500 INTRODUCTION TO EVOLUTION 



But we usually think of progress in terms of increase rather than de- 

 crease in complexity of structure. Especially are we prone to think of 

 progress in terms of increasing mental development. We are rather an- 

 thropomorphic about the matter. In our judgments of lower animals, the 

 more they approach us in endowments the more progressive are we likely 

 to consider them. Perhaps after all there is justification for such a point of 

 view. Unquestionably we ourselves are the finest product yet produced by 

 the evolutionary process. Has natural selection been a major directive force 

 in progress defined as this "upward trend" in evolution? 



As noted above (p. 18), a progressive step in such a trend depends upon 

 two things: (1) an opportunity open and (2) an organism capable of tak- 

 ing advantage of that opportunity. For example, the phenomenon of the 

 emergence of vertebrates from the water to take up life on land depended 

 upon ( 1 ) presence of a dry-land environment as yet unexploited by verte- 

 brates and (2) presence of vertebrates (the Crossopterygii) capable of 

 making the change (pp. 15-17, 157-162). Natural selection in its varied 

 aspects supplied the stimulus causing certain Crossopterygii to forsake the 

 water and causing their descendants to become more and more perfectly 

 adapted to life on land. Thus when the nature of opportunities open is 

 such that more complex structure and greater mental endowments are 

 needed, natural selection in its role of promoting adaptation is found to be 

 promoting progress in the sense of our second, more restricted, definition 

 of the word. 



RATES OF EVOLUTIONARY CHANGE 



Two groups of factors are involved in determining the 

 rate of evolutionary change: factors within the organism and factors ex- 

 ternal to the organism. 



Internal Factors 



Of the internal factors involved, primacy must be accorded the rate with 

 which mutations occur, since mutations are the raw materials of evolution- 

 ary change. Other things being equal, we should expect a population in 

 which mutations occurred at a high rate to change more rapidly than would 

 a population having a low mutation rate. In the former population the ge- 

 netic equilibrium would be much more radically modified (p. 436) by oc- 

 currence of new mutations than in the latter population. Unfortunately, 

 positive evidence of the actual importance of this factor in determining 



