404 THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 



forget that the diversity is general and multifarious, like that of 

 individual apple trees or men. Evolution is a continuous summary 

 or integration of this individual diversity, and is not a simple process, 

 but highly multiplex, as much so, indeed, as the lines of descent in 

 which the life of the species goes forward. A composite general 

 direction is maintained by the species because the multitudinous 

 strands of individual descent are bound together by interbreeding. 

 The variations take place in particular threads, but evolution signifies 

 rather the progressive change of the whole organic network. 



The evolution of a new type means changes in many directions and 

 of many kinds in the germ cells and in the various tissues and organs, 

 as Avell as in the external form of the complex cell colony which we 

 are accustomed to look upon as a single individual. 



Each cell, tissue, organ, and feature is undergoing evolution, and 

 for normal and permanent progress these manifold developments 

 must keep together. When single lines or slender strands of descent 

 are separated from the main network the congruence of type is lost. 

 The normal variation and individual diversity of the species having 

 been eliminated, the evolutionary coordination of cells, organs, and 

 functions breaks down, and abrupt changes or aberrations of heredity 

 appear. These degenerative mutations may not differ in their essen- 

 tial nature from normal variations, but the conditions of their appear- 

 ance are abnormal, and the results often disastrous." Mutations, like 

 hybrids, are sometimes completely sterile, and they may have at the 

 same time an increased vegetative vigor. The vegetative vigor of 

 many mutative varieties of domesticated plants has doubtless delayed 

 the recognition of their abnormal evolutionary status, though the 

 abnormality of infertile hybrids has long been appreciated. It is 

 paradoxical, indeed, that the increased vigor which accompanies nor- 

 mal variations and crosses should also attend degenerative changes, 

 but there is room for this apparent contradiction in so complex and 

 many-sided a process as evolution. 



A domestic variety may be " improved " by the further increase of 

 the one or two characters or qualities which render it valuable, but a 

 new specific or generic type is the compound or resultant of many 

 ■\ariations in many characters. By close selection, which restricts 

 evolutionary progress to a narrow line of descent, a " single charac- 

 ter " may push out farther in a decade than the natural nuUtiplex 

 evolution would carry it in a centurv or a millennium, but such a spe- 

 cialization weakens and unbalances the organisms, and is a process of 

 degeneration rather tlian a constructive evohition. Selective inbreed- 

 ing and other forms of isolation accentuate single characters, but the 



a See Cook, O. F., 1904. The Vegetative Vigor of Hybrids and Mutations. 

 Proc. Biological Society of Washington, 17 : 83. 



