THE VITAL FAHRIC OF DESCENT 3O3 



In line with'the previous teaching, that evolution is due to the 

 environment, it has been held that interbreeding hinders or pre- 

 vents evolution bv interfering with the preservation of new 

 variations ; sexuality, in other words, has been reckoned as 

 anti-evolutionar}'. In complete contrast with this is the kinetic 

 interpretation, that the continued interbreeding of the numerous 

 and diverse individuals of the species is essential to sustained 

 organic progress. Evolution becomes, in short, a sexual proc- 

 ess. This distinction is not merely a matter of terms and defi- 

 nitions, but is capable of being tested by application to estab- 

 lished facts of evolutionary history. 



In accordance with the earlier view, that sexuality was anti- 

 evolutionary, it has been assumed that the complex and special- 

 ized bodies of the higher plants and animals are asexual struc- 

 tures whose development has been accomplished by the suppres- 

 sion of sexuality in alternating generations of individuals. A 

 more careful inspection of the facts shows that instead of evolu- 

 tion having been accomplished through alternation of genera- 

 tions, or having been accompanied by a greater and greater 

 accentuation of asexual structures, it has remained closely at- 

 tached to the sexual process of cell-conjugation, and dependent 

 upon it. The bodies of the higher plants and animals are not 

 built up between conjugations or subsequent to the completion 

 of the conjugation of the parental reproductive cells, as often 

 supposed. The reproductive cells divide and build up the new 

 structure while still in the sexually double or conjugating con- 

 dition. 



This phase of the subject has been treated in a previous 

 publication.^ The present paper undertakes only a brief and 

 informal presentation of some of the general consequences and 

 applications which flow from the recognition of symbasic inter- 

 breeding as the normal condition of organic existence, and of 

 evolutionary progress. By emphasizing and applying the fact 

 that organic descent is a continuous network, it seeks to avoid 

 the danger of mistaking the results of violations of the law of 

 symbasis for examples of genuine, constructive evolution. All 



iCook, O. F., and Swingle, W. T., 1905. Evolution of Cellular Structures. 

 Bulletin 81, Bureau of Plant Industry, U. S. Dept. of Agriculture. 



