194 Causes and Course of Organic Evolution 



During the past half century many writers have drawn 

 attention to the great "betterment" process or steadily pro- 

 gressive principle that seems to run, as a guiding thread, through 

 the entire plan of evolutionary unfolding. For they have 

 realized vividly that, as well in the ontogeny of any plant or 

 animal individual as in the progressive advance of any great 

 organic group, there often is shown an apparent intrinsic ten- 

 dency toward improvement, betterment, or advance, which 

 if understood or recognized would largely explain to us the 

 entire evolution process.* 



The law of Proenvironment furnishes the needed principle. 

 From abundant and direct experiment we know that every 

 nucleate organism, and many non-nucleate organisms, show 

 an exact response to each environal stimulus. This is evi- 

 dently due to certain molecular changes wrought by the stimu- 

 lating agent on the living substance, and later it may be on 

 certain substances accessory to it, as can be observed during 

 movement in leaves of Drosera, DioncBa, or Mimosa^ amongst 

 many others. These molecular changes inevitably cause a 

 definite course or pathway of movement to be pursued, which 

 is determined during that measurable period of time that we 

 now call the latent period or period of excitation. At the close 

 of that period — amounting, it may be, in some plants or plant 

 tissues to a quarter second, in others to several minutes, but 

 in most animals of much shorter duration — the organism 

 has plotted a definite line of response or a pathway, even though 

 as yet it may be motionless, that is as exact for the future 

 result as if the response action had already taken place. Or 

 in other words a definite mode or line of response has been 

 determined on, that will cause the organism to reach out to 

 or occupy a definite environal relation. 



* One of the last to refer to this is Osborn {IfS: 282), who says: "This muta- 

 bility, or evolution movement, or whatever it is that is in progress in the blastos, 

 is the chief and most difficult matter either to explain or to form any concep- 

 tion of. 



"Emphasis may be laid on the words adaptive 7ired or nccessily because there 

 is distinct evidence in palaeontology that the blastos is not evolving alone 

 through its internal forces independently of the external forces of ontogeny 

 and of environment, but that there is some harmony or form of interaction, 

 the nature of which we do not understand." 



