408 THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 



.subdivision and are able to restore or regenerate the lost part, so 

 species may survive a certain amount of segregation; but. if too small 

 a group of individuals be cut off, it perishes through the reproductive 

 clebilit}^ long recognized as inherent in inbred or narrowly segregated 

 organisms. For taxonomy the tree motion of descent was sufficient 

 as a means of indicating the history and affinities of species and 

 higher groups, but evolution is a process which must be studied inside 

 the species, and here the diagram of relationship is not dendritic, but 

 reticular. 



SYMBASIS A CAUSE OF EVOLUTION. 



If reproduction by means of cell division is reckoned as an essen- 

 tial property of protoplasm, equally fundamental importance can 

 scarcely be denied to the property called symbasis," which requires 

 this interweaving of numerous lines of descent and this simultaneous 

 movement of organisms in specific groups. As organic complexity 

 increases there is a greater necessity for cross-breeding, as evidenced 

 by the accentuation of sexual diversity and by the decline of asexual 

 propagation and of the power of regenerating lost parts. Organ- 

 isms which have traveled farthest upon the evolutionary journey 

 are most dependent upon symbasis. Nowhere among the higher 

 animals, including many thousands of species of arthropods and 

 vertebrates, is there known to be a long-continued series of nonsexual 

 individuals.'' In comparison with the higher animals, plants are but 

 loose and unspecialized aggregations of cells, and yet among them 

 also sexual differentiation has made great progress, and in some 

 orders contrivances to insure cross-fertilization are highly developed. 



The extent to which conjugation exists among the lower groups is 

 not yet determined. That it may be omitted for many generations 

 of a simple organism should not be taken to mean that it is entirely 

 absent or has no importance, since among the higher animals, where 



a Symbasis signifies etymologically a moving together or in company, and 

 refers to the fact that organisms exist and make nonual evolutionary progress 

 in groups, rather than on simple or narrow lines of succession. The word may 

 be used also in a physiological sense, to indicate a normal and advantageous 

 range of interbreeding among the individuals of organic groups. It is to be 

 distinguished on the one side from wide cross-breeding and on the other from 

 narrow inbreeding, both of which i)roduce inferior offspring and interfere with 

 evolutionai'y progress. The confusion arising from the very frequent use of 

 interbreeding in the contrary sense of inbreeding would also compel the intro- 

 duction of a new and unambiguous term. 



6 Among the bees fertilization may be omitted for a single male generation, 

 and among the plant-lice for several wingless generations, but such instances are 

 admittedly excejjtional and specialized. 



