attach themselves on the surface, close to the epiglottidean region. 

 They finally pass down into the intestine and each species takes up 

 its abode in a particular region, some in the stomach, the others in 

 the rectum or the duodenum. They generally abandon their host 

 before pupating and are evacuated with feces (117). The life history 

 of ascaris is somewhat similar to this, indicating a similarity in their 

 parasitic nature existing between nematodes and insects. Insects, in 

 general, possess at their larval stage a form strikingly resembling 

 that of nematodes, a fact in which a relic of parasitism is seen. 



Metamorphosis, especially distinct in insects, may suggest the rapid 

 phylogenetic occurrence of the liberation of themselves from parasitism 

 just as the sudden release from water of a tadpole with the establish- 

 ment of metamorphosis. 



