452 POPULAR SCIENCE MONTHLY. 



Only among the simplest organisms, if anywhere, is indefinite repro- 

 duction possible without the assistance of conjugation. 



The new science of cytology has made us aware that the division of 

 cells is not a passive or a simple process, but is extremely active, com- 

 plex and varied. Living protoplasm is in motion, and the discovery 

 that cell walls are not hermetically closed, but are perforated by delicate 

 protoplasmic strands, lends strength to the belief of some biologists 

 that j)rotoplasm circulates, not only inside the individual cells, but 

 through the entire organism. Conjugation may signify that such a 

 circulation extends also throughout the species. Or, to vary the anal- 

 ogy, the net -like structure of protoplasm may be thought of as contin- 

 uous, not only in the individual, but as binding together the whole 

 species by the intercrossing of the lines of individual descent. As indi- 

 vidual organisms will in different degrees endure subdivision, and are 

 able to restore or regenerate the lost part, so species may survive a cer- 

 tain amount of segregation, but if too small a group of individuals be 

 cut off it perishes through the reproductive debility long recognized as 

 inherent in inbred or narrowly segregated organisms. For taxonomy 

 the tree notion 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 dentritic, but reticular. 



Symhasis a Cause of Evolution. 



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

 property of protoplasm, equally fundamental importance can scarcely 

 be denied to the property called symbasis* which requires this inter- 

 weaving of numerous lines of descent and this simultaneous movement 

 of organisms in specific groups. As organic complexity increases there 

 is 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. Organisms which have traveled 

 farthest upon the evolutionary journey are most dependent upon sym- 

 basis. 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, f In comparison with the 



* Popular Science Monthly, May, 1903. Symbasis may be defined fur- 

 ther as the property of which sexual diversity and cross-fertilization furnish 

 the phenomena. The word may also be used physiologically to signify a 

 normal and advantageous range of interbreeding among the individuals of 

 organic grovips. It is to be distinguished on the one side from wide cross- 

 breeding and on the other from narrow inbreeding, both of which produce 

 inferior offspring and interfere with evolutionary progress. 



-j- 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 exceptional and specialized. 



