SYMBIONTICISM AND ORIGIN OF SPECIES 113 



The influence of environmental factors on the response 

 to infection has repeatedly been demonstrated. Pasteur 

 rendered the naturally resistant hen susceptible to anthrax 

 by chilling the bird with cold water. Frogs, which are 

 inmaune to anthrax at ordinary room temperature, quickly 

 die after an inoculation if placed at a temperature of 25° 

 to 35°C. (Jordan). 



Obviously, not all symbiotic relationships result in the 

 origin of species. There are a number of microsymbiotic 

 relationships, apparently, which lack a morphological re- 

 sponse. This is especially true in extracellular symbiosis, 

 and probably obtains in some intracellular symbiotic rela- 

 tionships. It is practically impossible to draw a sharp 

 Une between parasitic relationships and symbiosis. Fur- 

 thermore, there may be responses in the host under these 

 conditions that are not permanent. 



It is evident that in those cases of symbiosis in which the 

 response in the host is permanent and produces new species, 

 a pecuhar combination of factors, both extrinsic and intrin- 

 sic, in two distinct symbionts have met and are harmonious. 

 It would be futile to attempt to analyze and determine the 

 nature of these factors. An analysis of fertihzation may 

 serve to elucidate their significance. The sperm of a 

 species possess a definite affinity for the ovum of the homol- 

 ogous species. In some instances the sperm may pene- 

 trate an ovum of a closely related species and fertiUze it. 

 The individual that develops from this fertilization is 

 usually sterile or at least abnormal. Changing the com- 

 position of the sea water surrounding the eggs of certain 

 marine animals, may be sufficient to prevent fertihzation 

 by the sperm. Certain temperatures may produce similar 

 results. In other words, if normal reproduction is to take 

 place, it is imperative that certain extrinsic and intrinsic 

 conditions be satisfied. 



The responses of a host to the presence of a micro- 



