Only among the simplest organisms, if anywhere, is indefinite reproduction 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, complex 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 protoplasm 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 analogy, the net-like structure of protoplasm may be thought of as continuous, not only in the individual, but as binding together the whole species by the intercrossing of the lines of individual descent. As individual organisms will in different degrees endure 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 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.
Symbasis 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[1] which requires this interweaving 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 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,[2] In comparison with the
- ↑ Popular Science Monthly, May, 1903. Symbasis may be defined further 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 groups. It is to be distinguished on the one side from wide crossbreeding and on the other from narrow inbreeding, both of which produce inferior offspring and interfere with evolutionary progress.
- ↑ 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.
