138 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



of a certain character or combination, may be essential for 

 the development of intimate symbiotic complexes. 



Pleomorphic responses of mitochondria to environmental 

 influences have been shown by a number of investigators. 

 The sensitive properties of mitochondria led Cowdry to 

 say: "The central thought which underUes all the recent 

 work on mitochondria in pathological conditions is the 

 conviction that we now have at our disposal a new criterion 

 of cell activity and cell injury. We do not know it or under 

 stand it, but it has been proven over and over again in the 

 last few months to be of great and surprising dehcacy, for 

 it responds (even before the nucleus) to injurious influences; 

 and it has the rare merit of being cytoplasmic. We may 

 expect environmental changes to act on it which would 

 make no impression at all upon the nucleus." It appears 

 highly improbable, however, that environmental forces by 

 themselves are capable of producing permanent modifica- 

 tions. In the previous chapters we have suggested the 

 possibiUty that environmental influences acting upon 

 mitochondria may be responsible for hereditary variations, 

 but we do not beheve that such variations, by themselves, 

 would be of evolutionary rank. 



We are not in a position to deny the possible influence of 

 environmental factors in the origin of species. It appears 

 probable that they play the role of at least secondary fac- 

 tors. Certainly we recognize the definite influence of 

 environmental factors in organic evolution. In that phase 

 of evolution that we have called the "retention and de- 

 struction of species" we recognize the significance of en- 

 viroimiental influences. We must also emphasize the 

 probabihty that Symbionticism may be a factor in the 

 "destruction of species," as well as in their origin. Sjon- 

 bionticism may lead to retrogressive as well as progressive 

 modifications. 



In the theories on the origin of species that have been 



