204 Causes and Course of Organic Evolution 



astrums, violets, and other groups. It is quite fair to conclude 

 that, if definite and recognizable advantage accrues when rather 

 diverse species are crossed, the more vigorous offspring that 

 Darwin and many observers since have recorded, as resulting 

 from crossing of varieties or even distinct individuals of a 

 species, clearly indicates that sex-differentiation and sex-cell 

 fusion constitute an important evolutionary factor. Some 

 possible explanations are set forth in chapter X. But, while 

 this fifth factor has a profound importance as an agent in evo- 

 lution, we still are so ignorant relatively of the more exact 

 details involved that we must look to the future for its fuller 

 development. 



If now one postulate the factors of heredity, environment, 

 proenvironment, selection, and reproduction as cooperative 

 agents in organic evolution, it necessarily follows that the 

 result or effect will be the gradual unfolding from the spore 

 cell, or from the fertilized egg of a plant or animal body, of 

 a mathematically exact structure, except that the second, 

 third, and fourth factors may more or less modify it during 

 the unfolding process, and in time we beheve modify its de- 

 scendants. Such plant or animal, moreover, represents an 

 absolutely continuous line of organic development, it may be 

 through millions of years, during which changing environment, 

 changing proenvironal response, changing degrees of selectional 

 strain, and changing reproductive blendings have all cooperated 

 to alter the organism along an evolving or devolving line, or 

 these factors have so acted in balanced relation that the type 

 of organism retains a stationary state. 



So, from the time when a spore-cell is set free, or when two 

 complemental sex-cells fuse to form a fertilized egg, and on- 

 ward through the life of each organism, the combined action 

 of the above five factors as a great cause gives rise to the grad- 

 ual unfolding or ontogeny of the organism as an effect. Such 

 a summated cause or cooperative agency then might be termed 

 pentaniorphogenij, or the action of the five organic factors that 

 are form-producing, in the history of every organism. 



