406 THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 



second does not account for adaptations. The '' selection " of Nageli, 

 Weisniann, and other believers in a " determining principle " or 

 ''hereditary mechanism" of evolution is a verj^ weak substitute for 

 the original Darwinian idea, since it is able to eliminate only the 

 hopelessly unfit, but is quite without means of influencing the 

 survivors. 



Segregation enables species to attain differential characters, and 

 selection assists their accommodation to environment, but both these 

 possibilities rest on the more fundamental fact that organic evolution 

 goes forward Avithout external causation in groups of diverse, inter- 

 breeding individuals. If a species stood still selection could effect 

 nothing except its partial extinction. In the recognition of a con- 

 tinuous and universal evolutionary motion the kinetic theory supplies 

 the long-sought explanation of selective influence. Selection ceases 

 to be a mysterious evolutionary cause, but retains a practical and 

 easily comprehensible evolutionary function. 



THE SPECIES A PROTOPLASMIC NETWORK. 



The traditional illustration of organic descent by a tree with ever- 

 dividing branches is entirely misleading as a suggestion of the nature 

 of evolutionary processes, because individuals do not follow each 

 other in simple series. Successive generations are connected by end- 

 less intergraf tings of the lines of descent. A species may be treated 

 systematically or statistically as an aggregation of individuals, and 

 may be described by an averaging of the characters of these, but from 

 an evolutionary point of view it does not exist as a species because of 

 the possession of a certain complex of characters, but because the com- 

 ponent individuals breed together; through this alone is the integrity 

 or coherence of the species maintained. For evolutionary purposes 

 we may think of the same species existing thousands of years hence, 

 and with any or all of its characters changed." It is not necessary 

 even that the individuals of a species remain alike; in many unre- 

 lated natural groups extremely diverse sexes, castes, and " forms " 

 remain associated in the same species and travel together on the evo- 

 lutionary journey, sharing the same environment, but without any 

 tendency to become " exacth^ alike." Moreover, we know that sexual 

 and other diversities inside the species are not casual or accidental, 

 but normal and advantageous, facts quite overlooked in static theo- 

 ries, which have viewed life from a narrowly systematic standpoint 



a Cook, O. F., 1899, Four Categories of Species, American Naturalist, 33:287. 

 The "species" into wbich i);ileontolosists ;irl)itraril.v divide geoloj^ical series of 

 organisms may l)e explaiiialilo i).v cvnlutioiiary i)rogr(»ss alone, l)ut tlie multipli- 

 cation of the contemporaneous si)ecies of a given horizon is a different question. 



